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1 ? fn9236.0 caution: these devices are sensitive to electrosta tic discharge; follow proper ic handling procedures. 1-888-intersil or 1-888-468-3774 | intersil (and design) is a registered trademark of intersil americas inc. dynamic vid? is a trademark of intersil americas inc. copyright ? intersil americas inc. 2006. all rights reserved all other trademarks mentioned are the property of their respective owners. isl6307a data sheet february 6, 200 6 ultra-high bandw idth 6-phase pwm controller with 8 bit vid code capable of precision r ds(on) or dcr differential current sensing the isl6307a controls micr oprocessor core voltage regulation by driving up to 6 synchronous-rectified buck channels in parallel. multiphase buck converter architecture uses interleaved timing to multiply channel ripple frequency and reduce input and output ripple currents. lower ripple results in fewer components, lower component cost, reduced power dissipation, and smaller implementation area. microprocessor loads can generate load transients with extremely fast edge rates. th e isl6307a features a high bandwidth control loop and ripple frequencies up to 12mhz to provide optimal response to the transients. the isl6307a senses current by utilizing patented techniques to measure the voltage across the on resistance, r ds(on) , of the lower mosfets or dcr, of the output inductor during the lower mosfet conduction intervals. current sensing provides the needed signals for precision droop, channel-current balancing, and overcurrent protection. a programmable internal temperature compensation function is implemented to effectively compensate for the temperatur e coefficient of the current sense element. a unity gain, differential amplifier is provided for remote voltage sensing. any potential difference between remote and local grounds can be completely eliminated using the remote-sense amplifier. eliminating ground differences improves regulation and protec tion accuracy. the threshold- sensitive enable input is available to accurately coordinate the start up of the isl6307a with any other voltage rail. dynamic-vid? technology allows seamless on-the-fly vid changes. the offset pin allows accurate voltage offset settings that are independent of vid setting. features ? precision multiphase core voltage regulation - differential remote voltage sensing - 0.5% system accuracy over life, load, line and temperature - adjustable precision reference-voltage offset ? precision r ds(on) or dcr current sensing - accurate load-line programming - accurate channel-current balancing - differential current sense ? microprocessor voltag e identification input - dynamic vid? technology - 8-bit vid icode with 6.25mv step - 0.5v to 1.600v operation range ? threshold-sensitive enable function for power sequencing and vtt enable ? driver enable output for application with drmos device ? thermal monitoring ? programmable temperature compensation ? overcurrent protection ? overvoltage protection with ovp output indication ? 2, 3, 4, 5 or 6 phase operation ? adjustable switching frequency up to 2mhz per phase ? qfn package option - qfn compliant to jedec pub95 mo-220 qfn - quad flat no leads - product outline - qfn near chip scale package footprint; improves pcb efficiency, thinner in profile ? pb-free plus anneal available (rohs compliant) ordering information part number (note) part marking temp. (c) package (pb-free) pkg. dwg. # ISL6307ACRZ ISL6307ACRZ 0 to 70 48 ld 7x7 qfn l48.7x7 isl6307airz isl6307airz -40 to 85 48 ld 7x7 qfn l48.7x7 *add ?-t? suffix to part number for tape and reel packaging. note: intersil pb-free plus anneal products employ special pb-free material sets; molding compounds/die attach materials and 100% matte tin plate termination finish, which are rohs compliant and compatible with both snpb and pb-free soldering operations. intersil pb-free products are msl classified at pb-free peak reflow temperatures that meet or exceed the pb-free requirements of ipc/jedec j std-020.
2 fn9236.0 february 6, 2006 pinout isl6307a (48 ld qfn) top view vid6 vid5 vid4 vid3 vid2 vid0 ofs vid1 tm vr_rdy vr_fan vr_hot fs en_vtt en_pwr drven vid7 vcc isen2+ pwm2 pwm3 isen3+ isen3- isen1- isen1+ pwm1 pwm4 isen4+ tcomp vsen rgnd fb comp pwm5 idroop iout dac isen2- isen4- isen5+ vdiff ovp isen6- isen6+ pwm6 ref isen5- ss 1 2 3 4 5 6 7 8 9 10 11 12 36 35 34 33 32 31 30 29 28 27 26 25 13 14 15 16 17 18 19 20 21 22 23 24 48 47 46 45 44 43 42 41 40 39 38 37 gnd isl6307a
3 fn9236.0 february 6, 2006 isl6307a block diagram i_trip ovp drive channel power-on reset (por) pwm1 pwm2 pwm3 pwm6 gnd vcc fb fs s clock and vid5 vid4 vid3 vid2 comp vsen generator sawtooth isen3- isen4+ vid1 rgnd vdiff vr_rdy ovp en_pwr 0.875v i_tot dynamic vid d/a temperature current balance channel detect ofs three-state isen1+ isen2- channel current sense ovp vid0 soft-start and fault logic offset ref +200mv r x1 e/a oc1 pwm pwm pwm pwm q en_vtt dac isen4- isen3+ isen2+ isen1- iout 0.875v vid6 vid7 pwm4 pwm pwm5 pwm isen5+ isen5- isen6+ isen6- idroop temperature compensation tcomp tm vr_hot vr_fan thermal monitoring ss 2v oc2 1 n gain compensation drven isl6307a
4 fn9236.0 february 6, 2006 typical application - 6-ph ase buck converter with r ds(on) sensing and external tcomp vid6 +5v pwm vcc boot ugate phase lgate gnd +5v vin vid7 vr_rdy vid5 vsen vdiff fb comp vcc gnd rgnd en_pwr pwm6 isen6- pwm4 isen4+ pwm2 isen2+ pwm1 isen1+ isl6307a p load isen6+ isen4- isen2- isen1- tcomp ref dac fs ofs en en_vtt vtt idroop isl6609 driver +12v r t vid4 vid3 vr_fan vr_hot tm +5v ntc r ofs pwm vcc boot ugate phase lgate gnd +5v vin en isl6609 driver pwm vcc boot ugate phase lgate gnd +5v vin en isl6609 driver pwm vcc boot ugate phase lgate gnd +5v vin en isl6609 driver pwm vcc boot ugate phase lgate gnd +5v vin en isl6609 driver pwm vcc boot ugate phase lgate gnd +5v vin en isl6609 driver isen3- pwm5 pwm3 isen3+ isen5- isen5+ ntc2 ss vid2 vid1 vid0 drven ovp iout r ss r iout external tcomp compensation network isl6307a
5 fn9236.0 february 6, 2006 typical application - 6-ph ase buck converter with r ds(on) sensing and integrated tcomp +5v pwm vcc boot ugate phase lgate gnd +5v vin vsen vdiff fb comp vcc gnd rgnd en_pwr pwm6 isen6- pwm4 isen4+ pwm2 isen2+ pwm1 isen1+ isl6307a p load isen6+ isen4- isen2- isen1- tcomp ref dac fs ofs en en_vtt vtt idroop isl6609 driver +12v r t vr_fan vr_hot tm +5v ntc r ofs pwm vcc boot ugate phase lgate gnd +5v vin en isl6609 driver pwm vcc boot ugate phase lgate gnd +5v vin en isl6609 driver pwm vcc boot ugate phase lgate gnd +5v vin en isl6609 driver pwm vcc boot ugate phase lgate gnd +5v vin en isl6609 driver pwm vcc boot ugate phase lgate gnd +5v vin en isl6609 driver isen3- pwm5 pwm3 isen3+ isen5- isen5+ ss +5v vid6 vid7 vr_rdy vid5 vid4 vid3 vid2 vid1 vid0 drven ovp iout r iout r ss isl6307a
6 fn9236.0 february 6, 2006 typical application - 6-phase buck converter with dcr se nsing and external tcomp +5v pwm vcc boot ugate phase lgate gnd +5v vin vsen vdiff fb comp vcc gnd rgnd en_pwr pwm6 isen6- pwm4 isen4+ pwm2 isen2+ pwm1 isen1+ isl6307a p load isen6+ isen4- isen2- isen1- tcomp ref dac fs ofs en en_vtt vtt idroop isl6609 driver +12v r t vr_fan vr_hot tm +5v ntc r ofs pwm vcc boot ugate phase lgate gnd +5v vin en isl6609 driver pwm vcc boot ugate phase lgate gnd +5v vin en isl6609 driver pwm vcc boot ugate phase lgate gnd +5v vin en isl6609 driver pwm vcc boot ugate phase lgate gnd +5v vin en isl6609 driver pwm vcc boot ugate phase lgate gnd +5v vin en isl6609 driver isen3- pwm5 pwm3 isen3+ isen5- isen5+ ntc2 ss vid6 vid7 vr_rdy vid5 vid4 vid3 vid2 vid1 vid0 ovp iout r iout r ss external tcomp compensation network drven isl6307a
7 fn9236.0 february 6, 2006 typical application - 6-phase buck converter with dcr se nsing and integrated tcomp +5v pwm vcc boot ugate phase lgate gnd +5v vin vsen vdiff fb comp vcc gnd rgnd en_pwr pwm6 isen6- pwm4 isen4+ pwm2 isen2+ pwm1 isen1+ isl6307a p load isen6+ isen4- isen2- isen1- tcomp ref dac fs ofs en en_vtt vtt idroop isl6609 driver +12v r t vr_fan vr_hot tm +5v ntc r ofs pwm vcc boot ugate phase lgate gnd +5v vin en isl6609 driver pwm vcc boot ugate phase lgate gnd +5v vin en isl6609 driver pwm vcc boot ugate phase lgate gnd +5v vin en isl6609 driver pwm vcc boot ugate phase lgate gnd +5v vin en isl6609 driver pwm vcc boot ugate phase lgate gnd +5v vin en isl6609 driver isen3- pwm5 pwm3 isen3+ isen5- isen5+ ss +5v vid6 vid7 vr_rdy vid5 vid4 vid3 vid2 vid1 vid0 ovp iout r iout r ss drven isl6307a
8 fn9236.0 february 6, 2006 absolute m aximum ratings supply voltage, vcc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .+6v all pins . . . . . . . . . . . . . . . . . . . . . . . . . . . gnd -0.3v to v cc + 0.3v esd (human body model). . . . . . . . . . . . . . . . . . . . . . . . . . . . .>2kv esd (machine model) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .>200v esd (charged device model) . . . . . . . . . . . . . . . . . . . . . . . . >1.5kv operating conditions supply voltage, vcc (5v bias mode, note 3) . . . . . . . . . . +5v 5% ambient temperature (ISL6307ACRZ) . . . . . . . . . . . . . 0c to 70c ambient temperature (isl6307airz) . . . . . . . . . . . . .-40c to 85c thermal information thermal resistance (typical, notes 1, 2) ja (c/w) jc (c/w) qfn package. . . . . . . . . . . . . . . . . . . . 32 6.5 maximum junction temperature . . . . . . . . . . . . . . . . . . . . . . . 150c maximum storage temperature range . . . . . . . . . . . -65c to 150c maximum lead temperature (soldering 10s) . . . . . . . . . . . . . 300c caution: stress above those listed in ?absolute maximum ratings? may cause permanent damage to the device. this is a stress onl y rating and operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. notes: 1. ja is measured in free air with the component mounted on a high ef fective thermal conductivity te st board with ?direct attach? fe atures. see tech brief tb379 2. for jc , the ?case temp? location is the center of the exposed metal pad on the package underside . electrical specifications operating conditions: vcc = 5v or icc < 25ma. unless otherwise specified parameter test conditions min typ max units vcc supply current nominal supply vcc = 5vdc; en_pwr = 5vdc; r t = 100k ?, isen1 = isen2 = isen3 = isen4 = -70 a -1518ma shutdown supply vcc = 5vdc; en_pwr = 0vdc; r t = 100k ? -1012ma power-on reset and enable por threshold vcc rising 4.3 4.5 4.70 v vcc falling 3.7 3.9 4.20 v en_pwr threshold rising 0.850 0.875 0.910 v hysteresis - 130 - mv falling 0.720 0.745 0.775 v en_vtt threshold rising 0.850 0.875 0.910 v hysteresis - 130 - mv falling 0.720 0.745 0.775 v reference voltage and dac system accuracy of ISL6307ACRZ (vid = 1v-1.6v), t j = 0c to 70c (note 3) -0.5 - 0.5 %vid system accuracy of ISL6307ACRZ (vid = 0.5v-1v), t j = 0c to 70c (note 3) -0.9 - 0.9 %vid system accuracy of isl6307airz (vid = 1v-1.6v), t j = -40c to 85c (note 3) -0.6 - 0.6 %vid system accuracy of isl6307airz (vid = 0.5v-1v), t j = -40c to 85c (note 3) -1 - 1 %vid vid pull up -60 -40 -20 a vid input low level --0.4v vid input high level 0.8 - - v dac source current -47ma dac sink current - - 300 a ref source current 45 50 55 a ref sink current 45 50 55 a isl6307a
9 fn9236.0 february 6, 2006 pin-adjustable offset voltage at ofs pin for ISL6307ACRZ offset resistor connected to ground 392 400 408 mv voltage below vcc, offset resistor connected to vcc 1.568 1.600 1.632 v voltage at ofs pin for isl6307airz offset resistor connected to ground 388 400 412 mv voltage below vcc, offset resistor connected to vcc 1.552 1.600 1.648 v oscillators accuracy of switching frequency setting r t = 100k ? 225 250 275 khz adjustment range of switching frequency (note 4) 0.08 - 2.0 mhz soft-start ramp rate (note 5, 6) r ss = 100k ? - 1.563 - mv/ s adjustment range of soft-start ramp rate (note 4) 0.625 - 6.25 mv/ s pwm generator sawtooth amplitude -1.5- v max duty cycle - 66.7 - % error amplifier open-loop gain r l = 10k ? to ground (note 4) - 96 - db open-loop bandwidth c l = 100pf, r l = 10k ? to ground (note 4) - 100 - mhz slew rate c l = 100pf (note 4) - 20 - v/ s maximum output voltage 3.8 4.3 4.9 v output high voltage @ 2ma 3.6 - - v output low voltage @ 2ma --1.2v remote-sense amplifier bandwidth (note 4) - 20 - mhz output high current vsen - rgnd = 2.5v -500 - 500 a output high current vsen - rgnd = 0.6 -500 - 500 a pwm output pwm output voltage low threshold iload = 500 a--0.5v pwm output voltage high threshold iload = 500 a4.3--v driver enable output drven output voltage low with 1.250k ? resistor pull up to 5v, i vr_hot = 4ma - - 0.9 v drven output voltage high with 1.250k ? resistor pull up to 5v, i vr_hot = 4ma 3.8 - - v sense current output (idroop and iout) sensed current tolerance isen1 = isen2 = isen3 = isen4 = isen5 = isen6 = 80 a 768084 a overcurrent trip level 90 100 110 a maximum voltage at idroop and iout pins -2- v thermal monitoring tm input voltage for vr_fan trip 1.6 1.65 1.69 v tm input voltage for vr_fan reset 1.89 1.93 1.98 v tm input voltage for vr_hot trip 1.35 1.4 1.44 v tm input voltage for vr_hot reset 1.6 1.65 1.69 v leakage current of vr_hot with external pull-up resistor connected to 5v - - 30 a vr_hot low voltage with 1.250k ? resistor pull up to 5v, i vr_hot = 4ma - - 0.3 v leakage current of vr_fan with external pull-up resistor connected to 5v - - 30 a vr_fan low voltage with 1.250k ? resistor pull up to 5v, i vr_fan = 4ma - - 0.3 v electrical specifications operating conditions: vcc = 5v or icc < 25ma. unless otherwise specified (continued) parameter test conditions min typ max units isl6307a
10 fn9236.0 february 6, 2006 vr ready and protection monitors leakage current of vr_rdy with external pull-up resistor connected to 5v - - 30 a vr_rdy low voltage i vr_rdy = 4ma - - 0.3 v under voltage trip of vr-rdy vsen falling 48 50 52 %vid vr-rdy reset voltage vsen rising 58 60 62 %vid overvoltage protection threshold before valid vid 1.250 1.275 1.300 v after valid vid, the voltage above vid 150 175 200 mv overvoltage reset threshold 0.38 0.40 0.42 v ovp output high voltage i ovp = 4ma 4.5 - - v ovp output low voltage i ovp = 4ma - - 0.25 v notes: 3. these parts are designed and adjusted for accuracy with all errors in the voltage loop included. 4. spec guaranteed by design. 5. during soft-start, vdac rises from 0 to 1.1v first and then ramp to vid voltage after receiving valid vid input. 6. soft-start ramp rate is determined by the adjustable soft -start oscillator frequency at the speed of 6.25mv per cycle. electrical specifications operating conditions: vcc = 5v or icc < 25ma. unless otherwise specified (continued) parameter test conditions min typ max units isl6307a
11 fn9236.0 february 6, 2006 functional pin description vcc - supplies all the power necessary to operate the chip. the controller starts to operate when the voltage on this pin exceeds the rising por threshold and shuts down when the voltage on this pin drops below the falling por threshold. connect this pin directly to a +5v supply. gnd - bias and reference ground for the ic. the bottom metal base of isl6307a is the gnd. en_pwr - this pin is a threshold- sensitive enable input for the controller. connecting the 12v supply to en_pwr through an appropriate resistor divider provides a means to synchronize power-up of the controller and the mosfet driver ics. when en_pwr is driven above 0.875v, the isl6307a is active depending on status of en_vtt, the internal por, and pending fault states. driving en_pwr below 0.745v will clear all fault states and prime the isl6307a to soft-start when re-enabled. en_vtt - this pin is another th reshold-sensitive enable input for the controller. it?s typically connected to vtt output of vtt voltage regulator in the computer mother board. when en_vtt is driven above 0.875v, the isl6307a is active depending on status of enll, the internal por, and pending fault states. driving en_vtt below 0.745v will clear all fault states and prime the isl6307a to soft-start when re- enabled . fs - use this pin to set up the desired switching frequency. a resistor, placed from fs to gr ound will set the switching fre- quency. the relationship between the value of the resistor and the switching frequency will be described by an approxi- mate equation 40. ss - use this pin to set up the desired start-up oscillator fre- quency. a resistor, placed from ss to ground will set up the soft-start ramp rate. the relationship between the value of the resistor and the soft-start ramp up time will be described by an approximate equation 14. vid7, vid6, vid5, vid4, vid3 , vid2, vid1 and vid0 - these are the inputs to the internal dac that provide the reference voltage for output regulation. connect these pins either to open-drain outputs with or without external pull-up resistors or to active pull-up outputs. vid7-vid0 have 40a internal pull-up current sources that diminish to zero as the voltage rises above the logic-high level. these inputs can be pulled up as high as vcc plus 0.3v. when a vid code causes a shut-off, the controller needs to be reset before it will start again. vsen and rgnd - vsen and rgnd form the precision differential remote-sense amplifier. this amplifier converts the differential voltage of the remote output to a single-ended voltage referenced to local ground. connect vsen and rgnd to the sense pins of the remote load. vdiff - vdiff is the amplifier?s output and the input to the regulation and protection circuitry. it should be connected to fb through a resistor. fb and comp - inverting input and output of the error amplifier respectively. fb is connected to vdiff through a resistor. a negative current, proportional to output current is present on the fb pin. a properly sized resistor between vdiff and fb sets the load-line (droop). the droop scale factor is set by the ratio of the isen resistors and the lower mosfet r ds(on) . comp is tied back to fb through an external r-c network to compensate the regulator. dac and ref - the dac output pin is the output of the precision internal dac reference. the ref input pin is the positive input of the error amp. in typical applications, a 1k ? , 1% resistor is used between dac and ref to generate a precise offset voltage. this vo ltage is proportional to the offset current determined by the offset resistor from ofs to ground or vcc. a capacitor is used between ref and ground to smooth the voltage transition during dynamic vid? operations. pwm1, pwm2, pwm3, pw m4, pwm5, pwm6 - pulse- width modulation outputs. co nnect these pins to the pwm input pins of the intersil driver ic. the number of active channels is determined by the state of pwm3, pwm4, pwm5 and pwm 6. tie pwm3 to vcc to configure for 2-phase operation. tie pwm4 to vcc to configure for 3-phase operation. tie pwm5 to vcc to configure for 4-phase operation. tie pwm6 to vcc to configure for 5-phase operation. isen1+, isen1-; isen2+, isen2-; isen3+, isen3-; isen4+, isen4-; isen5+, isen5-; isen6+, isen6- - the isen+ and isen- pins are current sense inputs to individual differential amplifiers. the sensed current is used as a reference for channel balancing, protection, and regulation. inactive channels should have their respective current sense inputs left open (for example, for 3-phase operation open isen4+). for dcr sensing, connect each isen- pin to the node between the rc sense elements. tie the isen+ pin to the other end of the sense capacitor through a resistor, r isen . the voltage across the sense capacitor is proportional to the inductor current. the sense current is proportional to the output current, and scaled by the dcr of the inductor and r isen . when configured for r ds(on) current sensing, the isen1-, isen2-, isen3-, isen4-, isen5- , isen6- pins are grounded at the lower mosfet sour ces. the isen1+, isen2+, isen3+, isen4+, isen5+, isen6+ pins are then held at a virtual ground, such that a re sistor connected between them, and the drain terminal of the associated lower mosfet, will carry a current proportional to the current flowing through that channel. the current is determined by the negative isl6307a
12 fn9236.0 february 6, 2006 voltage developed across the lower mosfet?s r ds(on) , which is the channel current scaled by r ds(on) and r isen . vr_rdy - vr_rdy is used as an indication of the end of soft-start with certain delay per intel vr11. it is an open- drain logic output that is low impedance until the soft-start is completed. it will be pulled low again once the undervoltage point is reached. ofs - the ofs input pin provides means to program a dc offset current for generating a dc offset voltage at the ref input. the offset current is gen erated via an external resistor and precision internal voltage references. the polarity of the offset is selected by connecting the resistor to gnd or vcc. for no offset, the ofs pin should be left unterminated. tcomp - temperature compensation scaling input. the voltage sensed on the tm pin is utilized as the temperature input to adjust ldroop and the over current protection limit to effectively compensate for the temperature coefficient of the current sense element. to implement the integrated temperature compensation, a re sistor divider circuit is needed as shown in the typical application diagrams. changing the ratio of the resistor values will set the gain of the integrated thermal comp ensation. when integrated temperature compensation function is not used, connect tcomp to gnd. ovp - the overvoltage protection output indication pin. this pin can be pulled to vcc and is latched when an overvoltage condition is detected. when not used, keep this pin open. idroop - the output pin of se nsed average channel current which is proportional to load current. in the application which does not requ ire load-line, leave this pin open. in the application which requires load-line, connect this pin to fb so that the sensed average current will flow through the resistor between fb and vdiff to create a voltage drop which is proportional to load current. iout - iout has the same output as idroop with additional ocp adjustment functi on. in actual application, a resistor needs to be placed between iout and gnd to ensure the proper operation. the voltage at iout pin will be proportional to the load current. if the voltage is higher than 2v, isl6307a will go into ocp mode. the ocp trip level can be adjusted by changing the resistor value. drven - driver enable output pin. this pin can be used to enable the mosfet drivers which have enable pins such as isl6609, isl6608 or other drmo s devices. if isl6307a is used with intersil?s isl6612 drivers, it?s not necessary to use this pin. tm - tm is an input pin for vr temperature measurement. connect this pin through a ntc thermistor to gnd and a resistor to 5v. the voltage at this pin is proportional to the vr temperature. isl6307a monitors the vr temperature based on the voltage at tm and triggers vr_fan and vr_hot signals based on the temperature thresholds. vr_hot - an indication output pin of high vr temperature. it is an open-drain logic out put with low impedance. it will be pulled high when measured vr temperature reaches certain level. vr_fan - an indication output pin of vr temperature high warning with open-drain logic. it will be pulled high when measured vr temperature reaches certain level. vr_fan will be pulled high before vr_hot. operation multiphase power conversion microprocessor load current profiles have changed to the point that the advantages of multiphase power conversion are impossible to ignore. the technical challenges associated with producing a single-phase converter which is both cost-effective and thermally viable, have forced a change to the cost-saving approach of multiphase. the isl6307a controller helps reduce the complexity of implementation by integrating vital functions and requiring minimal output components. the block diagrams on pages 4, 5, 6 and 7 provide top level views of multiphase power conversion using the isl6307a controller. interleaving the switching of each channel in a multiphase converter is timed to be symmetrically out of phase with each of the other channels. in a 3-phase converter, each channel switches 1/3 cycle after the previous channel and 1/3 cycle before the following channel. as a result, the three-phase converter has a combined ripple frequency three times greater than the ripple frequency of any one phase. in addition, the peak-to- peak amplitude of the combined inductor current is reduced in proportion to the number of phases (equations 1 and 2). increased ripple frequency and lower ripple amplitude mean that the designer can use less per-channel inductance and lower total output capacitance for any performance specification. figure 1. pwm and inductor-current waveforms for 3-phase converter 1s/div pwm2, 5v/div pwm1, 5v/div il2, 7a/div il1, 7a/div il1 + il2 + il3, 7a/div il3, 7a/div pwm3, 5v/div isl6307a
13 fn9236.0 february 6, 2006 figure 1 illustrates the multiplica tive effect on output ripple frequency. the three channel currents (il1, il2, and il3) combine to form the ac ripple current and the dc load current. the ripple component has three times the ripple frequency of each individual channel current. each pwm pulse is terminated 1/3 of a cycle after the pwm pulse of the previous phase. the peak-to- peak current for each phase is about 7a, and the dc components of the inductor currents combine to feed the load. to understand the reduction of ripple current amplitude in the multiphase circuit, examine the equation representing an individual channel?s peak-to-peak inductor current. in equation 1, v in and v out are the input and output voltages respectively, l is the single-channel inductor value, and f s is the switching frequency. the output capacitors conduct the ripple component of the inductor current. in the case of multiphase converters, the capacitor current is the sum of the ripple currents from each of the individual channels. compare equation 1 to the expression for the peak-to-peak current after the summation of n symmetrically phase-shi fted inductor currents in equation 2. peak-to-peak ripple current decreases by an amount proportional to the number of channels. output- voltage ripple is a function of capacitance, capacitor equivalent series resistance (esr), and inductor ripple current. reducing the inductor ripple current allows the designer to use fewer or less costly output capacitors. another benefit of interleaving is to reduce input ripple current. input capacitance is determined in part by the maximum input ripple current. multiphase topologies can improve overall system cost and size by lowering input ripple current and allowing the designer to reduce the cost of input capacitance. the example in figure 2 illustrates input currents from a three-phase converter combining to reduce the total input ripple current. the converter depicted in figure 2 delivers 36a to a 1.5v load from a 12v input. the rms input capacitor current is 5.9a. compare this to a single-phase converter also stepping down 12v to 1.5v at 36a. the si ngle-phase converter has 11.9a rms input capacitor current. the single-phase converter must use an input capacitor bank with twice the rms current capacity as the equivalent three-phase converter. figures 25, 26 and 27 in the section entitled input capacitor selection can be used to determine the input-capacitor rms current based on load current , duty cycle, and the number of channels. they are provided as aids in determining the optimal input capacitor solution. figure 28 shows the single phase input-capacitor rms current for comparison. pwm operation the timing of each converter leg is set by the number of active channels. the default channel setting for the isl6307a is four. one switching cycle is defined as the time between pwm1 pulse termination signals. the pulse termination signal is an internally generated clock signal which triggers the falling edge of pwm1. the cycle time of the pulse termination signal is the inverse of the switching frequency set by the resistor between the fs pin and ground. each cycle begins when the clock signal commands the channel-1 pwm output to go low. the pwm1 transition signals the channel-1 mosfet driver to turn off the channel- 1 upper mosfet and turn on the channel-1 synchronous mosfet. in the default channel configuration, the pwm2 pulse terminates 1/4 of a cycle after pwm1. the pwm3 output follows anot her 1/4 of a cycle after pwm2. pwm4 terminates another 1/4 of a cycle after pwm3. if pwm3 is connected to vcc, two channel operation is selected and the pwm2 pulse te rminates 1/2 of a cycle later. connecting pwm4 to vcc selects three channel operation and the pulse-termination ti mes are spaced in 1/3 cycle increments. connecting both pwm3 and pwm4 to vcc selects single-channel operation. once a pwm signal transitions low, it is held low for a minimum of 1/3 cycle. this forc ed off time is required to ensure an accurate current sample. current sensing is described in the next section. after the forced off time expires, the pwm output is enab led. the pwm output state is driven by the position of th e error amplifier output signal, v comp , minus the current correction signal relative to the sawtooth ramp as illustrated in figure 7. when the modified v comp voltage crosses the sawtoot h ramp, the pwm output transitions high. the mosfet driver detects the change in state of the pwm signal and turns off the synchronous (lower) mosfet and turns on the upper mosfet. the i pp v in v out ? () v out lf s v in ----------------------------------------------------- - = (eq. 1) figure 2. channel input currents and input- capacitor rms current for 3-phase converter channel 1 input current 10a/div channel 2 input current 10a/div channel 3 input current 10a/div input-capacitor current, 10a/div 1s/div i cpp , v in nv out ? () v out lf s v in ----------------------------------------------------------- - = (eq. 2) isl6307a
14 fn9236.0 february 6, 2006 pwm signal will remain high until the pulse termination signal marks the begi nning of the next cycl e by triggering the pwm signal low. current sampling during the forced off-time following a pwm transition low, the associated channel current sense amplifier uses the isen inputs to reproduce a signal proportional to the inductor current, i l . this current gets sampled starting 1/6 period after each pwm goes low and continuously gets sampled for 1/3 period, or until the pwm goes high, whichever comes first. no matter the current sense method, the sense current, i sen , is simply a scaled version of the inductor current. coincident with the falling edge of the pwm signal, the sample and hold circuitry samples the sensed current signal i sen , as illustrated in figure 3. therefore, the sample current, i n , is proportional to the output current and held for one switching cycle. the sample current is used for current balance, load-line regulation, and overcurrent protection. current sensing the isl6307a supports inductor dcr sensing, mosfet r ds(on) sensing, or resistive sensing techniques. the internal circuitry, shown in figures 4, 5, and 6, represents one channel of an n-channel converter. this circuitry is repeated for each channel in the converter, but may not be active depending on the status of the pwm3 and pwm4 pins, as described in the pwm operation section. inductor dcr sensing an inductor?s winding is characteristic of a distributed resistance as measured by the dcr (direct current resistance) parameter. consider the inductor dcr as a separate lumped quantity, as shown in figure 4. the channel current i l , flowing through the inductor, will also pass through the dcr. equation 3 shows the s-domain equivalent voltage across the inductor v l . a simple r-c network across th e inductor extracts the dcr voltage, as shown in figure 4. the voltage on the capacitor v c , can be shown to be proportional to the channel current i l , see equation 4. if the r-c network components are selected such that the rc time constant (= r*c) matches the inductor time constant (= l/dcr), the voltage across the capacitor v c is equal to the voltage drop across the dcr, i.e. proportional to the channel current. with the internal low-offset current amplifier, the capacitor voltage v c is replicated across the sense resistor r isen . therefore the current out of isen+ pin, i sen , is proportional to the inductor current. equation 5 shows that the ratio of the channel current to the sensed current i sen is driven by the value of the sense resistor and the dcr of the inductor. figure 3. sample and hold timing time pwm i l switching period i sen 0.5tsw sample current, i n v l i l sl dcr + ? () ? = (eq. 3) v c s l dcr ------------- ? 1 + ?? ?? dcr i l ? () ? src 1 + ? () -------------------------------------------------------------------- - = (eq. 4) figure 4. dcr sensing configuration i n i sen i l dcr r isen ----------------- - = - + isen-(n) sample & hold isl6307a internal circuit v in isen+(n) pwm(n) isl6609 r isen(n) dcr l inductor r v out c out (ptc) - + v c (s) c i l s () - + v l i sen i l dcr r isen ----------------- - ? = (eq. 5) isl6307a
15 fn9236.0 february 6, 2006 resistive sensing for accurate current sense, a dedicated current-sense resistor r sense in series with each output inductor can serve as the current sense element (see figure 5). this technique is more accurate, but reduces overall converter efficiency due to the additional power loss on the current sense element r sense . equation 6 shows the ratio of the channel current to the sensed current i sen . mosfet r ds(on) sensing the controller can also sense the channel load current by sampling the voltage across the lower mosfet r ds(on) (see figure 6). the amplifier is ground-reference by connecting the isen- pin to the source of the lower mosfet. isen+ pin is connected to the phase node through the current sense resistor r isen . the voltage across r isen is equivalent to the voltage drop across the r ds(on) of the lower mosfet while it is conducting. the resulting current out of the isen+ pin is proportional to the channel current i l . equation 7 shows the ratio of the channel current to the sensed current i sen . both inductor dcr and mosfet r ds(on) value will increase as the temperatur e increases. therefore the sensed current will increase as the temperature of the current sense element increases . in order to compensate the temperature effect on the sensed current signal, a positive temperature coeffici ent (ptc) resistor can be selected for the sense resistor r isen , or the integrated temperature compensation function of isl6307a should be utilized. the integrated temper ature compensation function is described in the temperature compensation section. channel-current balance the sensed current i n from each active channel are summed together and divided by the num ber of active channels. the resulting average current i avg provides a measure of the total load current. channel current balance is achieved by comparing the sampled current of each channel to the average current to make an appropriate adjustment to the pwm duty cycle of each channel. intersil?s patented current- balance method is illustrat ed in figure 7. the average current combines with the channel 1 current i 1 to create an error signal i er . the filtered error signal modifies the pulse width commanded by v comp to correct any unbalance and force i er toward zero. the same method for error signal correction is applied to each active channel. i sen i l r sense r isen ----------------------- ? = (eq. 6) figure 5. sense resistor in series with inductors i n i sen i l r sense r isen -------------------------- = - + isen-(n) sample & hold isl6307a internal circuit isen+(n) r isen(n) r sense l v out c out i l figure 6. mosfet r ds(on) current-sensing circuit i n i sen i l r ds on () r isen --------------------------- - = - + isen+(n) r isen sample & hold isl6307a internal circuit external circuit v in n-channel mosfets - + i l x r ds on () i l isen-(n) (ptc) i sen i l r ds on () r isen ------------------------ - = (eq. 7) isl6307a
16 fn9236.0 february 6, 2006 channel current balance is essential in achieving the thermal advantage of multiphase operation. with good current balance, the power loss is equally dissipated over multiple devices and a greater area. voltage regulation the integrating compensation network shown in figure 8 assures that the steady-state e rror in the output voltage is limited only to the error in t he reference voltage (output of the dac) and offset errors in the ofs current source, remote-sense and error amplifiers. intersil specifies the guaranteed tolerance of the isl6307a to include the combined tolerances of each of these elements. the output of the error amplifier, v comp , is compared to the sawtooth waveform to generate the pwm signals. the pwm signals control the timing of the intersil mosfet drivers and regulate the converter output to the specified reference voltage. the internal and external circuitry which control voltage regulation is illustrated in figure 8. the isl6307a incorporates an internal differential remote- sense amplifier in the feedba ck path. the amplifier removes the voltage error encountered when measuring the output voltage relative to the local controller ground reference point, resulting in a more accurate means of sensing output voltage. connect the microprocessor sense pins to the non- inverting input, vsen, and inverting input, rgnd, of the remote-sense amplifier. t he remote-sense output, v diff , is connected to the inverting input of the error amplifier, fb, through an external resistor, r fb . a digital to analog converter (dac) generates a reference voltage based on the state of logic signals at pins vid7 through vid0. the dac decodes the 8-bit logic signal (vid) into one of the discrete voltages shown in table 1. each vid input offers a 45 a pull-up to an internal 2.5v source for use with open-drain outputs. the pul l-up current diminishes to zero above the logic threshold to protect voltage-sensitive output devices. external pull-up resistors can augment the pull-up current sources if case leakage into the driving device is greater than 45 a. load-line regulation some microprocessor manufacturers require a precisely- controlled output resistance. this dependence of output voltage on load current is often termed ?droop? or ?load-line? regulation. by adding a well controlled output impedance, the output voltage can effectively be level shifted in a direction which works to achieve the load-line regulation required by these manufacturers. figure 7. channel-1 pwm function and current- balance adjustment n i avg i 5 i 4 i 3 - + + - + - f(j ) pwm1 i 1 v comp sawtooth signal i er filter i 6 i 2 figure 8. output voltage and load-line regulation with offset adjustment i avg external circuit isl6307a internal circuit comp r c r fb fb vdiff vsen rgnd - + v droop error amplifier - + v out + differential remote-sense amplifier v comp c c ref dac r ref c ref - + v out - idroop isl6307a
17 fn9236.0 february 6, 2006 table 1. vid code vid7 800mv vid6 400mv vid5 200mv vid4 100mv vid3 50mv vid2 25mv vid1 12.5mv vid0 6.25mv dac voltage 00000000off 00000001off 000000101. 60000 000000111. 59375 000001001. 58750 000001011. 58125 000001101. 57500 000001111. 56875 000010001. 56250 000010011. 55625 000010101. 55000 000010111. 54375 000011001. 53750 000011011. 53125 000011101. 52500 000011111. 51875 000100001. 51250 000100011. 50625 000100101. 50000 000100111. 49375 000101001. 48750 000101011. 48125 000101101. 47500 000101111. 46875 000110001. 46250 000110011. 45625 000110101. 45000 000110111. 44375 000111001. 43750 000111011. 43125 000111101. 42500 000111111. 41875 001000001. 41250 001000011. 40625 001000101. 40000 001000111. 39375 001001001. 38750 001001011. 38125 001001101. 37500 001001111. 36875 001010001. 36250 001010011. 35625 001010101. 35000 001010111. 34375 001011001. 33750 001011011. 33125 001011101. 32500 001011111. 31875 001100001. 31250 001100011. 30625 001100101. 30000 001100111. 29375 001101001. 28750 001101011. 28125 001101101. 27500 001101111. 26875 001110001. 26250 001110011. 25625 001110101. 25000 001110111. 24375 001111001. 23750 001111011. 23125 001111101. 22500 001111111. 21875 010000001. 21250 010000011. 20625 010000101. 20000 010000111. 19375 010001001. 18750 010001011. 18125 010001101. 17500 010001111. 16875 010010001. 16250 010010011. 15625 010010101. 15000 010010111. 14375 010011001. 13750 010011011. 13125 table 1. vid code (continued) vid7 800mv vid6 400mv vid5 200mv vid4 100mv vid3 50mv vid2 25mv vid1 12.5mv vid0 6.25mv dac voltage isl6307a
18 fn9236.0 february 6, 2006 010011101. 12500 010011111.1 1875 010100001.1 1250 010100011. 10625 010100101. 10000 010100111. 09375 010101001. 08750 010101011. 08125 010101101. 07500 010101111. 06875 010110001. 06250 010110011. 05625 010110101. 05000 010110111. 04375 010111001. 03750 010111011. 03125 010111101. 02500 010111111. 01875 011000001. 01250 011000011. 00625 011000101. 00000 011000110. 99375 011001000. 98750 011001010. 98125 011001100. 97500 011001110. 96875 011010000. 96250 011010010. 95625 011010100. 95000 011010110. 94375 011011000. 93750 011011010. 93125 011011100. 92500 011011110. 91875 011100000. 91250 011100010. 90625 011100100. 90000 011100110. 89375 011101000. 88750 table 1. vid code (continued) vid7 800mv vid6 400mv vid5 200mv vid4 100mv vid3 50mv vid2 25mv vid1 12.5mv vid0 6.25mv dac voltage 011101010. 88125 011101100. 87500 011101110. 86875 011110000. 86250 011110010. 85625 011110100. 85000 011110110. 84375 011111000. 83750 011111010. 83125 011111100. 82500 011111110. 81875 100000000. 81250 100000010. 80625 100000100. 80000 100000110. 79375 100001000. 78750 100001010. 78125 100001100. 77500 100001110. 76875 100010000. 76250 100010010. 75625 100010100. 75000 100010110. 74375 100011000. 73750 100011010. 73125 100011100. 72500 100011110. 71875 100100000. 71250 100100010. 70625 100100100. 70000 100100110. 69375 100101000. 68750 100101010. 68125 100101100. 67500 100101110. 66875 100110000. 66250 100110010. 65625 100110100. 65000 100110110. 64375 table 1. vid code (continued) vid7 800mv vid6 400mv vid5 200mv vid4 100mv vid3 50mv vid2 25mv vid1 12.5mv vid0 6.25mv dac voltage isl6307a
19 fn9236.0 february 6, 2006 in other cases, the designer may determine that a more cost-effective solution can be achieved by adding droop. droop can help to reduce the output-voltage spike that results from fast load-current demand changes. the magnitude of the spike is dictated by the esr and esl of the output capacitors selected. by positioning the no-load voltage level near the upper s pecification limit, a larger negative spike can be sustained without crossing the lower limit. by adding a well cont rolled output impedance, the output voltage under load can effectively be level shifted down so that a larger positive spike can be sustained without crossing the upper specification limit. as shown in figure 8, a current proportional to the average current in all active channels, i avg , flows from fb through a load-line regulation resistor, r fb . the resulting voltage drop across r fb is proportional to the output current, effectively creating an output voltage droo p with a steady-state value defined as the regulated output voltage is reduced by the droop voltage v droop . the output voltage, as a f unction of load current, is derived by combining equation 8 with the appropriate sample current expression defined by the current sense method employed. v ref is the reference voltage and v ofs is the programmed offset voltage. i out is the total output current of the converter, r isen is the sense resistor in the isen line, n is the number of active channels, and r fb is the feedback resistor. r x has a value of dcr, resistor or r ds(on) , or r sense , depending on the sensing method. therefore, the equivalent lo ad-line impedance, i.e. droop impedance, is equal to: output-voltage offset programming the isl6307a allows the designer to accurately adjust the offset voltage. when a resistor, r ofs , is connected between ofs to vcc, the voltage across it is regulated to 1.6v. this causes a proportional current (i ofs ) to flow into ofs. if r ofs is connected to ground, the voltage across it is regulated to 0.4v, and i ofs flows out of ofs. a resistor between dac and ref, r ref , is selected so that the product (i ofs x r ofs ) is equal to the desired offset voltage. these functions are shown in figure 9. once the desired output offset voltage has been determined, use the following formulas to set r ofs : for positive offset (connect r ofs to vcc): for negative offset (connect r ofs to gnd): 100111000. 63750 100111010. 63125 100111100. 62500 100111110. 61875 101000000. 61250 101000010. 60625 101000100. 60000 101000110. 59375 101001000. 58750 101001010. 58125 101001100. 57500 101001110. 56875 101010000. 56250 101010010. 55625 101010100. 55000 101010110. 54375 101011000. 53750 101011010. 53125 101011100. 52500 101011110. 51875 101100000. 51250 101100010. 50625 101100100. 50000 11111110off 11111111off table 1. vid code (continued) vid7 800mv vid6 400mv vid5 200mv vid4 100mv vid3 50mv vid2 25mv vid1 12.5mv vid0 6.25mv dac voltage v droop i avg r fb = (eq. 8) v out v ref v offset ? i out n ------------- r x r isen ----------------- -r fb ?? ?? ?? ? = (eq. 9) r ll r fb n ------------ r x r isen ----------------- - = (eq. 10) r ofs 1.6 r ref v offset ----------------------------- - = (eq. 11) r ofs 0.4 r ref v offset ----------------------------- - = (eq. 12) isl6307a
20 fn9236.0 february 6, 2006 dynamic vid modern microprocessors need to make changes to their core voltage as part of normal operation. they direct the core-voltage regulator to do this by making changes to the vid inputs during regulator operation. the power management solution is required to monitor the dac inputs and respond to on-the-fly vid changes in a controlled manner. supervising the safe ou tput voltage transition within the dac range of the proce ssor without discontinuity or disruption is a necessary function of the core-voltage regulator. the isl6307a checks the vid inputs six times every switching cycle. if the vid co de is found to have been changed, the controller waits ha lf of a complete cycle before executing a 12.5mv change. if during the half-cycle wait period, the difference between dac level and the new vid code changes, no change is made. if the vid code is more than 1 bit higher or lower than the dac (not recommended), the controller will execute 12.5mv changes six times per cycle until vid and dac are equal. it is for this reason that it is important to carefully contro l the rate of vid stepping in 1- bit increments. in order to ensure the smooth transition of output voltage during vid change, a vid step change smoothing network composed of r ref and c ref is required for an isl6307a based voltage regulator (see figure 8). the selection of r ref is based on the desired offset as detailed above in output-voltage offset programming section. the selection of c ref is based on the time duration for 1 bit vid change and the allowable delay time. assuming the microprocessor co ntrols the vid change at 1 bit every t vid , the relationship between the time constant of r ref and c ref network and t vid is given by equation 13. operation initialization prior to converter initialization, proper conditions must exist on the enable inputs and vcc. when the conditions are met, the controller begins soft-sta rt. once the output voltage is within the proper window of operation, vr_rdy asserts a logic 1. enable and disable while in shutdown mode, the pwm outputs are held in a high-impedance state to assure the drivers remain off. the following input conditions must be met before the isl6307a is released from shutdown mode. 1. the bias voltage applied at vcc must reach the internal power-on reset (por) rising threshold. once this threshold is reached, proper operation of all aspects of the isl6307a is guaranteed . hysteresis between the rising and falling thresholds assure that once enabled, the isl6307a will not inadvertently turn off unless the bias voltage drops substantially (see electrical specifications ). 2. the isl6307a features an enable input (en_pwr) for power sequencing between the controller bias voltage dynamic vid d/a e/a vcc dac fb ref ofs vcc gnd + - + - 0.4v 1.6v or gnd r ofs r ref isl6307acr figure 9. output voltag e offset programming with isl6307a c ref r re f t vid = (eq. 13) figure 10. power sequencing using threshold- sensitive enable (en) function - + 0.875v external circuit isl6307a internal circuit en_pwr +12v por circuit 10k ? 91 0? enable comparator soft-start and fault logic en_vtt vcc + - 0.875v isl6307a
21 fn9236.0 february 6, 2006 and another voltage rail. the enable comparator holds the isl6307a in shutdown until the voltage at en_pwr rises above 0.875v. the enable comparator has about 130mv of hysteresis to prevent bounce. it is important that the driver ics reach their por level before the isl6307a becomes enabled. the schematic in figure 10 illustrates the sequencing of the isl6307a with the isl66xx family of intersil mo sfet drivers, which require 12v bias. 3. the voltage on en_vtt must be higher than 0.875v to enable the controller. this pin is typically connected to the output of vtt vr. when all conditions above are satisfied, isl6307a begins the soft-start and ramps the output voltage to 1.1v first. after remaining at 1.1v for a fixed delay time, isl6307a reads the vid code at the vid input pins . if the vid code is valid, isl6307a will regulate the output to the final vid setting. if the vid code is an off code, isl6307a will shut down. cycling vcc, en_pwr or en_vtt is needed to restart. soft-start isl6307a based vr has four periods during soft-start, as shown in figure 11. after vcc, en_vtt and en_pwr reach their por and enable thresholds, there?s a fixed delay period td1. after this delay period, the vr will begin first soft-start ramp until the output voltage reaches 1.1v, vboot voltage. then, the controller will regulate the vr voltage at 1.1v for another fixed period, td 3. at the end of td3 period, isl6307a will read the vid signals . if the vid code is valid, isl6307a will initiate the second soft-start ramp until the voltage reaches the vid voltage minus offset voltage. the soft-start time is the sum of the four periods as shown in the following equation. td1 is the fixed delay with typical value as 1.36ms. td3 is determined by the fixed 85s time plus the time to obtain valid vid voltage. if the vid is valid before the output reaches the 1.1v, the minimum time to check the vid input is 500ns. therefore, the mi nimum td3 is about 86s. during td2 and td4, isl6307a digitally controls the dac voltage change at 6.25mv per step. the time for each step is determined by the frequency of the soft-start oscillator which is defined by the resistor r ss from ss pin to gnd. the two soft-start ramp times, td2 and td4, can be calculated based on the following equations. for example, when vid is set to 1.5v and rss is set at 100k ? , the first soft-start ramp time td2 will be 704s and the second soft-start ramp time td4 will be 256s. fault monitoring and protection the isl6307a actively monitors output voltage and current to detect fault conditions. fault monitors trigger protective measures to prevent damage to a microprocessor load. one common power good indicator is provided for linking to external system monitors. the schematic in figure 12 outlines the interaction between the fault monitors and the power good signal, vr_rdy. vr_rdy signal the vr_rdy pin is an open-drain logic output to indicate that the soft-start period has completed and the output voltage is within the regulated range. vr_rdy is pulled low during shutdown and releases high after a successful soft- start and a delay time, td5. td5 is typically 85s. vr_rdy will be pulled low when an undervoltage or overvoltage condition is detected, or the co ntroller is disabled by a reset from en_pwr, en_vtt, por, or a vid off-code. t ss td1td2td3td4 +++ = (eq. 14) td2 1.1xr ss 6.25x25 ----------------------- - s () = (eq. 15) td4 v vid 1.1 ? () xr ss 6.25x25 ------------------------------------------------ s () = (eq. 16) figure 11. soft-start waveforms vout, 500mv/div en_vtt, 1v/div 500s/div td1 td2 td3 td4 td5 vr_rdy, 5v/div isl6307a
22 fn9236.0 february 6, 2006 undervoltage detection the undervoltage threshold is set at 50% of the vid voltage. when the output vo ltage at vsen is bel ow the undervoltage threshold, vr_rdy gets pulled low. when the output voltage comes back to 60% of the vid voltage, vr_rdy will return back to high. overvoltage protection regardless of the vr being e nabled or not, the over voltage protection (ovp) circuit will be active after its por. the ovp thresholds are different under different operation conditions. when vr is not enabled and before the second soft-start, the ovp threshold is 1.275v. on ce the controller detects a valid vid input, the ovp trip point will be changed to the vid voltage plus 175mv. two actions are taken by the isl6307a to protect the microprocessor load when an overvoltage condition occurs. at the inception of an overvoltage event, all pwm outputs are commanded low instantly (less than 20ns) until the voltage at vdiff falls below 0.4v. this causes the intersil drivers to turn on the lower mosfets and pull the output voltage below a level that might cause damage to the load. the pwm outputs remain low until vdiff falls below 0.4v, and then pwm signals enter a high-impedance state. the intersil drivers respond to the high-impedance input by turning off both upper and lower mosfets. if the overvoltage condition reoccurs, the isl6307a will again command the lower mosfets to turn on. the isl6307a will continue to protect the load in this fashion as long as the overvoltage condition recurs. once an overvoltage condition is detected, normal pwm operation ceases until the isl6307a is reset. cycling the voltage on en_pwr, en_vtt or vcc below the por- falling threshold will reset the controller. cycling the vid codes will not reset the controller. overcurrent protection isl6307a has two levels of ov ercurrent protection. each phase is protected from a sust ained overcurrent condition on a delayed basis, while the combined phase currents are protected on an instantaneous basis. in instantaneous protection mode, the isl6307a takes advantage of the proportionality between the load current and the average current, i avg , to detect an overcurrent condition. see the channel-current balance section for more detail on how the average current is measured. the average current is continually compared with a constant 100 a reference current, as shown in figure 12. once the average current exceeds the reference current, a comparator triggers the converter to shutdown. in individual overcurrent pr otection mode, the isl6307a continuously compares the curre nt of each channel with the same 100 a reference current. if any channel current exceeds the reference current continuously for eight consecutive cycles, the comparator triggers the converter to shutdown. the overcurrent protection level for the above two ocp modes can be adjusted by changing the value of current sensing resistors. in addition, isl6307a can also adjust the average ocp threshold level by adjusting the value of the resistor from iout to gnd, as seen in figure 12. this provides additional safety for the voltage regulator. the following equation can be used to calculate the value of the resistor r iout based on the desired ocp level i avg, ocp2 . figure 12. power good and protection circuitry ovp - + vid + 0.175v vdiff - + 100a i avg - + dac reference ov oc1 uv vr_rdy 50% soft-start, fault and control logic - + oc i 1 repeat for each channel 100a - + oc2 iout r iout i avg 2v r iout 2 i avg ocp2 , ------------------------------- = (eq. 17) isl6307a
23 fn9236.0 february 6, 2006 at the beginning of overcurrent shutdown, the controller places all pwm signals in a high-impedance state within 20ns, commanding the intersil mosfet driver ics to turn off both upper and lower mosfets. the system remains in this state for a period of 4096 switching cycles. if the controller is still enabled at the end of this waiting period, it will attempt a soft-start. if the fault rema ins, the trip-retry cycles will continue indefinitely (as shown in figure 13) until either the controller is disabled or the f ault is cleared. note, the energy delivered during trip-retry cycling is much less than during full-load operation. there is no thermal hazards during this kind of operation. driver enable output the isl6307a has a driver enable output pin, drven. drven is designed for applications where isl6307a need to work with drivers that can not recognize three-state pwm input. in normal operation, drven remains low until isl6307a begins soft-start ramp and then is pulled high (figure 14). when an overcurrent event occurs, drven is pulled low instantly, less than 20ns, to disable the driver so that both upper and lower fets turn off (figure 15). during an overvoltage condition, drven remains high to allow the drivers to turn on the lower fets to discharge the energy stored in the output inductor. once the output voltage is reduced to 0.4v, drven is pulled low, as shown in figure 16. there?s no need to use drven when isl6307a is used with intersil?s 12v drivers, such as isl6612 and isl6614. for drivers such as isl6609 a nd isl6605, drven output of isl6307a can be connected to the en pin of the driver. 0a 0v 2ms/div output current, 50a/div figure 13. overcurrent behavior in hiccup mode. f sw = 500khz output voltage, 500mv/div figure 14. drven waveform at startup vout, 1v/div en, 5v/div 500s/div drven, 5v/div 0a 0v 2ms/div output current, 50a/div figure 15. drven during overcurrent operation output voltage, 500mv/div drven, 5v/div 0v 2ms/div figure 16. drven during overvoltage operation output voltage, 500mv/div drven, 5v/div isl6307a
24 fn9236.0 february 6, 2006 current sense output the isl6307a has two current sense output pins, idroop and iout and they are identical. in a typical application where load-line is required, the idroop pin is connected to the fb pin. the iout pin was designed for load current measurement. as shown in typical application schematics on pages 4 to 7, load current information can be obtained by measuring the voltage at iout pin with a resistor connected to ground. when the progr ammable temperature compensation function of isl 6307a is properly used, the output current at iout pin is proportional to load current, as shown in figure 17. thermal monitoring (vr_hot/vr_fan) there are two thermal signals to indicate the temperature status of the voltage regulator: vr_hot and vr_fan. both vr_fan and vr_hot are open-drain outputs, and external pull-up resistors are required. vr_fan indicates that the temperature of the voltage regulator is high and more cooling airflow is needed. vr_hot can be used to in form the system that the temperature of the voltage regu lator is too high and the cpu should reduce its power consumption. vr_hot may be tied to the cpu?s prochot# signal. the diagram of thermal monitoring function block is shown in figure 18. one ntc resistor should be placed close to the power stage of the voltage regulator to sense the operational temperature, and one pull-up resi stor is needed to form the voltage divider for the tm pin. as the temperature of the power stage increases, the resistance of the ntc will reduce, resulting in the redu ced voltage at the tm pin. figure 19 shows the tm voltage over temperature for a typical design with a recommended 6.8k ? ntc (p/n: nths0805n02n6801 from vishay) and 1k ? resistor rtm1. those numbers are recommended for accurate temperature compensation. there are two comparators with hysteresis to compare the tm pin voltage to the fixed thresholds for vr_fan and vr_hot signals, respectively. vr_fan is set high when tm is lower than 33% of vcc, and is pulled to gnd when tm increases to above 39% of vcc. vr_hot is set high when tm goes below 28% of vcc, an d is pulled to gnd when tm goes back to above 33% of vcc. figure 20 shows operation of thermal monitoring and indication. figure 17. voltage at iout pin with aresistor to gnd when load current changes v_iout, 200mv/div 100a 50a 0a figure 18. block diagram of thermal monitoring function 0.28v cc 0.33v cc o c r tm1 r ntc vcc tm vr_fan vr_hot figure 19. the ratio of tm voltage to ntc temperature with recommended parts v tm / v cc vs. tem perature 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 20 40 60 80 100 120 140 temperature ( o c) v tm / v cc isl6307a
25 fn9236.0 february 6, 2006 based on the ntc temperatur e characteristics and the desired threshold of vr_hot, the pull-up resistor r tm1 of the tm pin is given by: r ntc(t3) is the ntc resistance at the vr_hot threshold temperature t3. the ntc resistance at the set point t2 and release point t1 of vr_fan can be calculated as: with the ntc resistance value obtained from equations 19 & 20, the temperature t2 and t1 can be found from the ntc datasheet. temperature compensation isl6307a supports inductor dcr sensing, mosfet r ds(on) sensing, or resistive sensing techniques. both inductor dcr and mosfet r ds(on) have positive temperature coefficient, which is about +0.38%/c. because the voltage across the inductor or mosfet is sensed for output current information, the sensed current has the same positive temperature coefficient as the inductor dcr or mosfet r ds(on) . in order to obtain the correct current information, there should be a way to correct th e temperature impact on the current sense component. isl6307a provides two methods: integrated temperature compensation and external temperature compensation. integrated temperature compensation when tcomp voltage is equal or greater than vcc/15, isl6307a will utilize the voltage at tm and tcomp pins to compensate the temperature impact on the sensed current. the block diagram of this f unction is shown in figure 21. when the tm ntc is placed close to the current sense component (inductor or mosfet ), the temperature of the ntc will track the temperat ure of the current sense component. therefore, the tm voltage can be utilized to obtain the temperature of the current sense component. isl6307a converts the tm pin voltage to a 6-bit tm digital signal for temperature compensation. with the non-linear a/d converter of isl6307a, tm digital signal is linearly proportional to the ntc temperature. for accurate temperature compensation, the ratio of the tm voltage to the ntc temperature of the practical design should be similar to that in figure 19. depending on the location of the ntc and the air flow, the ntc may be cooler or hotter than the current sense component. tcomp voltage can be utilized to correct the temperature difference betwe en ntc and the current sense component. when a different ntc type or different voltage divider is used for the tm f unction, tcomp voltage can also be used to compensate for the difference between the recommended tm voltage curve in figure 20 and that of the actual design. isl6307a converts the tcomp pin voltage to a 4-bit tcomp digital signal as the tcomp factor n. tcomp factor n is an integer between 0 and 15. the integrated temperature compensati on function is disabled for n = 0. for n = 4, the ntc temperature is equal to the temperature of the current sense component. for n < 4, the ntc is hotter than the current sense component. the ntc is cooler than the current sense component for n > 4. when n > 4, the larger the tcomp factor n, the larger the difference between the ntc temperature and the temperature of the cu rrent sense component. figure 20. vr_hotand vr_fan signal vs tm voltage tm vr_fan vr_hot 0.39*vcc 0.33*vcc 0.28*vcc temperature t1 t2 t3 r tm1 2.75xr ntc t3 () = (eq. 18) r ntc t2 () 1.267xr ntc t3 () = (eq. 19) r ntc t1 () 1.644xr ntc t3 () = (eq. 20) figure 21. block diagram of integrated temperature compensation o c r tm1 r ntc tm r tc1 r tc2 tcomp v cc non-linear a/d 4-bit a/d droop, iout & over current protection k i d/a v cc i sen4 i sen3 i sen2 i sen1 i 1 i 2 i 3 i 4 i 5 i 6 i sen6 i sen5 channel current sense isl6307a
26 fn9236.0 february 6, 2006 isl6307a multiplexes the tcomp factor n with the tm digital signal to obtain the adjustment gain to compensate the temperature impact on the sensed channel current. the compensated channel current signal is used for droop and overcurrent protection functions. design procedure: 1. properly choose the voltage divider for tm to match the tm voltage vs. temperatur e curve with the recommended curve in figure 19. 2. run the actual board under th e full load and the desired cooling conditions. 3. after the board reaches the th ermal steady state, record the temperature (t csc ) of the current sense component (inductor or mosfet) and tm voltage and vcc. 4. use the following equation to calculate the resistance of the tm ntc, and find out the corresponding ntc temperature t ntc from the ntc datasheet. 5. use the following equation to calculate the tcomp factor n. 6. choose an integral number close to the above result for the tcomp factor. if this factor is higher than 15, use n=15. if it is less than 1, use n=1. 7. choose the pull-up resistor r tc1 (typical 10k ? ). 8. if n=15, do not use the pull-down resistor r tc2. otherwise obtain r tc2 by the following equation. 9. run the actual board under full load again with the proper resistors to tcomp. 10. record the output voltage as v1 immediately after the output voltage is stable with the full load. record the output voltage as v2 after the vr reaches the thermal steady state. 11. if the output voltage increases over 2mv as the temperature increases, i.e. v2-v1>2mv, reduce n and redesign r tc2 . if the output voltage decreases over 2mv as the temperature increases, i.e. v1-v2>2mv, increase n and redesign r tc2 . the design spreadsheet is available for those calculations. please contact intersil to get the spreadsheet. external temperature compensation by setting the voltage of tcomp to 0, the integrated temperature compensation function is disabled. in this case, an external temperature com pensation network, as shown in figure 22, can be used to cancel the temperatur e impact on the droop (i.e. load-line). the sensed current will flow out of the idroop pin and develop the droop voltage across the resistor, r fb, between fb and vdiff. if r fb resistance reduces as the temperature increases, the temperature impact on the droop can be compensated. an ntc resistor can be placed close to the power stage and used to form r fb . due to the non-linear temperature characteristics of t he ntc, a resistor network is needed to make the equivalent resistance between fb and vdiff pins reverse propor tional to the temperature. the external temperature co mpensation network can only compensate the temperature impact on the droop, while it has no impact to the sensed current inside isl6307a. therefore this network cannot compensate for the temperature impact on the over current protection function. to simplify the design for external ntc network parameters, excel calculation tools have been developed. please contact intersil to get the tools. general design guide this design guide is intended to provide a high-level explanation of the steps neces sary to create a multiphase power converter. it is assumed th at the reader is familiar with many of the basic skills and te chniques referenced below. in addition to this guide, intersil provides complete reference designs that include schematics, bills of materials, and example board layouts for all common microprocessor applications. power stages the first step in designing a multiphase converter is to determine the number of phases. this determination depends heavily on the cost analysis which in turn depends on system constraints that differ from one design to the next. principally, the designer will be concerned with whether components can be mounted on both sides of the circuit r ntc t ntc () v tm xr tm1 v cc v ? tm ------------------------------- - = (eq. 21) n 209x t csc t ? ntc () 3xt ntc 400 + ------------------------------------------------------- - 4 + = (eq. 22) r tc2 nxr tc1 15 n ? ----------------------- = (eq. 23) figure 22. temperature compensation with external ntc network isl6307a comp c c r c r fb fb idroop vdiff - + v droop c 2 (optional) ntc external tcomp compensation network r 1 r t r 3 r 2 isl6307a
27 fn9236.0 february 6, 2006 board; whether through-hole components are permitted; and the total board space available for power-supply circuitry. generally speaking, the most economical solutions are those in which each phase handles between 20 and 30a. all surface-mount designs will tend toward the lower end of this current range. if through-hole mosfets and inductors can be used, higher per-phase currents are possible. in cases where board space is the limiting constraint, current can be pushed as high as 40a per phase, but these designs require heat sinks and forced air to cool the mosfets, inductors and heat-dissipating surfaces. mosfets the choice of mosfets depends on the current each mosfet will be required to conduct; the switching frequency; the capability of the mosfets to dissipate heat; and the availability and nature of heat sinking and air flow. lower mosfet power calculation the calculation for heat dissipated in the lower mosfet is simple, since virtually all of the heat loss in the lower mosfet is due to current conducted through the channel resistance (r ds(on) ). in equation 24, i m is the maximum continuous output current; i pp is the peak-to-peak inductor current (see equation 1); d is the duty cycle (v out /v in ); and l is the per-channel inductance. an additional term can be added to the lower-mosfet loss equation to account for additional loss accrued during the dead time when inductor current is flowing through the lower-mosfet body diode. this term is dependent on the diode forward voltage at i m , v d(on) ; the switching frequency, f s ; and the length of dead times, t d1 and t d2 , at the beginning and the end of the lower-mosfet conduction interval respectively. thus the total maximum power dissipated in each lower mosfet is approximated by the summation of p low,1 and p low,2 . upper mosfet power calculation in addition to r ds(on) losses, a large portion of the upper- mosfet losses are due to currents conducted across the input voltage (v in ) during switching. since a substantially higher portion of the upper-mosfet losses are dependent on switching frequency, the power calculation is more complex. upper mosfet loss es can be divided into separate components in volving the upper-mosfet switching times; the lower-mo sfet body-diode reverse- recovery charge, q rr ; and the upper mosfet r ds(on) conduction loss. when the upper mosfet turns of f, the lower mosfet does not conduct any portion of th e inductor current until the voltage at the phase node falls below ground. once the lower mosfet begins conducting, the current in the upper mosfet falls to zero as the current in the lower mosfet ramps up to assume the full inductor current. in equation 26, the required time for this commutation is t 1 and the approximated associated power loss is p up,1 . at turn on, the upper mosfet begins to conduct and this transition occurs over a time t 2 . in equation 27, the approximate power loss is p up,2 . a third component involves the lower mosfet?s reverse- recovery charge, q rr . since the inductor current has fully commutated to the upper mosfet before the lower- mosfet?s body diode can draw all of q rr , it is conducted through the upper mosfet across vin. the power dissipation, p up,3 , can be calculated from equation 28. approximately finally, the resistive part of t he upper mosfet?s is given in equation 29 as p up,4 . the total power dissipated by the upper mosfet, at full load, can now be approximated as the summation of the results from equations 26, 27, 28 and 29. since the power equations depend on the mosfet parameters, choosing the correct mosfets, can be an iterative process involving repetitive solutions to the loss equations for different mosfets and different switching frequencies. current sensing resistor the resistors connected betwe en these isen+ pins and the respective phase nodes (r ds(on) sensing) or the output side of inductor (dcr sensing) determine the gains in the load-line regulation loop and the channel-current balance loop as well as setting the overcurrent trip point. select values for these resistors based on the room temperature r ds(on) of the lower mosfets, the dcr of inductor or additional resistor; the full-load operating current, i fl ; and the number of phases, n, using equation 30. p low 1 , r ds on () i m n ----- - ?? ?? ?? 2 1d ? () i lpp , 2 1d ? () 12 -------------------------------- + = (eq. 24) p low 2 , v don () f s i m n ----- - i pp 2 -------- - + ?? ?? t d1 i m n ----- - i pp 2 -------- - ? ?? ?? ?? t d2 + = (eq. 25) p up 1 , v in i m n ----- - i pp 2 -------- - + ?? ?? t 1 2 ---- ?? ?? ?? f s (eq. 26) p up 2 , v in i m n ----- - i pp 2 -------- - ? ?? ?? t 2 2 ---- ?? ?? ?? f s (eq. 27) p up 3 , v in q rr f s (eq. 28) p up 4 , r ds on () i m n ----- - ?? ?? ?? 2 d i pp 2 12 --------- - d + (eq. 29) r isen r x 50 10 6 ? ----------------------- i fl n ------- - = (eq. 30) isl6307a
28 fn9236.0 february 6, 2006 in certain circumstances, it ma y be necessary to adjust the value of one or more isen resistors. when the components of one or more channels are inhibited from effectively dissipating their heat so that the affected channels run hotter than desired, choose new, sma ller values of risen for the affected phases (see the section entitled channel-current balance ). choose r isen,2 in proportion to the desired decrease in temperature rise in order to cause proportionally less current to flow in the hotter phase. in equation 31, make sure that ? t 2 is the desired temperature rise above the ambient temperature, and ? t 1 is the measured temperature rise above the am bient temperature. while a single adjustment according to equation 31 is usually sufficient, it may occasionally be necessary to adjust r isen two or more times to achieve optimal thermal balance between all channels. load-line regulation resistor the load-line regulation resistor is labelled r fb in figure 8. its value depends on the desired full-load droop voltage (v droop in figure 8). if equation 30 is used to select each isen resistor, the load-line regulation resistor is as shown in equation 32. if one or more of the isen re sistors are adjusted for thermal balance, as in equation 31, the load-line regulation resistor should be selected according to equation 33 where i fl is the full-load operating current and r isen(n) is the isen resistor connected to the n th isen pin. compensation the two opposing goals of compensating the voltage regulator are stability and speed. depending on whether the regulator employs the optiona l load-line regulation as described in load-line regulati on, there are two distinct methods for achieving these goals. compensating load-line regulated converter the load-line regulated converter behaves in a similar manner to a peak-current mode controller because the two poles at the output-filter l- c resonant frequency split with the introduction of current information into the control loop. the final locations of these poles are determined by the system function, the gain of the current signal, and the value of the compensation components, r c and c c . since the system poles and zero are affected by the values of the components that are me ant to compensate them, the solution to the system equation becomes fairly complicated. fortunately there is a simple approximation that comes very close to an optimal solution. treating the system as though it were a voltage-mode regulator by compensating the l-c poles and the esr zero of t he voltage-mode approximation yields a solution that is always stable with very close to ideal transient performance. the feedback resistor, r fb , has already been chosen as outlined in load-line regulation resistor . select a target bandwidth for the compensated system, f 0 . the target bandwidth must be large enough to assure adequate transient performance, but smaller than 1/3 of the per- channel switching frequency. the values of the compensation components depend on the relationships of f 0 to the l-c pole frequency and the esr zero frequency. for each of the three cases which follow, there are a separate set of equations for the compensation components r isen 2 , r isen ? t 2 ? t 1 ---------- = (eq. 31) r fb v droop 50 10 6 ? ------------------------ - = (eq. 32) r fb v droop i fl r ds on () -------------------------------- r isen n () n = (eq. 3 3 figure 23. compensation configuration for load-line regulated isl6307a circuit isl6307a comp c c r c r fb fb idroop vdiff - + v droop c 2 (optional) 1 2 lc ------------------- f 0 > r c r fb 2 f 0 v pp lc 0.75v in ----------------------------------- - = c c 0.75v in 2 v pp r fb f 0 ------------------------------------ = case 1: isl6307a
29 fn9236.0 february 6, 2006 . in equation 34, l is the per-channel filter inductance divided by the number of active channels; c is the sum total of all output capacitors; esr is the equivalent-series resistance of the bulk output-filter capacitance; and v pp is the peak-to- peak sawtooth signal amplitude as described in figure 7 and electrical specifications . the optional capacitor c 2 , is sometimes needed to bypass noise away from the pwm comparator (see figure 24). keep a position available for c 2 , and be prepared to install a high- frequency capacitor of between 22pf and 150pf in case any leading-edge jitter problem is noted. once selected, the compensation values in equation 34 assure a stable converter with reasonable transient performance. in most cases, transient performance can be improved by making adjustments to r c . slowly increase the value of r c while observing the transient performance on an oscilloscope until no further improvement is noted. normally, c c will not need adjustment. keep the value of c c from equation 34 unless some performance issue is noted. compensation without load-line regulation the non load-line regulated converter is accurately modeled as a voltage-mode regulator with two poles at the l-c resonant frequency and a zero at the esr frequency. a type iii controller, as shown in figure 24, provides the necessary compensation. the first step is to choose the desired bandwidth, f 0 , of the compensated system. choose a frequency high enough to assure adequate transient per formance but not higher than 1/3 of the switching frequency. the type-iii compensator has an extra high-frequency pole, f hf . this pole can be used for added noise rejection or to assure adequate attenuation at the error-amplifier high-order pole and zero frequencies. a good general rule is to choose f hf =10f 0 , but it can be higher if desired. choosing f hf to be lower than 10f 0 can cause problems with too much phase shift below the system bandwidth. in the solutions to the compensation equations, there is a single degree of freedom. for the solutions presented in equation 35, r fb is selected arbitrarily. the remaining compensation components are then selected according to equation 35. in equation 35, l is the per-channel filter inductance divided by the number of active channel s; c is the sum total of all 1 2 lc ------------------- f 0 1 2 cesr () ----------------------------- - < r c r fb v pp 2 () 2 f 0 2 lc 0.75 v in -------------------------------------------- = c c 0.75v in 2 () 2 f 0 2 v pp r fb lc ------------------------------------------------------------- = case 2: (eq. 34) f 0 1 2 c esr () ----------------------------- - > r c r fb 2 f 0 v pp l 0.75 v in esr () ------------------------------------------ = c c 0.75v in esr () c 2 v pp r fb f 0 l ------------------------------------------------- = case 3: figure 24. compensation circuit for isl6307a based converter without load-line regulation isl6307a comp c c r c r fb fb idroop vdiff c 2 c 1 r 1 c c 0.75v in 2 f hf lc 1 ? ?? ?? 2 () 2 f 0 f hf lcr fb v pp ------------------------------------------------------------------ - = r c v pp 2 ?? ?? 2 f 0 f hf lcr fb 0.75 v in 2 f hf lc 1 ? ?? ?? -------------------------------------------------------------------- - = r 1 r fb c esr () lc c esr () ? ---------------------------------------- - = c 1 lc c esr () ? r fb ---------------------------------------- - = c 2 0.75v in 2 () 2 f 0 f hf lcr fb v pp ------------------------------------------------------------------ - = (eq. 35) isl6307a
30 fn9236.0 february 6, 2006 output capacitors; esr is the equivalent-series resistance of the bulk output-filter capacitance; and v pp is the peak-to- peak sawtooth signal amplitude as described in figure 7 and electrical specifications . output filter design the output inductors and the output capacitor bank together form a low-pass filter responsible for smoothing the pulsating voltage at the phase nodes. the output filter also must provide the transient energy until the regulator can respond. because it has a low bandwidth compared to the switching frequency, the output filter necessarily limits the system transient response. the output ca pacitor must supply or sink load current while the current in the output inductors increases or decreases to meet the demand. in high-speed converters, the output capacitor bank is usually the most costly (and often the largest) part of the circuit. output filter design begins with minimizing the cost of this part of the circuit. the critical load parameters in choosing the output capacitors are the maximum size of the load step, ? i; the load-current slew rate, di/dt; and the maximum allowable output-volt age deviation under transient loading, ? v max . capacitors are characterized according to their capacitance, esr, and esl (equivalent series inductance). at the beginning of the load tr ansient, the output capacitors supply all of the transient current. the output voltage will initially deviate by an amount approximated by the voltage drop across the esl. as the load current increases, the voltage drop across the esr increases linearly until the load current reaches its final value. the capacitors selected must have sufficiently low esl and esr so that the total output- voltage deviation is less than the allowable maximum. neglecting the contribution of i nductor current and regulator response, the output voltage initially deviates by an amount the filter capacitor must have sufficiently low esl and esr so that ? v < ? v max . most capacitor solutions rely on a mixture of high-frequency capacitors with relatively low capacitance in combination with bulk capacitors having high capacitance but limited high-frequency performance. minimizing the esl of the high-frequency capacitors allows them to support the output voltage as the current increases. minimizing the esr of the bulk capacitors allows them to supply the increased current with less output voltage deviation. the esr of the bulk capacitors also creates the majority of the output-voltage ripple. as the bulk capacitors sink and source the inductor ac ripple current (see interleaving and equation 2), a voltage develops across the bulk-capacitor esr equal to i c,pp (esr). thus, once the output capacitors are selected, the maximum allowable ripple voltage, v pp(max) , determines the lower limit on the inductance. since the capacitors are supplying a decreasing portion of the load current while the regulator recovers from the transient, the capacitor voltage becomes slightly depleted. the output inductors must be capable of assuming the entire load current before the output voltage decreases more than ? v max . this places an upper limit on inductance. equation 38 gives the upper limit on l for the cases when the trailing edge of the current transient causes a greater output-voltage deviation than the leading edge. equation 39 addresses the leading edge. normally, the trailing edge dictates the selection of l because duty cycles are usually less than 50%. nevertheless, both inequalities should be evaluated, and l should be se lected based on the lower of the two results. in each equation, l is the per-channel inductance, c is the total output capacitance, and n is the number of active channels. switching frequency there are a number of variables to consider when choosing the switching frequency, as there are considerable effects on the upper-mosfet loss calcul ation. these effects are outlined in mosfets , and they establish the upper limit for the switching frequency. the lowe r limit is established by the requirement for fast transi ent response and small output- voltage ripple as outlined in output filter design . choose the lowest switching frequency that allows the regulator to meet the transient-response requirements. switching frequency is determi ned by the selection of the frequency-setting resistor, r t (see the figures labelled typical application on pages 4, 5, 6 and 7). equation 40 is provided to assist in selecting the correct value for r t . input capacitor selection the input capacitors are responsible for sourcing the ac component of the input curr ent flowing into the upper mosfets. their rms current capa city must be sufficient to handle the ac component of the current drawn by the upper mosfets that is related to duty cycle and the number of active phases. ? v esl () di dt ---- -esr ()? i + (eq. 36) l esr () v in nv out ? ?? ?? v out f s v in v pp max () ----------------------------------------------------------- - (eq. 37) l 2ncv o ? i () 2 --------------------- ? v max ? i esr () ? (eq. 38) l 1.25 () nc ? i () 2 ------------------------- - ? v max ? iesr () ? v in v o ? ?? ?? (eq. 39) (eq. 40) r t 2.5x 10 10 f sw ------------------------- - 600 ? = isl6307a
31 fn9236.0 february 6, 2006 for a two phase design, use figure 25 to determine the input-capacitor rms current requirement given the duty cycle, maximum sustained output current (i o ), and the ratio of the per-phase peak-to-peak inductor current (i l,pp ) to i o . select a bulk capacitor with a ripple current rating which will minimize the total number of input capacitors required to support the rms current calculated. the voltage rating of the capacitors should also be at least 1.25 times greater than the maximum input voltage. figures 26 and 27 provide the same input rms current information for three and four phase designs respectively. use the same approach to selecting the bulk capacitor type and number as described above. low capacitance, high-frequency ceramic capacitors are needed in addition to the bulk capacitors to suppress leading and falling edge voltage spikes. this is a result from the high current slew rates produced by the upper mosfets turning on and off. select low esl ceramic capacitors and place one as close as possible to each upper mosfet drain to minimize board parasitic impedances and maximize suppression. multiphase rms improvement figure 28 is provided as a reference to demonstrate the dramatic reductions in input-capacitor rms current upon the implementation of the multipha se topology. for example, compare the input rms current requirements of a two-phase converter versus that of a single phase. assume both converters have a duty cycle of 0.25, maximum sustained output current of 40a, and a ratio of i l,pp to i o of 0.5. the single phase converter would require 17.3arms current capacity while the two-phase converter would only require 10.9arms. the advantages become even more pronounced when output current is increas ed and additional phases are added to keep the component cost down relative to the single phase approach. 0.3 0.1 0 0.2 input-capacitor current (i rms /i o ) figure 25. normalized input-capacitor rms current vs duty cycle for 2-phase converter 00.4 1.0 0.2 0.6 0.8 duty cycle (v o /v in ) i l,pp = 0 i l,pp = 0.5 i o i l,pp = 0.75 i o duty cycle (v o/ v in ) figure 26. normalized input-capacitor rms current vs duty cycle for 3-phase converter 00.4 1.0 0.2 0.6 0.8 input-capacitor current (i rms/ i o ) 0.3 0.1 0 0.2 i l,pp = 0 i l,pp = 0.25 i o i l,pp = 0.5 i o i l,pp = 0.75 i o input-capacitor current (i rms/ i o ) figure 27. normalized input-capacitor rms current vs duty cycle for 4-phase converter 00.4 1.0 0.2 0.6 0.8 duty cycle (v o/ v in ) 0.3 0.1 0 0.2 i l,pp = 0 i l,pp = 0.25 i o i l,pp = 0.5 i o i l,pp = 0.75 i o isl6307a
32 fn9236.0 february 6, 2006 layout considerations the following layout strategies are intended to minimize the impact of board parasitic impedances on converter performance and to optimize the heat-dissipating capabilities of the printed-circuit board. t hese sections highlight some important practices which should not be overlooked during the layout process. component placement within the allotted implementation area, orient the switching components first. the switching components are the most critical because they carry larg e amounts of energy and tend to generate high levels of noise. switching component placement should take into account power dissipation. align the output inductors and mosfets such that spaces between the components are minimized while creating the phase plane. place the intersil mo sfet driver ic close to the mosfets they control, to reduce the parasitic impedances due to trace length between critical driver input and output signals. if possi ble, duplicate the same placement of these components for each phase. next, place the input and output capacitors. position one high-frequency ceramic input capacitor next to each upper mosfet drain. place the bulk input capacitors as close to the upper mosfet drains as dictated by the component size and dimensions. long distances between input capacitors and mosfet drains result in too much trace inductance and a reduction in capacitor performance. locate the output capacitors between the inductors and the load, while keeping them in close proximity to the microprocessor socket. the isl6307a can be placed off to one side or centered relative to the individual phase switching components. routing of sense lines and pwm signals will guide final placement. critical small signal components to place close to the controller include the isen resistors, r t resistor, feedback resistor, and compensation components. bypass capacitors for the isl6307a and isl66xx driver bias supplies must be placed next to their respective pins. trace parasitic impedances will reduce their effectiveness. plane allocation and routing dedicate one solid layer, usually a middle layer, for a ground plane. make all critical comp onent ground connections with vias to this plane. dedicate one additional layer for power planes; breaking the plane up into smaller islands of common voltage. use the remaining layers for signal wiring. route phase planes of copper fi lled polygons on the top and bottom once the switching component placement is set. size the trace width between the driver gate pins and the mosfet gates to carry 4a of current. when routing components in the switching path, use short wide traces to reduce the associated parasitic impedances. figure 28. normalized input-capacitor rms current vs duty cycle for single-phase converter 00.4 1.0 0.2 0.6 0.8 duty cycle (v o/ v in ) input-capacitor current (i rms/ i o ) 0.6 0.2 0 0.4 i l,pp = 0 i l,pp = 0.5 i o i l,pp = 0.75 i o isl6307a
33 all intersil u.s. products are manufactured, asse mbled and tested utilizing iso9000 quality systems. intersil corporation?s quality certifications ca n be viewed at www.intersil.com/design/quality intersil products are sold by description only. intersil corpor ation reserves the right to make changes in circuit design, soft ware and/or specifications at any time without notice. accordingly, the reader is cautioned to verify that data sheets are current before placing orders. information furnishe d by intersil is believed to be accurate and reliable. however, no responsibility is assumed by intersil or its subsidiaries for its use; nor for any infringements of paten ts or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of intersil or its subsidiari es. for information regarding intersil corporation and its products, see www.intersil.com fn9236.0 february 6, 2006 isl6307a quad flat no-lead plastic package (qfn) micro lead frame pl astic package (mlfp) l48.7x7 48 lead quad flat no-lead plastic package (compliant to jedec mo-220vkkd-2 issue c) symbol millimeters notes min nominal max a 0.80 0.90 1.00 - a1 - - 0.05 - a2 - - 1.00 9 a3 0.20 ref 9 b 0.18 0.23 0.30 5, 8 d 7.00 bsc - d1 6.75 bsc 9 d2 4.15 4.30 4.45 7, 8 e 7.00 bsc - e1 6.75 bsc 9 e2 4.15 4.30 4.45 7, 8 e 0.50 bsc - k0.25 - - - l 0.30 0.40 0.50 8 l1 - - 0.15 10 n482 nd 12 3 ne 12 3 p- -0.609 --129 rev. 1 10/02 notes: 1. dimensioning and tolerancing conform to asme y14.5-1994. 2. n is the number of terminals. 3. nd and ne refer to the number of terminals on each d and e. 4. all dimensions are in millimeters. angles are in degrees. 5. dimension b applies to the meta llized terminal and is measured between 0.15mm and 0.30mm from the terminal tip. 6. the configuration of the pin #1 identifier is optional, but must be located within the zone indicated. the pin #1 identifier may be either a mold or mark feature. 7. dimensions d2 and e2 are fo r the exposed pads which provide improved electrical and thermal performance. 8. nominal dimensions are provided to assist with pcb land pattern design efforts, see intersil technical brief tb389. 9. features and dimensions a2, a3, d1, e1, p & are present when anvil singulation method is used and not present for saw singulation. 10. depending on the method of lead termination at the edge of the package, a maximum 0.15mm pull back (l1) maybe present. l minus l1 to be equal to or greater than 0.3mm.

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