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| rev5.0_01_28_10 1 ir3856wmpbf features ? greater than 95% maximum efficiency ? wide input voltage range 1.5v to 16v ? wide output voltage range 0.7v to 0.9*vin ? continuous 6a load capability ? integrated bootstrap-diode ? high bandwidth e/a for excellent transient performance ? programmable switching frequency up to 1.5mhz ? programmable over current protection ? pgood output ? hiccup current limit ? precision reference voltage (0.7v, +/-1%) ? programmable soft-start ? enable input with voltage monitoring capability ? enhanced pre-bias start-up ? seq input for tracking applications ? -40 o c to 125 o c operating junction temperature ? thermal protection ? 4mm x 5mm power qfn package ? halogen free, lead free and rohs compliant applications ? server applications ? storage applications ? embedded telecom systems ? distributed point of load power architectures fig. 1. typical application diagram description the ir3856w supirbuck tm is an easy-to-use, fully integrated and highly efficient dc/dc synchronous buck regulator. the mosfets co- packaged with the on-chip pwm controller make ir3856w a space-efficient solution, providing accurate power delivery for low output voltage applications. ir3856w is a versatile regulator which offers programmability of start up time, switching frequency and current limit while operating in wide input and output voltage range. the switching frequency is programmable from 250khz to 1.5mhz for an optimum solution. it also features important protection functions, such as pre-bias startup, hiccup current limit and thermal shutdown to give required system level security in the event of fault conditions. highly efficient integrated 6a, synchro nous buck regulator sup ir buck tm ? netcom applications ? computing peripheral voltage regulators ? general dc-dc converters
rev5.0_01_28_10 2 ir3856wmpbf absolute maximum ratings (voltages referenced to gnd unless otherwise specified) ? vin ???????????????.?????. -0.3v to 25v ? pvcc, vcc ??????.??..?.??..???.?? -0.3v to 8v (note2) ? boot ??????????????..???.?. -0.3v to 33v ? sw ???????????? ????..??? -0.3v to 25v(dc), -4v to 25v(ac, 100ns) ? boot to sw ??..???????????.?..?.. -0.3v to vcc+0.3v (note1) ? ocset ????????????????.??. -0.3v to 30v (max 30ma) ? input / output pins ????????????.. ... -0.3v to vcc+0.3v (note1) ? pgnd to gnd ?????...??????????.. -0.3v to +0.3v ? storage temperature range ................. .................. -55c to 150c ? junction temperature range .......... ......................... -40c to 150c (note2) ? esd classification ??????????? ??? jedec class 1c ? moisture sensitivity level??????...??????jedec level 3@260 c stresses beyond those listed under ?absolute maxi mum ratings? may cause pe rmanent damage to the device. these are stress ratings only and functiona l operation of the device at these or any other conditions beyond those indicated in the operationa l sections of the specifications are not implied. note1: must not exceed 8v note2: vcc must not exceed 7.5v for junction temperature between -10 o c and -40 o c package information 4mm x 5mm power qfn 4000 17 IR3856WMTRPBF m 750 parts per reel 17 pin count ir3856wmtr1pbf package description m package designator ordering information 13 v in 12 sw 11 pgnd 17 gnd 1 23 4 5 6 7 8 9 16 15 fb nc comp gnd rt ss/sd ocset pgood v cc enable boot seq pvcc 10 14 rev5.0_01_28_10 3 ir3856wmpbf block diagram fig. 2. simplified block diagram of the ir3856w rev5.0_01_28_10 4 ir3856wmpbf pin description pin name description 1 fb inverting input to the error amplifier. this pin is connected directly to the output of the regulator via resistor divider to set the output voltage and provide feedback to the error amplifier. 2 nc no connect 3 comp output of error amplifier. an external resistor and capacitor network is typically connected from this pin to fb pin to provide loop compensation. 4 gnd signal ground for internal reference and control circuitry. 5 rt set the switching frequency. connect an external resistor from this pin to gnd to set the switching frequency. 6 ss/sd soft start / shutdown. this pin provides user programmable soft-start function. connect an external capacitor from this pin to gnd to set the start up time of the output voltage. the converter can be shutdown by pulling this pin below 0.3v. 7 ocset current limit set point. a resistor from this pin to sw pin will set the current limit threshold. 8 pgood power good status pin. output is open drain. connect a pull up resistor from this pin to vcc. 9 v cc this pin powers the internal ic. a minimum of 1uf high frequency capacitor must be connected from this pin to analog ground (agnd). 10 pvcc this pin powers the drivers. a minimum of 1uf high frequency capacitor must be connected from this pin to the power ground (pgnd). 11 pgnd power ground. this pin serves as a separated ground for the mosfet drivers and should be connected to the system?s power ground plane. 12 sw switch node. this pin is connected to the output inductor. 13 v in input voltage connection pin. 14 boot supply voltage for high side driver. a 0.1uf capacitor must be connected from this pin to sw. 15 enable enable pin to turn on and off the device. use two external resistors to set the turn on threshold (see enable section). connect this pin to vcc if it is not used. 16 seq sequence pin. use two external resistors to set simultaneous power up sequencing. if this pin is not used connect to vcc. 17 gnd signal ground for internal reference and control circuitry. rev5.0_01_28_10 5 ir3856wmpbf recommended operating conditions parameter symbol test condition min typ max units power loss power loss p loss vcc=5v, v in =12v, v o =1.8v, i o =6a, fs=600khz, l=tbduh, note4 tbd w mosfet r ds(on) top switch r ds(on)_top v boot -v sw =5v, i d =6a, tj= 25 o c 22.6 29.0 bottom switch r ds(on)_bot v cc =5v, i d =6a, tj= 25 o c 14.3 19 m reference voltage feedback voltage v fb 0.7 v 0 o c rev5.0_01_28_10 9 ir3856wmpbf rdson of mosfets over temperature at vcc=5v 12 14 16 18 20 22 24 26 28 30 -40-20 0 20406080100120140 temperature [c] resistance [m ] sync-fet ctrl-fet rev5.0_01_28_10 10 ir3856wmpbf typical efficiency and power loss curves vin=12v, vcc=5v, io=1a-6a, f s =600khz, room temperature, no air flow the table below shows the inductors used for each of the output voltages in the efficiency measurement. v o (v) l (uh) p/n dcr (mohm) 0.9 0.68 pcmb065t-r68ms 3.9 1.0 0.68 pcmb065t-r68ms 3.9 1.1 0.82 ihlp2525ezerr82m 4.6 1.2 0.82 ihlp2525ezerr82m 4.6 1.5 1.0 spm6550t-1r0m100a 4.7 1.8 1.0 spm6550t-1r0m100a 4.7 2.5 1.5 pcmb065t-1r5ms 6.7 3.3 1.5 pcmb065t-1r5ms 6.7 5.0 2.2 pcmb065t-2r2ms 11.2 82 84 86 88 90 92 94 96 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 load current (a) efficiency (%) 0.9v 1.0v 1.1v 1.2v 1.5v 1.8v 2.5v 3.3v 5.0v 0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 load current (a) power loss (w) 0.9v 1.0v 1.1v 1.2v 1.5v 1.8v 2.5v 3.3v 5.0v rev5.0_01_28_10 11 ir3856wmpbf typical efficiency and power loss curves vin=5v, vcc=5v, io=1a-6a, f s =600khz, room temperature, no air flow the table below shows the inductors used for each of the output voltages in the efficiency measurement. 80 82 84 86 88 90 92 94 96 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 load current (a) efficiency (%) 0.7v 0.75v 0.9v 1.0v 1.1v 1.2v 1.5v 1.8v 2.5v 3.3v 0.1 0.3 0.5 0.7 0.9 1.1 1.3 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 load current (a) power loss (w) 0.7vout 0.75vout 0.9vout 1.0vout 1.1vout 1.2vout 1.5vout 1.8vout 2.5vout 3.3vout v o (v) l (uh) p/n dcr (mohm) 0.7 0.56 ihlp2525ezerr56m 3.4 0.75 0.56 ihlp2525ezerr56m 3.4 0.9 0.56 ihlp2525ezerr56m 3.4 1.0 0.56 ihlp2525ezerr56m 3.4 1.1 0.68 pcmb065t-r68ms 3.9 1.2 0.68 pcmb065t-r68ms 3.9 1.5 0.68 pcmb065t-r68ms 3.9 1.8 0.82 ihlp2525ezerr82m 4.6 2.5 0.82 ihlp2525ezerr82m 4.6 3.3 0.82 ihlp2525ezerr82m 4.6 rev5.0_01_28_10 12 ir3856wmpbf circuit description theory of operation introduction the ir3856w uses a pwm voltage mode control scheme with external compensation to provide good noise immunity and maximum flexibility in selecting inductor values and capacitor types. the switching frequency is programmable from 250khz to 1.2mhz and provides the capability of optimizing the design in terms of size and performance. ir3856w provides precisely regulated output voltage programmed via two external resistors from 0.7v to 0.9*vin. the ir3856w operates with an external bias supply from 4.5v to 5.5v, allowing an extended operating input voltage range from 1.5v to 21v. the device utilizes the on-resistance of the low side mosfet as current sense element, this method enhances the converter?s efficiency and reduces cost by eliminating the need for external current sense resistor . ir3856w includes two low r ds(on) mosfets using ir?s hexfet technology. these are specifically designed for high efficiency applications. under-voltage lockout and por the under-voltage lockout circuit monitors the input supply vcc and the enable input. it assures that the mosfet driver outputs remain in the off state whenever either of these two signals drop below the set thresholds. normal operation resumes once vcc and enable rise above their thresholds. the por (power on ready) signal is generated when all these signals reach the valid logic level (see system block diagram). when the por is asserted the soft start sequence starts (see soft start section). enable the enable features another level of flexibility for start up. the enable has precise threshold which is internally monitored by under-voltage lockout (uvlo) circuit. therefore, the ir3856w will turn on only when the voltage at the enable pin exceeds this threshold, typically, 1.2v. if the input to the enable pin is derived from the bus voltage by a suitably programmed resistive divider, it can be ensured that the ir3856w does not turn on until the bus voltage reaches the desired level. only after the bus voltage reaches or exceeds this level will the voltage at enable pin exceed its threshold, thus enabling the ir3856w. therefore, in addition to being a logic input pin to enable the ir3856w, the enable feature, with its precise threshold, also allows the user to implement an under-voltage lockout for the bus voltage v in . this is desirable particularly for high output voltage applications, where we might want the ir3856w to be disabled at least until v in exceeds the desired output voltage level. figure 3b. shows the recommended start-up sequence for the non-sequenced operation of ir3856w. figure 3c. shows the recommended startup sequence for sequenced operation of ir3856w fig. 3a. normal start up, device turns on when the bus voltage reaches 10.2v fig. 3b. recommended startup sequence, non-sequenced operation rev5.0_01_28_10 13 ir3856wmpbf soft-start the ir3856w has a programmable soft-start to control the output voltage rise and to limit the current surge at the start-up. to ensure correct start-up, the soft-start sequence initiates when the enable and vcc rise above their uvlo thresholds and generate the power on ready (por) signal. the internal current source (typically 20ua) charges the external capacitor c ss linearly from 0v to 3v. figure 6 shows the waveforms during the soft start. the start up time can be estimated by: during the soft start the ocp is enabled to protect the device for any short circuit and over current condition. pre-bias startup ir3856w is able to start up into pre-charged output, which prevents oscillation and disturbances of the output voltage. the output starts in asynchronous fashion and keeps the synchronous mosfet off until the first gate signal for control mosfet is generated. figure 4 shows a typical pre-bias condition at start up. the synchronous mosfet always starts with a narrow pulse width and gradually increases its duty cycle with a step of 25%, 50%, 75% and 100% until it reaches the steady state value. the number of these startup pulses for the synchronous mosfet is internally programmed. figure 5 shows a series of 32, 16, 8 startup pulses. fig. 5. pre-bias startup pulses fig. 6. theoretical operation waveforms during soft-start ( ) (1) - - - - - - - - - - - - - - - - - - - - a 20 * 0.7 - 1.4 ss start c t = fig. 4. pre-bias startup fig. 3c. recommended startup sequence, sequenced operation rev5.0_01_28_10 14 ir3856wmpbf an internal current source sources current ( i ocset ) out of the ocset pin. this current is a function of the switching frequency and hence, of r t . shutdown the ir3856w can be shutdown by pulling the enable pin below its 1 v threshold. this will tri- state both, the high side driver as well as the low side driver. alternatively, the output can be shutdown by pulling the soft-start pin below 0.3v. normal operation is resumed by cycling the voltage at the soft start pin. over-current protection the over current protection is performed by sensing current through the r ds(on) of low side mosfet. this method enhances the converter?s efficiency and reduces cost by eliminating a current sense resistor. as shown in figure 7, an external resistor (r ocset ) is connected between ocset pin and the switch node (sw) which sets the current limit set point. table 1. shows i ocset at different switching frequencies. the internal current source develops a voltage across r ocset . when the low side mosfet is turned on, the inductor current flows through the q2 and results in a voltage at ocset which is given by: an over current is detected if the ocset pin goes below ground. hence, at the current limit threshold, v ocset =0. then, for a current limit setting i limit ,r ocset is calculated as follows: operating frequency the switching frequency can be programmed between 250khz ? 1200khz by connecting an external resistor from r t pin to gnd. table 1 tabulates the oscillator frequency versus r t . fig. 7. connection of over current sensing resistor an overcurrent detection trips the ocp comparator, latches ocp signal and cycles the soft start function in hiccup mode. the hiccup is performed by shorting the soft-start capacitor to ground and counting the number of switching cycles. the soft start pin is held low until 4096 cycles have been completed. the ocp signal resets and the converter recovers. after every soft start cycle, the converter stays in this mode until the overload or short circuit is removed. i ) r r i v l (on ds ocset ocset ocset .(3) .......... ) ( ) ( ? ? ? = i i r r ocset limit on ds ocset (4) .... .......... .......... ) ( * = ) 2 .....( .......... .......... .......... ) (k ) a ( = t ocset r i 1400 table 1. switching frequency and i ocset vs. external resistor ( r t ) the ocp circuit starts sampling current typically 160 ns after the low gate drive rises to about 3v. this delay functions to filter out switching noise. 110.2 1100 12.7 97.9 1000 14.3 121.7 1200 11.5 88.6 900 15.8 78.6 800 17.8 68.2 700 20.5 59.07 600 23.7 48.7 500 28.7 39.2 400 35.7 29.4 300 47.5 i ocset ( a) f s (khz) r t (k ? ) 110.2 1100 12.7 97.9 1000 14.3 121.7 1200 11.5 88.6 900 15.8 78.6 800 17.8 68.2 700 20.5 59.07 600 23.7 48.7 500 28.7 39.2 400 35.7 29.4 300 47.5 i ocset ( a) f s (khz) r t (k ? ) rev5.0_01_28_10 15 ir3856wmpbf thermal shutdown temperature sensing is provided inside ir3856w. the trip threshold is typically set to 140 o c. when trip threshold is exceeded, thermal shutdown turns off both mosfets and discharges the soft start capacitor. automatic restart is initiated when the sensed temperature drops within the operating range. there is a 20 o c hysteresis in the thermal shutdown threshold. fig. 8b. application circuit for simultaneous sequencing simultaneous powerup vo1 vo2 output voltage sequencing the ir3856w can accommodate user programmable sequencing using seq, enable and power good pins. boot vcc fb comp gnd pgnd sw ocset ss/ sd 4.5v rev5.0_01_28_10 17 ir3856wmpbf minimum on time considerations the minimum on time is the shortest amount of time for which the control fet may be reliably turned on, and this depends on the internal timing delays. for the ir3856w, the typical minimum on-time is specified as 50 ns. any design or application using the ir3856w must ensure operation with a pulse width that is higher than this minimum on-time and preferably higher than 100 ns. this is necessary for the circuit to operate without jitter and pulse- skipping, which can cause high inductor current ripple and high output voltage ripple. in any application that uses the ir3856w, the following condition must be satisfied: the minimum output voltage is limited by the reference voltage and hence v out(min) = 0.7 v. therefore, for v out(min) = 0.7 v, therefore, at the maximum recommended input voltage 21v and minimum output voltage, the converter should be designed at a switching frequency that does not exceed 330 khz. conversely, for operation at the maximum recommended operating frequency 1.2 mhz and minimum output voltage, any voltage above 5.83v may not be stepped down reliably without pulse-skipping. v/s ns 100 v 0.7 v v in (min) (min) in 6 10 7 = s on out s f t v f maximum duty ratio considerations a fixed off-time of 200 ns maximum is specified for the ir3856w. this provides an upper limit on the operating duty ratio at any given switching frequency. it is clear t hat, higher the switching frequency, the lower is the maximum duty ratio at which the ir3856w can operate. to allow a margin of 50 ns, the maximum operating duty ratio in any application using the ir3856w should still accommodate about 250 ns off-time. fig 10. shows a plot of the maximum duty ratio v/s the switching frequency, with 250 ns off-time. s out s on f v f d t v in = = (min) (min) (min) on out s in s in out on on on t v f v f v v t t t fig. 10. maximum duty cycle v/s switching frequency. 70 75 80 85 90 95 300 400 500 600 700 800 900 1000 1100 1200 switching frequency (khz) max duty cycle (%) rev5.0_01_28_10 18 ir3856wmpbf when an external resistor divider is connected to the output as shown in figure 11. equation (7) can be rewritten as: for the calculated values of r8 and r9 see feedback compensation section. soft-start programming the soft-start timing can be programmed by selecting the soft-start capacitance value. from (1), for a desired start-up time of the converter, the soft start capacitor can be calculated by using: where t start is the desired start-up time (ms). for a start-up time of 3.5ms, the soft-start capacitor will be 0.099 f. choose a 0.1 f ceramic capacitor. bootstrap capacitor selection to drive the control fet, it is necessary to supply a gate voltage at least 4v greater than the voltage at the sw pin, which is connected the source of the control fet . this is achieved by using a bootstrap configuration, which comprises the internal bootstrap diode and an external bootstrap capacitor (c6), as shown in fig. 12. the operation of the circuit is as follows: when the lower mosfet is turned on, the capacitor node connected to sw is pulled down to ground. the capacitor charges towards v cc through the internal bootstrap diode, which has a forward voltage drop v d . the voltage v c across the bootstrap capacitor c6 is approximately given as when the upper mosfet turns on in the next cycle, the capacitor node connected to sw rises to the bus voltage v in . however, if the value of c6 is appropriately chosen, application information design example: the following example is a typical application for ir3856w. the application circuit is shown on page 23. enabling the ir3856w as explained earlier, the precise threshold of the enable lends itself well to implementation of a uvlo for the bus voltage. for a typical enable threshold of v en = 1.2 v for a v in (min) =10.2v, r 1 =49.9k and r 2 =7.5k is a good choice. programming the frequency for f s = 600 khz, select r t = 23.7 k ? , using table. 1. output voltage programming output voltage is programmed by reference voltage and external voltage divider. the fb pin is the inverting input of the error amplifier, which is internally referenced to 0.7v. the divider is ratioed to provide 0.7v at the fb pin when the output is at its desired value. the output voltage is defined by using the following equation: khz 600 = 54mv a 6 = v 1.8 = max) 13.2v ( v 12 = s o o o in f v i v v .....(7) .......... .......... .......... ? ? ? ? ? ? ? ? + ? = 9 8 1 r r v v ref o fig. 11. typical application of the ir3856w for programming the output voltage (8) .... .......... .......... .......... v v v r r ref o ref ? ? ? ? ? ? ? ? ? ? = 8 9 v v v d cc c (10) ...... .......... .......... ? ? t c start ss (9) .......... 0.02857 ) ms ( ) f ( = ir3856w enable v in r 2 r 1 v r r r v en in (5) .......... 1.2 * (min) = = + 2 1 2 v v v r r en ) in( en (6) .......... min ? = 1 2 rev5.0_01_28_10 19 ir3856wmpbf the voltage vc across c6 remain s approximately unchanged and the voltage at the boot pin becomes a bootstrap capacitor of value 0.1uf is suitable for most applications. for applications with 21v input voltage, the switch node may ring above the 25v absolute maximum voltage rating. to prevent this, in addition to using best layout practices, it may be necessary to provide a 10ohm resistor in series with the boot capacitor. input capacitor selection the ripple current generated during the on time of the upper mosfet should be provided by the input capacitor. the rms value of this ripple is expressed by: where: d is the duty cycle i rms is the rms value of the input capacitor current. io is the output current. for i o =6a and d = 0.15, the i rms = 2.14 a. ceramic capacitors are recommended due to their peak current capabilities. they also feature low esr and esl at higher frequency which enables better efficiency. for this application, it is advisable to have 2x10uf 25v ceramic capacitors c3216x5r1e106m from tdk. in addition to these, although not mandatory, a 1x330uf, 25v smd capacitor eev-fk1e331p may also be used as a bulk capacitor and is recommended if the input power supply is not located close to the converter. inductor selection the inductor is selected based on output power, operating frequency and efficiency requirements. a low inductor value causes large ripple current, resulting in the smaller size, faster response to a load transient but poor efficiency and high output noise. generally, the selection of the inductor value can be reduced to the desired maximum ripple current in the inductor . the optimum point is usually found between 20% and 50% ripple of the output current. for the buck converter, the inductor value for the desired operating ripple current can be determined using the following relation: where: if i 42%( i o ), then the output inductor is calculated to be 1.01 h. select l =1 h. the spm6550t-1r0m100a from tdk provides a compact inductor suitable for this application ....(12) .......... .......... ) ( d d i i o rms ? ? ? = 1 (13) .. .......... .......... .......... in o v v d = ) ( i cycle duty time on turn frequency switching current ripple inductor voltage output voltag e input maximum = = = = = = d t f i v v s o in () (14) . .......... .......... .......... * ; s in o o in s o in f i v v v v l f d t t i l v v ? ? ? = ? = ? = ? 1 fig. 12. bootstrap circuit to generate vc voltage (11) .......... .......... .......... .......... d cc in boot v v v v ? + ? rev5.0_01_28_10 20 ir3856wmpbf phase 0 0 f lc -180 0 frequency gain f lc 0 db frequency -40db/decade (15) ..... .......... .......... current ripple inductor ripple voltage output * * * * ) ( ) ( ) ( ) ( ) ( ) ( = = = ? ? ? ? ? ? ? ? ? = = + + = l o s o l c o o in esl o l esr o c o esl o esr o o i v f c i v esl l v v v esr i v v v v v 8 since the output capacitor has a major role in the overall performance of the converter and determines the result of transient response, selection of the capacitor is critical. the ir3856w can perform well with all types of capacitors. as a rule, the capacitor must have low enough esr to meet output ripple and load transient requirements. the goal for this design is to meet the voltage ripple requirement in the smallest possible capacitor size. therefore it is advisable to select ceramic capacitors due to their low esr and esl and small size. four of the panasonic ecj- 2fb0j226ml (22uf, 6.3v, 3mohm) capacitors is a good choice. feedback compensation the ir3856w is a voltage mode controller. the control loop is a single voltage feedback path including error amplifier and error comparator. to achieve fast transient response and accurate output regulation, a compensation circuit is necessary. the goal of the compensation network is to provide a closed-loop transfer function with the highest 0 db crossing frequency and adequate phase margin (greater than 45 o ). the output lc filter introduces a double pole, ?40db/decade gain slope above its corner resonant frequency, and a total phase lag of 180 o (see figure 13). the resonant frequency of the lc filter is expressed as follows: figure 13 shows gain and phase of the lc filter. since we already have 180 o phase shift from the output filter alone, the system runs the risk of being unstable. the ir3856w uses a voltage-type error amplifier with high-gain (110db) and wide-bandwidth. the output of the amplifier is available for dc gain control and ac phase compensation. the error amplifier can be compensated either in type ii or type iii compensation. local feedback with type ii compensation is shown in fig. 14. this method requires that the output capacitor should have enough esr to satisfy stability requirements. in general the output capacitor?s esr generates a zero typically at 5khz to 50khz which is essential for an acceptable phase margin. the esr zero of the output capacitor is expressed as follows: (16) .. .......... .......... .......... o o lc c l f ? ? = 2 1 fig. 13. gain and phase of lc filter (17) ....... .......... .......... o esr *esr*c f ? = 2 1 output capacitor selection the voltage ripple and transient requirements determine the output capacitors type and values. the criteria is normally based on the value of the effective series resistance (esr). however the actual capacitance value and the equivalent series inductance (esl) are other contributing components. these components can be described as rev5.0_01_28_10 21 ir3856wmpbf the transfer function ( v e /v o ) is given by: the (s) indicates that the transfer function varies as a function of frequency. this configuration introduces a gain and zero, expressed by: first select the desired zero-crossover frequency ( f o ): use the following equation to calculate r3: v out v ref r 9 r 8 c pole c 4 r 3 ve f z f pole e/a z f frequency gain(db) h(s) db fb comp z in fig. 14. type ii compensation network and its asymptotic gain plot (18) ..... ) ( 4 8 4 3 1 c sr c sr z z s h v v in f o e + ? = ? = = () (20) ........ .......... .......... * * (19) ......... .......... .......... ......... 4 3 8 3 2 1 c r f r r s h z = = () s esr o f f f * 1/10 ~ 1/5 f and o > (21) ....... .......... .......... * * * * 2 8 3 lc in esr o osc f v r f f v r = where: v in = maximum input voltage v osc = oscillator ramp voltage f o = crossover frequency f esr = zero frequency of the output capacitor f lc = resonant frequency of the output filter r 8 = feedback resistor to cancel one of the lc filter poles, place the zero before the lc filter resonant frequency pole: use equations (20), (21) and (22) to calculate c4. one more capacitor is sometimes added in parallel with c4 and r3. this introduces one more pole which is mainly used to suppress the switching noise. the additional pole is given by: the pole sets to one half of the switching frequency which results in the capacitor c pole : for a general solution for unconditional stability for any type of output capacitors, and a wide range of esr values, we should implement local feedback with a type iii compensation network. the typically used compensation network for voltage-mode controller is shown in figure 15. again, the transfer function is given by: by replacing z in and z f according to figure 15, the transfer function can be expressed as: (22) ....... .......... .......... .......... * * . % o o z lc z c l f f f 2 1 75 0 75 = = ...(23) .......... .......... .......... * * * pole pole p c c c c r f + = 4 4 3 2 1 (24) .. .......... .......... s s pole *f *r c *f *r c 3 4 3 1 1 1 ? ? = in f o e z z s h v v ? = = ) ( () [ ] (25) .... ) ( * ) ( ) ( ) ( 7 10 3 4 3 4 3 3 4 8 10 8 7 4 3 1 1 1 1 c sr c c c c sr c c sr r r sc c sr s h + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? + + + + + + ? = rev5.0_01_28_10 22 ir3856wmpbf tantalum ceramic f lc rev5.0_01_28_10 24 ir3856wmpbf application diagram: fig. 16. application circuit diagram for a 12v to 1.8 v, 6 a point of load converter suggested bill of materials for the application circuit: part reference quantity value description manufacturer part number 1 330uf smd elecrolytic, fsize, 25v, 20% panasonic eev-fk1e331p 2 10uf 1206, 16v, x5r, 20% tdk c3216x5r1e106m 1 0.1uf 0603, 25v, x7r, 10% panasonic ecj-1vb1e104k lo 1 1.0uh 6.9x6.5x5mm, 20%, 4.7mohm tdk spm6550t-1r0m100a co 4 22uf 0805, 6.3v, x5r, 20% panasonic ecj-2fb0j226ml r1 1 49.9k thick film, 0603,1/10 w,1% rohm mcr03ezpfx4992 r2 1 7.5k thick film, 0603,1/10w,1% rohm mcr03ezpfx7501 r t 1 23.7k thick film, 0603,1/10w,1% rohm mcr03ezpfx2372 r ocset 1 2.67k thick film, 0603,1/10w,1% rohm mcr03ezpfx2671 r pg 1 10k thick film, 0603,1/10w,1% rohm mcr03ezpfx1002 c ss c6 2 0.1uf 0603, 25v, x7r, 10% panasonic ecj-1vb1e104k r3 1 2.05k thick film, 0603,1/10w,1% rohm mcr03ezpfx2051 c3 1 220pf 50v, 0603, npo, 5% panasonic ecj-1vc1h221j c4 1 10nf 0603, 50v, x7r, 10% panasonic ecj-1vb1h103k r8 1 4.02k thick film, 0603,1/10w,1% rohm mcr03ezpfx4021 r9 1 2.55k thick film, 0603,1/10w,1% rohm mcr03ezpfx2551 r10 1 130 thick film, 0603,1/10w,1% rohm erj-3ekf1300v c7 1 2200pf 0603, 50v, x7r, 10% panasonic ecj-1vb1h222k c vcc c pvcc 2 1.0uf 0603, 16v, x5r, 20% panasonic ecj-bvb1c105m u1 1 ir3856w supirbuck, 6a, pqfn 4x5mm international rectifier ir3856wmpbf cin rev5.0_01_28_10 25 ir3856wmpbf typical operating waveforms vin=12.0v, vcc=5v, vo=1.8v, io=0-6a, room temperature, no air flow fig. 20. output voltage ripple, 6a load ch 3 : v o fig. 21. inductor node at 6a load ch 3 :lx fig. 22. short (hiccup) recovery ch 2 :v o , ch 3 :v ss fig. 18. start up at 6a load, ch 1 :v in , ch 2 :v o , ch 3 :v ss , ch 4 :v pgood fig. 17. start up at 6a load ch 1 :v in , ch 2 :v o , ch 3 :v ss , ch 4 :enable fig. 19. start up with 1.62v pre bias, 0a load, ch 2 :v o , ch 3 :v ss rev5.0_01_28_10 26 ir3856wmpbf typical operating waveforms vin=12v, vcc=5v, vo=1.8v, io=3a-6a, room temperature, no air flow fig. 23. transient response, 3a to 6a step 2.5a/ s ch 3 :v o , ch 4 :i o rev5.0_01_28_10 27 ir3856wmpbf typical operating waveforms vin=12v, vcc=5v, vo=1.8v, io=6a, room temperature, no air flow fig. 24. bode plot at 6a load shows a bandwidth of 104khz and phase margin of 54 degrees rev5.0_01_28_10 28 ir3856wmpbf simultaneous tracking at power up and power down vin=12v, vo=1.8v, io=6a, room temperature, no air flow fig. 25: simultaneous tracking a 3.3v input at power-up and shut-down with 6a load. ch2: vo2 (1.8vout) ch3:ss ch4: vo1 (3.3v) fb ir3624 v out r 9 r 8 ir3856w 3.3v r s2 r s1 seq 4.02k 4.02k 2..55k 2.55k rev5.0_01_28_10 29 ir3856wmpbf layout considerations the layout is very important when designing high frequency switching converters. layout will affect noise pickup and can cause a good design to perform with less than expected results. make all the connections for the power components in the top layer with wide, copper filled areas or polygons. in general, it is desirable to make proper use of power planes and polygons for power distribution and heat dissipation. the inductor, output capacitors and the ir3856w should be as close to each other as possible. this helps to reduce the emi radiated by the power traces due to the high switching currents through them. place the input capacitor directly at the vin pin of ir3856w. the feedback part of the system should be kept away from the inductor and other noise sources. the critical bypass components such as capacitors for vcc should be close to their respective pins. it is important to place the feedback components including feedback resistors and compensation components close to fb and comp pins. pgnd vin agnd vout pgnd vin agnd vout the connection between the ocset resistor and the sw pin should not share any trace with the connection between the bootstrap capacitor and the sw pin. instead, it is recommended to use a kelvin connection of the trace from the ocset resistor and the trace from the bootstrap capacitor at the sw pin. in a multilayer pcb use one layer as a power ground plane and have a control circuit ground (analog ground), to which all signals are referenced. the goal is to localize the high current path to a separate loop that does not interfere with the more sensitive analog control function. these two grounds must be connected together on the pc board layout at a single point. the power qfn is a thermally enhanced package. based on thermal performance it is recommended to use at least a 4-layers pcb. to effectively remove heat from the device the exposed pad should be connected to the ground plane using vias. figure 26 illustrates the implementation of the layout guidelines outlined above, on the irdc3856w 4 layer demoboard. vin all bypass caps should be placed as close as possible to their connecting pins. resistors rt, rocset, and ss capacitor should be placed as close as possible to their pins. pgnd vin agnd vout compensation parts should be placed as close as possible to the comp pin . fig. 26a. irdc3856w demo-board layout considerations ? top layer rev5.0_01_28_10 30 ir3856wmpbf pgnd vin agnd power ground plane analog ground plane single point connection between agnd & pgnd, should be close to the supirbuck, kept away from noise sources. fig. 26c. irdc3856 demoboard layout considerations ? mid layer 1 fig. 26d. irdc3856 demoboard layout considerations ? mid layer 2 fig. 26b. irdc3856 demoboard layout considerations ? bottom layer feedback trace should be kept away form noise sources rev5.0_01_28_10 31 ir3856wmpbf ir world headquarters: 233 kansas st., el segundo, california 90245, usa tel: (310) 252-7105 tac fax: (310) 252-7903 visit us at www.irf.com for sales contact information data and specifications subject to change without notice. 05/07 |
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