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jan28-08 revg SP6125: tsot-6 pfet buck controller ? 2007 exar corporation 1 features wide 4.5v C 29v input voltage range internal compensation built-in high current pmos driver adjustable overcurrent protection internal soft-start 300khz constant frequency operation 0.6v reference voltage 1% output setpoint accuracy lead free, rohs compliant package: small 6 pin tsot description the SP6125 is a pwm controlled step down (buck) vol tage mode regulator with v in feedforward and internal type-ii compensation. it operates from 4.5 v to 29v, making is suitable for 5v, 12v, and 24v applications. by using a pmos driver, this device i s capable of operating at 100% duty cycle. the high side driver is designed to drive the gate 5v b elow v in . the programmable overcurrent protection is based on high-side mosfets on resist ance sensing and allows setting the overcurrent protection value up to 300mv threshold (measured from v in -lx). the SP6125 is available in a space-saving 6-pin tsot package maki ng it the smallest controller available capable of operating from 24vdc supplies. typical application circuit high=of f l1, ihlp-2525cz 8.2uh, 68mohm, 4a c4 22uf ds mbra340t3g r1 300k, 1% rz 2k r2 66.5k, 1% q1 fds4685 cz 47pf c6 0.1uf c1 4.7uf rs 2k d1 1n4148 24v shdn vin vout gnd gnd vfb vin lx gnd vdr gate SP6125 3.3v 0-3a 3 1 4 5 26 c5 22uf c2 4.7uf SP6125 6 5 4 1 2 3 SP6125 6 pintsot gnd lx gate fb v in vdr high-voltage, step down controller in tsot6
jan28-08 revg SP6125: tsot-6 pfet buck controller ? 2007 exar corporation 2 absolute maximum ratings input voltage.................................... ............-0.3v to 30v lx .. . -2v to 30v fb ................................................... .-0.3v to 5.5v storage temperature.. .. ... -65 c to 150 c junction temperature............................... ......-40 c to 125 c lead temperature (soldering, 10 sec)...300 c esd rating ..1kv lx, 2kv all other nodes, hbm electrical specifications specifications are for t amb =t j =25 c, and those denoted by ? apply over the full operating range, -40 c< t j <125 c. unless otherwise specified: v in =4.5v to 29v, c in = 4.7 m f. parameter min typ max units ? conditions uvlo turn-on threshold 4.2 4.35 4.5 v 0 c< t j <125 c uvlo turn-off threshold 4.0 4.2 4.4 v 0 c< t j <125 c uvlo hysterisis 0.2 v operating input voltage range 4.5 29 v 0 c< t j <125 c operating input voltage range 7 29 v ? operating vcc current 0.3 3 ma vfb=1.2v reference voltage accuracy 0.5 1 % reference voltage accuracy 0.5 2 % ? reference voltage 0.594 0.6 0.606 v reference voltage 0.588 0.6 0.612 v ? switching frequency 255 300 345 khz peak-to-peak ramp modulator v in /5 v minimum on-pulse duration 40 100 ns ? minimum duty cycle 0 % maximum duty cycle 100 % gate driver turn-off resistance 50 60 k internal resistor between gate and v in gate driver pull-down resistance 4 8 v in =12v, v fb =0.5v, measure resistance between gate and vdr gate driver pull-up resistance 3 6 v in =12v, v fb =0.7v, measure resistance between gate and v in v in - vdr voltage difference 4.5 5.5 v ? measure v in C vdr, v in >7v overcurrent threshold 270 300 330 mv measure v in - lx lx pin input current 25 30 35 ua v lx = v in off interval during hiccup 200 ms soft start time 3 5 9 ms vfb=0.58v, measure between v in =4.5v and first gate pulse shdn threshold 0.9 1.0 1.1 v ? apply voltage to fb shdn threshold hysteresis 100 mv these are stress ratings only, and functional operation of the device at these ratings or any oth er above those indicated in the operation sections of the specifications below is not implied. exposure to absolute maximum rating conditions for extended periods of time may affect reliability.
jan28-08 revg SP6125: tsot-6 pfet buck controller ? 2007 exar corporation 3 pin description pin # pin name description 1 v in input power supply for the controller. place input decoupling capacitor as close as possible to this pin. 2 gate connect to the gate terminal of the external p-channel mosfet. 3 vdr power supply for the internal driver. this voltage is internally regulated to about 5v below v in . place a 0.1uf decoupling capacitor between vdr an d vin as close as possible to the ic. 4 fb regulator feedback input. connect to a resistive vo ltage-divider network to set the output voltage. this pin can be also used for o n/off control. if this pin is pulled above 1v the p-channel driver is disabled an d controller resets internal soft start circuit. 5 gnd ground pin. 6 lx this pin is used as a current limit input for the i nternal current limit comparator. connect to the drain pin of the external mosfet thr ough an optional resistor. internal threshold is pre-set to 300mv nominal and can be decreased by changing the external resistor based on the followi ng formula: v trshld = 300mv C 30ua * r block diagram fault register set dominant por +- enbl 4-bit counter vdr vin gate 5v internal ldo vin - 0.3v overcurrent comparator lx vdr 30ua gnd 5v +- +- vref fault fb oscillator vin sr i = k x vin error amplifier pwm comparator pwm latch reset dominant vin - 5v ldo fault +- 1v fault 200ms delay uvlo s r r
jan28-08 revg SP6125: tsot-6 pfet buck controller ? 2007 exar corporation 4 the SP6125 is a fixed frequency, voltage- mode, non-synchronous pwm controller optimized for minimum component, small form factor and cost effectiveness. it has been designed for single-supply operation ranging from 4.5v to 29v. SP6125 has type-ii internal compensation for use with electrolytic or tantalum output capacitors. for ceramic capacitors type-iii compensation can be implemented by simply adding an r and c between output and feedback. a precision 0.6v reference, present on the positive terminal of the error amplifier, permits programming of the output voltage down to 0.6v via the fb pin. the output of the error amplifier is internally compared to a feed-forward (v in /5 peak-to- peak) ramp and generates the pwm control. timing is governed by an internal oscillator that sets the pwm frequency at 300khz. SP6125 contains useful protection features. overcurrent protection is based on high-side mosfets r ds(on) and is programmable via a resistor placed at lx node. under-voltage lock-out (uvlo) ensures that the controller starts functioning only when sufficient voltage exists for powering ics internal circuitry. SP6125 loop compensation the SP6125 includes type-ii internal compensation components for loop compen- sation. external compensation components are not required for systems with tantalum or aluminum electrolytic output capacitors with sufficiently high esr. use the condition below as a guideline to determine whether or not the internal compensation is sufficient for your design. type-ii internal compensation is sufficient if the following condition is met: dbpole esrzero f f < . (1) where: out esr esrzero c r f . . .2 1 p = .. (2) out dbpole c l f = . .2 1 p (3) 1 3 . 1 r c l cz = .. (4) creating a type-iii compensation network the above condition requires the esr zero to be at a lower frequency than the double-pole from the lc filter. if this condition is not met, type-iii compensation should be used and can be accomplished by placing a series rc combination in parallel with r1 as shown below. the value of cz can be calculated as follows and rz selected from table 1. table1- selection of rz f esrzero f dbpole rz 1x 50k 2x 40k 3x 30k 5x 10k >= 10x 2k general overview
jan28-08 revg SP6125: tsot-6 pfet buck controller ? 2007 exar corporation 5 r1 300k, 1% r2 SP6125 vfb +- rz2 200k cz2 130pf cp1 2pf error amplif ier vref =0.6v vout rz cz figure 1- rz and cz in conjunction with internal compensation components form a type-iii compensatio n loop compensation example 1- a converter utilizing a SP6125 has a 8.2uh inductor and two 22uf/5m ceramic capacitor. determine whether type-iii compensation is needed. from equation (2) f esrzero = 1.45mhz. from equation (3) f dbpole = 8.4khz. since the condition specified in (1) is not met, type-iii compensation has to be used by adding external components rz and cz. using equation (4) cz is calculated 48.7pf (use 47 pf). following the guideline given in table 1, a 2k rz should be used. the steps followed in example 1 were used to compensate the typical application circuit shown on page 1. satisfactory frequency response of the circuit, seen in figure 2, validates the above procedure. loop compensation example 2- a converter utilizing a SP6125 has a 8.2uh inductor and a 150uf, 82m aluminum electrolytic capacitor. determine whether type-iii compensation is needed. from equation (2) f esrzero = 13khz. from equation (3) f dbpole = 4.5khz. since the condition specified in (1) is not met, type-iii compensation has to be used by adding external components rz and cz. using equation (4) cz is calculated 89.9pf (use 100pf). since f esrzero f dbpole is approximately 3, rz has to be set at 30k . figure 2- satisfactory frequency response of typica l application circuit shown on page 1. crossover frequency fc is about 35khz with a corres ponding phase margin of 60 degrees. the two sets of curves, which are essentially identical , correspond to load current of 1a and 2.5a. general overview
jan28-08 revg SP6125: tsot-6 pfet buck controller ? 2007 exar corporation 6 overcurrent protection figure 3- overcurrent protection circuit the overcurrent protection circuit functions by monitoring the voltage across the high-side fet q1. when this voltage exceeds 0.3v, the overcurrent comparator triggers and the controller enters hiccup mode. for example if q1 has rds(on)=0.1 , then the overcurrent will trigger at i = 0.3v/0.1 =3a. to program a lower overcurrent use a resistor rs as shown in figure 1. calculate rs from: ( ) ua on rds iout rs 30 ) ( 15.1 3.0 - = (5) the overcurrent circuit triggers at peak current through q1 which is usually about 15% higher than average output current. hence the multiplier 1.15 is used in (5). example: a switching mosfet used with SP6125 has rds(on) of 0.1 . program the over- current circuit so that maximum output is 2a. ( ) ua ohm a rs 30 1.0 2 15.1 3.0 - = rs = 2333 using the above equation there is good agreement between calculated and test results for rs in the range of 0.5k to 3k . for rs larger than 3k test results are lower than those predicted by (5), due to circuit parasitics. therefore maximum value of rs should be limited to 3k . using the on/off function the feedback pin serves a dual role of on/off control. the mosfet driver is disabled when a voltage greater than 1v is applied at the fb pin. maximum voltage rating of this pin is 5.5v. the controlling signal should be applied through a small signal diode as shown on page 1. please note that an optional 10k bleeding resistor across the output helps keep the output capacitor discharged under no load condition. programming the output voltage to program the output voltage, calculate r2 using the following equation: ? ?? ? ? ?? ? - = 1 1 2 vref vout r r where: vref=0.6 is the reference voltage of the SP6125 r1=200k is a fixed-value resistor that, in addition to being a voltage divider, it is part of the compensation network. in order to simplify compensation calculations, r1 is fixed at 200k . soft start soft start is preset internally to 5ms (nominal). internal soft start eliminates the need for the external capacitor css that is commonly used to program this function. mosfet gate drive p-channel drive is derived through an internal regulator that generates v in -5v. this pin (vdr) has to be connected to v in with a 0.1uf decoupling capacitor. the gate drive circuit swings between v in and v in -5 and employs powerful drivers for efficient switching of the p- channel mosfet. ds q1 rs SP6125 gate lx 30ua +- vin - 0.3v vin ov er-current comparator general overview
jan28-08 revg SP6125: tsot-6 pfet buck controller ? 2007 exar corporation 7 power mosfet selection select the power mosfet for voltage rating bv dss , on resistance r ds(on) , and thermal resistance rthja. bv dss should be about twice as high as v in in order to guard against switching transients. recommended mosfet voltage rating for v in of 5v, 12v and 24v is 12v, 30v and 40v respectively. r ds(on) , must be selected such that when operating at peak current and junction temperature the overcurrent threshold of the SP6125 is not exceeded. allowing 50% for temperature coefficient of r ds(on) and 15% for inductor current ripple, the following expression can be used: ?? ? ?? ? iout mv on rds 15.1 5.1 300 ) ( within this constraint, selecting mosfets with lower r ds(on) will reduce conduction losses at the expense of increased switching losses. as a rule of thumb select the highest r ds(on) mosfet that meets the above criteria. switching losses can be assumed to roughly equal the conduction losses. a simplified expression for conduction losses is given by: ?? ? ?? ? = vin vout on rds iout pcond ) ( mosfets junction temperature can be estimated from: ( ) tambient rthja pc t + = 2 schottky rectifier selection select the schottky for voltage rating v r , forward voltage v f , and thermal resistance rthja. voltage rating should be selected using the same guidelines outlined for mosfet voltage selection. for a low duty cycle application such as the circuit shown on first page, the schottky is conducting most of the time and its conduction losses are the largest component of losses in the converter. conduction losses can be estimated from: ?? ? ?? ? - = vin vout iout vf pc 1 where: vf is diode forward voltage at i out schottkys ac losses due to its switching capacitance are negligible. inductor selection select the inductor for inductance l and saturation current isat. select an inductor with isat higher than the programmed overcurrent. calculate inductance from: ( ) ? ?? ? ? ?? ? ? ?? ? ? ?? ? ?? ? ?? ? - = irip f vin vout vout vin l 1 1 where: v in is converter input voltage v out is converter output voltage f is switching frequency i rip is inductor peak-to-peak current ripple (nominally set to 30% of i out ) keep in mind that a higher i rip results in a smaller inductor which has the advantages of small size, low dc equivalent resistance dcr, high saturation current isat and allows the use of a lower output capacitance to meet a given step load transient. a higher irip, however, increases the output voltage ripple and increases the current at which converter enters discontinuous conduction mode. the output current at which converter enters dcm is ? of i rip . note that a negative current step load that drives the converter into dcm will result in a large output voltage transient. therefore the lowest current for a step load should be larger than ? of i rip . output capacitor selection select the output capacitor for voltage rating, capacitance and equivalent series resistance (esr). nominally the voltage rating is selected to be twice as large as the output voltage. select the capacitance to satisfy the specification for output voltage overshoot or undershoot caused by current step load. a steady-state output current i out corresponds to inductor stored energy of ? l i out 2 . a sudden decrease in i out forces the energy surplus in l to be absorbed by c out . general overview
jan28-08 revg SP6125: tsot-6 pfet buck controller ? 2007 exar corporation 8 this causes an overshoot in output voltage that is corrected by power switch reduced duty cycle. use the following equation to calculate c out : ? ?? ? ? ?? ? - = 2 2 2 1 2 2 vout - vos i i l cout where: l is the output inductance i 2 is the step load high current i 1 is the step load low current vos is output voltage including overshoot v out is steady state output voltage output voltage undershoot calculation is more complicated. test results for SP6125 buck circuits show that undershoot is approximately equal to overshoot. therefore above equation provides a satisfactory method for calculating c out . select esr such that output voltage ripple (v rip ) specification is met. there are two components to v rip : first component arises from charge transferred to and from c out during each cycle. the second component of v rip is due to inductor ripple current flowing through output capacitors esr. it can be calculated from: 2 2 8 1 ? ?? ? ? ?? ? + = fs cout esr irip vrip where: i rip is inductor ripple current f s is switching frequency c out is output capacitor calculated above note that a smaller inductor results in a higher i rip , therefore requiring a larger c out and/or lower esr in order to meet v rip . input capacitor selection select the input capacitor for voltage, capacitance, ripple current, esr and esl. voltage rating is nominally selected to be twice the input voltage. the rms value of input capacitor current, assuming a low inductor ripple current (irip), can be calculated from: ( ) d d iout icin - = 1 in general total input voltage ripple should be kept below 1.5% of v in (not to exceed 180mv). input voltage ripple has three components: esr and esl cause a step voltage drop upon turn on of the mosfet. during on time capacitor discharges linearly as it supplies i out - i in . the contribution to input voltage ripple by each term can be calculated from: ( ) 2 , vin cin fs vout vin vout iout cin v - = d ( ) irip iout esr esr v 5.0 , - = d ( ) trise irip iout esl esl v 5.0 , - = d where trise is the rise time of current through capacitor total input voltage ripple is sum of the above: esl v esr v cin v tot v , , , , d + d + d = d in circuits where converter input voltage is applied via a mechanical switch excessive ringing may be present at turn-on that may interfere with smooth startup of sp6126. addition of an inexpensive 100 f aluminum electrolytic capacitor at the input will help reduce ringing and restore a smooth startup. general overview
jan28-08 revg SP6125: tsot-6 pfet buck controller ? 2007 exar corporation 9 high=of f l1, ihlp-2525cz 8.2uh, 68mohm, 4a c4 22uf ds mbra340t3g r1 300k, 1% rz 2k r2 66.5k, 1% q1 fds4685 cz 47pf c6 0.1uf c1 4.7uf rs 2k d1 1n4148 24v shdn vin vout gnd gnd vfb vin lx gnd vdr gate SP6125 3.3v 0-3a 3 1 4 5 26 c5 22uf c2 4.7uf figure 4- application circuit for vin=24v SP6125 efficiency versus iout, vin=24v,ta=25c 50 60 70 80 90 0.0 0.5 1.0 1.5 2.0 2.5 3.0 iout (a) efficiency (%) vout=3.3v figure 5- efficiency at v in = 24 v , t a = 25?c, natural convection typical performance characteristics
jan28-08 revg SP6125: tsot-6 pfet buck controller ? 2007 exar corporation 10 figure 6- step load 1.2-2.8a, ch1: v in ; ch2: v out ; ch3: i out figure 7- startup no load, ch1: v in ; ch2: v out , ch3: i out figure 8- start up 3a, ch1: v in ; ch2: v out ; ch3: i out figure 9- output ripple at 0a is 12mv, ch1: v in ; ch2: v out ; ch3: i out figure 10- output ripple at 3a is 32mv, ch1: v in ; ch2: v out ; ch3: i out typical performance characteristics
jan28-08 revg SP6125: tsot-6 pfet buck controller ? 2007 exar corporation 11 package: 6pin tsot exar
jan28-08 revg SP6125: tsot-6 pfet buck controller ? 2007 exar corporation 12 ordering information part number temperature range package SP6125ek1-l.-40 c to +125 c ...(lead free) 6 pin tsot SP6125ek1-l/tr....-40 c to +125 c .. ...(lead free) 6 pin tsot /tr = tape and reel pack quantity for tape and reel is 2500 for further assistance: email: customersupport@exar.com exar technical documentation: http://www.exar.com/ techdoc/default.aspx? exar corporation headquarters and sales office 48720 kato road fremont, ca 94538 main: 510-668-7000 fax: 510-668-7030 exar corporation reserves the right to make changes to the products contained in this publication in order to improve design, performance or reliabil ity. exar corporation assumes no responsibility for the use of any circuits described herein, conve ys no license under any patent or other right, and makes no representation that the circuits are free of patent infringement. charts and schedules contained here in are only for illustration purpose s and may vary depending upon a users specific application. while the information in this publicat ion has been carefully checked; no responsibility, however, is assumed for inaccuracies. exar corporation does not recommend the use of any of its products in life support applications where the failure or malfunction of the product can reasonably be expected to cause failure of the lif e support system or to significantly affect its safet y or effectiveness. products are not authorized for use in such applications unless exar corporation re ceives, in writing, assurances to its satisfaction that: (a) the risk of injury or damage has been min imized; (b) the user assumes all such risks; (c) potential liability of exar corporation is adequate ly protected under the circumstances.

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