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mp28315 3a, 16v, 340khz synchronous rectified step-down converter mp28315 rev. 1.2 www.monolithicpower.com 1 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. the future of analog ic technology tm tm description the mp28315 is a monolithic synchronous buck regulator. the device integrates 100m ? mosfets that provide 3a continuous load current over a wide operating input voltage of 5v to 16v. current mode control provides fast transient response and cycle-by-cycle current limit. an adjustable soft-start prevents inrush current at turn-on. in shutdown mode, the supply current drops below 1a. this device, available in an 8-pin soic package, provides a very compact system solution with minimal reliance on external components. features ? 3a output current ? wide 5v to 16v operating input range ? integrated 100m ? power mosfet switches ? output adjustable from 0.925v to 13v ? up to 95% efficiency ? programmable soft-start ? stable with low esr ceramic output capacitors ? fixed 340khz frequency ? cycle-by-cycle over current protection ? input under-voltage lockout ? thermally enhanced 8-pin soic package applications ? distributed power systems ? networking systems ? fpga, dsp, asic power supplies ? green electronics/appliances ? notebook computers ?mps? and ?the future of analog ic technology? are trademarks of monolithic power systems, inc. typical application input 5v to 16v output 3.3v 3a c3 3.9nf d1 b130 (optional) c5 10nf mp28315 bs in fb sw ss gnd comp en 1 2 3 5 6 4 8 7 c6 (optional) mp28315_tac_s01 100 95 90 85 80 75 70 65 60 55 50 efficiency (%) 0.1 1.0 10 load current (a) mp28315_ec01 efficiency vs load current v in = 5v v in = 12v
mp28315 ? 3a, 16v, 340khz synchrono us rectified step -down converter mp28315 rev. 1.2 www.monolithicpower.com 2 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. tm package reference bs in sw gnd ss en comp fb 1 2 3 4 8 7 6 5 top view mp28315_pd01_soic8n exposed pad on backside part number* package temperature MP28315CN soic8n 0 to +70c * for tape & reel, add suffix ?z (eg. MP28315CN?z) for lead free, add suffix ?lf (eg. MP28315CN?lf?z) absolute maxi mum ratings (1) supply voltage v in ....................... ?0.3v to +22v switch voltage v sw ................. ?1v to v in + 0.3v boost voltage v bs ..........v sw ? 0.3v to v sw + 6v all other pins................................. ?0.3v to +6v junction temperature...............................150c lead temperature ....................................260c storage temperature .............?65c to +150c recommended operating conditions (2) input voltage v in ................................. 5v to 16v output voltage v out .................... 0.925v to 13v ambient operating temp ...................0c to +70c thermal resistance (3) ja jc soic8n (exposed pad) ......... 50 ...... 10... c/w notes: 1) exceeding these ratings may damage the device. 2) the device is not guaranteed to function outside of its operating conditions. 3) measured on approximately 1? square of 1 oz copper. electrical characteristics v in = 12v, t a = +25c, unless otherwise noted. parameter symbol condition min typ max units shutdown supply current v en = 0v 0.3 10 a supply current v en = 2.0v, v fb = 1.0v 1.3 1.5 ma feedback voltage v fb 0.900 0.925 0.951 v feedback overvoltage threshold 1.1 v error amplifier voltage gain (4) a ea 400 v/v error amplifier transconductance g ea i c = 10 a 820 a/v high-side switch on-resistance (4) r ds(on)1 100 m ? low-side switch on-resistance (4) r ds(on)2 100 m ? high-side switch leakage current v en = 0v, v sw = 0v 0 10 a upper switch current limit minimum duty cycle 4.0 5.8 a oscillation frequency f osc1 300 340 380 khz short circuit oscillation frequency f osc2 v fb = 0v 110 khz maximum duty cycle d max v fb = 1.0v 90 % minimum on time (4) t on 220 ns en shutdown threshold voltage v en rising 1.1 1.5 2.0 v en shutdown threshold voltage hysterisis 220 mv en lockout threshold voltage 2.2 2.5 2.7 v en lockout hysteresis 210 mv
mp28315 ? 3a, 16v, 340khz synchrono us rectified step -down converter mp28315 rev. 1.2 www.monolithicpower.com 3 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. tm electrical characteristics (continued) v in = 12v, t a = +25c, unless otherwise noted. parameter symbol condition min typ max units input under voltage lockout threshold v in rising 3.80 4.05 4.40 v input under voltage lockout threshold hysteresis 210 mv soft-start current v ss = 0v 6 a soft-start period c ss = 0.1 f 15 ms thermal shutdown (4) 130 160 c note: 4) guaranteed by design, not tested. pin functions pin # name description 1 bs high-side gate drive boost input. bs supplies the drive for the high-side n-channel mosfet switch. connect a 0.01 f or greater capacitor from sw to bs to power the high side switch. 2 in power input. in supplies the power to the ic, as well as the step-down converter switches. drive in with a 5v to 16v power source. bypass in to gnd with a suitably large capacitor to eliminate noise on the input to the ic. see input capacitor . 3 sw power switching output. sw is the switching node that supplies power to the output. connect the output lc filter from sw to the output load. note that a capacitor is required from sw to bs to power the high-side switch. 4 gnd ground (connect exposed pad to pin 4). 5 fb feedback input. fb senses the output voltage to regulate that voltage. drive fb with a resistive voltage divider from the output volt age. the feedback threshold is 0.925v. see setting the output voltage . 6 comp compensation node. comp is used to compensate the regulation control loop. connect a series rc network from comp to gnd to compensate the regulation control loop. in some cases, an additional capacitor from comp to gnd is required. see compensation components. 7 en enable input. en is a digital input that turns t he regulator on or off. drive en high to turn on the regulator, drive it low to turn it off. pull up with 100k ? resistor for automatic startup. 8 ss soft-start control input. ss contro ls the soft-start peri od. connect a capacitor from ss to gnd to set the soft-start period. a 0.1 f capacitor sets the soft-start period to 15ms. to disable the soft-start feature, leave ss unconnected.
mp28315 ? 3a, 16v, 340khz synchrono us rectified step -down converter mp28315 rev. 1.2 www.monolithicpower.com 4 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. tm typical performanc e characteristics c1 = 2 x 10f, c2 = 2 x 22f, l= 10h, c ss = 0.1f, unless otherwise noted. v in 20mv/div. v out 20mv/div. v sw 10v/div. i l 1a/div. mp28315-tpc01 steady state test waveforms v in = 12v, v out = 3.3v, i out = 0a v in 200mv/div. v out 20mv/div. v sw v/div. i l 2a/div. mp28315-tpc02 steady state test waveforms v in = 12v, v out = 3.3v, i out = 3a v en 5v/div. v out 2v/div. v sw 10v/div. i l 1a/div. 2ms/div. mp28315-tpc03 startup through enable waveforms v in = 12v, v out = 3.3v, no load v en 5v/div. v out 2v/div. v sw 10v/div. i l 2a/div. 2ms/div. mp28315-tp04 startup through enable waveforms v in = 12v, v out = 3.3v, i out = 3a (resistance load) v en 5v/div. v out 2v/div. v sw 10v/div. i l 1a/div. 2ms/div. mp28315-tpc05 shutdown through enable waveforms v in = 12v, v out = 3.3v, no load v out 2v/div. v sw 10v/div. v en 5v/div. i l 2a/div. mp28315-tpc06 shutdown through enable waveforms v in = 12v, v out = 3.3v, i out = 3a (resistance load) v out 200mv/div. i l 1a/div. i load 1a/div. mp28315-tpc07 load transient test waveforms v in = 12v, v out = 3.3v, i out = 1a to 2a step v out 2v/div. i l 2a/div. mp28315-tpc08 short circuit test waveforms v in = 12v, v out = 3.3v v out 2v/div. i l 2a/div. mp28315-tpc09 short circuit recovery waveforms v in = 12v, v out = 3.3v
mp28315 ? 3a, 16v, 340khz synchrono us rectified step -down converter mp28315 rev. 1.2 www.monolithicpower.com 5 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. tm operation functional description the mp28315 is a synchronous rectified, current-mode, step-down regulator. it regulates input voltages from 5v to 16v down to an output voltage as low as 0.925v, and supplies up to 3a of load current. the mp28315 uses current-mode control to regulate the output voltage. the output voltage is measured at fb through a resistive voltage divider and amplified through the internal transconductance error amplifier. the voltage at the comp pin is compared to the switch current measured internally to control the output voltage. the converter uses internal n-channel mosfet switches to step-down the input voltage to the regulated output voltage. since the high-side mosfet requires a gate voltage greater than the input voltage, a boost capacitor connected between sw and bs is needed to drive the high side gate. the boost capacitor is charged from the internal 5v rail when sw is low. when the mp28315 fb pin exceeds 20% of the nominal regulation voltage of 0.925v, the over voltage comparator is tripped and the comp pin and the ss pin are discharged to gnd, forcing the high-side switch off. mp28315_bd01 lockout comparator internal regulators in en + error amplifier 1.2v ovp ramp clk 0.925v 7v 0.3v current comparator current sense amplifier 1.1v shutdown comparator 7 comp 6 ss 8 fb 5 gnd 4 oscillator 110/340khz s r q sw 3 bs 1 in 5v 2 ovp in < 4.10v en ok zener + q + + 1.5v + + 2.5v + + -- -- -- -- -- -- -- figure 1?functional block diagram
mp28315 ? 3a, 16v, 340khz synchrono us rectified step -down converter mp28315 rev. 1.2 www.monolithicpower.com 6 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. tm applications information component selection setting the output voltage the output voltage is set using a resistive voltage divider from the output voltage to fb pin. the voltage divider divides the output voltage down to the feedback voltage by the ratio: 2 r 1 r 2 r v v out fb + = thus the output voltage is: 2 r 2 r 1 r 925 . 0 v out + = r2 can be as high as 100k ? , but a typical value is 10k ? . using the typical value for r2, r1 is determined by: ) 925 . 0 v ( 81 . 10 1 r out ? = (k ? ) for example, for a 3.3v output voltage, r2 is 10k ? , and r1 is 26.1k ? . table 1 lists recommended resistance values of r1 and r2 for standard output voltages. table 1?recommended resistance values vout r1 r2 1.8v 9.53k ? 10k ? 2.5v 16.9k ? 10k ? 3.3v 26.1k ? 10k ? 5v 44.2k ? 10k ? 12v 121k ? 10k ? inductor the inductor is required to supply constant current to the output load while being driven by the switched input voltage. a larger value inductor will result in less ripple current that will in turn result in lower output ripple voltage. however, the larger value inductor will have a larger physical size, higher series resistance, and/or lower saturation current. a good rule is to allow the inductor peak-to-peak ripple current to be approximately 30% of the maximum switch current limit. also, make sure that the peak inductor current is below the maximum switch current limit. the inductance value can be calculated by: ? ? ? ? ? ? ? ? ? = in out l s out v v 1 i f v l where v out is the output voltage, v in is the input voltage, f s is the switching frequency, and i l is the peak-to-peak inductor ripple current. choose an inductor that will not saturate under the maximum inductor peak current. the peak inductor current can be calculated by: ? ? ? ? ? ? ? ? ? + = in out s out load lp v v 1 l f 2 v i i where i load is the load current. the choice of which style inductor to use mainly depends on the price vs. size requirements and any emi constraints. optional schottky diode during the transition between high-side switch and low-side switch, the body diode of the low- side power mosfet conducts the inductor current. the forward voltage of this body diode is high. an optional schottky diode may be paralleled between the sw pin and gnd pin to improve overall efficiency. table 2 lists example schottky diodes and their manufacturers. table 2?diode selection guide part number voltage/current rating vendor b130 30v, 1a diodes, inc. sk13 30v, 1a diodes, inc. mbrs130 30v, 1a international rectifier input capacitor the input current to the step-down converter is discontinuous, therefore a capacitor is required to supply the ac current to the step-down converter while maintaining the dc input voltage. use low esr capacitors for the best performance. ceramic capacitors are preferred, but tantalum or low-esr electrolytic capacitors may also suffice. choose x5r or x7r dielectrics when using ceramic capacitors.
mp28315 ? 3a, 16v, 340khz synchrono us rectified step -down converter mp28315 rev. 1.2 www.monolithicpower.com 7 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. tm since the input capacitor (c1) absorbs the input switching current it requires an adequate ripple current rating. the rms current in the input capacitor can be estimated by: ? ? ? ? ? ? ? ? ? = in out in out load 1 c v v 1 v v i i the worst-case condition occurs at v in = 2v out , where i c1 = i load /2. for simplification, choose the input capacitor whose rms current rating greater than half of the maximum load current. the input capacitor can be electrolytic, tantalum or ceramic. when using electrolytic or tantalum capacitors, a small, high quality ceramic capacitor, i.e. 0.1 f, should be placed as close to the ic as possible. when using ceramic capacitors, make sure that they have enough capacitance to provide sufficient charge to prevent excessive voltage ripple at input. the input voltage ripple for low esr capacitors can be estimated by: ? ? ? ? ? ? ? ? ? = in out in out s load in v v 1 v v f 1 c i v where c1 is the input capacitance value. output capacitor the output capacitor (c2) is required to maintain the dc output voltage. ceramic, tantalum, or low esr electrolytic capacitors are recommended. low esr capacitors are preferred to keep the output voltage ripple low. the output voltage ripple can be estimated by: ? ? ? ? ? ? ? ? + ? ? ? ? ? ? ? ? ? = 2 c f 8 1 r v v 1 l f v v s esr in out s out out where c2 is the output capacitance value and r esr is the equivalent series resistance (esr) value of the output capacitor. in the case of ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance. the output voltage ripple is mainly caused by the capacitance. for simplification, the output voltage ripple can be estimated by: ? ? ? ? ? ? ? ? ? = in out 2 s out out v v 1 2 c l f 8 v v in the case of tantalum or electrolytic capacitors, the esr dominates the impedance at the switching frequency. for simplification, the output ripple can be approximated to: esr in out s out out r v v 1 l f v v ? ? ? ? ? ? ? ? ? = the characteristics of the output capacitor also affect the stability of the regulation system. the mp28315 can be optimized for a wide range of capacitance and esr values. compensation components mp28315 employs current mode control for easy compensation and fast transient response. the system stability and transient response are controlled through the comp pin. comp pin is the output of the internal transconductance error amplifier. a series capacitor-resistor combination sets a pole-zero combination to control the characteristics of the control system. the dc gain of the voltage feedback loop is given by: out fb ea cs load vdc v v a g r a = where v fb is the feedback voltage, 0.925v; a vea is the error amplifier voltage gain; g cs is the current sense transconductance and r load is the load resistor value. the system has two poles of importance. one is due to the compensation capacitor (c3) and the output resistor of the error amplifier, and the other is due to the output capacitor and the load resistor. these poles are located at: vea ea 1 p a 3 c 2 g f = load 2 p r 2 c 2 1 f = where g ea is the error amplifier transconductance.
mp28315 ? 3a, 16v, 340khz synchrono us rectified step -down converter mp28315 rev. 1.2 www.monolithicpower.com 8 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. tm the system has one zero of importance, due to the compensation capacitor (c3) and the compensation resistor (r3). this zero is located at: 3 r 3 c 2 1 f 1 z = the system may have another zero of importance, if the output capacitor has a large capacitance and/or a high esr value. the zero, due to the esr and capacitance of the output capacitor, is located at: esr esr r 2 c 2 1 f = in this case, a third pole set by the compensation capacitor (c6) and the compensation resistor (r3) is used to compensate the effect of the esr zero on the loop gain. this pole is located at: 3 r 6 c 2 1 f 3 p = the goal of compensation design is to shape the converter transfer function to get a desired loop gain. the system crossover frequency where the feedback loop has the unity gain is important. lower crossover frequencies result in slower line and load transient responses, while higher crossover frequencies could cause system instability. a good rule of thumb is to set the crossover frequency below one-tenth of the switching frequency. to optimize the compensation components, the following procedure can be used. 1. choose the compensation resistor (r3) to set the desired crossover frequency. determine the r3 value by the following equation: fb out cs ea s fb out cs ea c v v g g f 1 . 0 2 c 2 v v g g f 2 c 2 3 r < = where f c is the desired crossover frequency which is typically below one tenth of the switching frequency. 2. choose the compensation capacitor (c3) to achieve the desired phase margin. for applications with typical inductor values, setting the compensation zero, f z1 , below one-forth of the crossover frequency provides sufficient phase margin. determine the c3 value by the following equation: c f 3 r 2 4 3 c > where r3 is the compensation resistor. 3. determine if the second compensation capacitor (c6) is required. it is required if the esr zero of the output capacitor is located at less than half of the switching frequency, or the following relationship is valid: 2 f r 2 c 2 1 s esr < if this is the case, then add the second compensation capacitor (c6) to set the pole f p3 at the location of the esr zero. determine the c6 value by the equation: 3 r r 2 c 6 c esr = external bootstrap diode it is recommended that an external bootstrap diode be added when the system has a 5v fixed input or the power supply generates a 5v output. this helps improve the efficiency of the regulator. the bootstrap diode can be a low cost one such as in4148 or bat54. mp28315 sw bs 10nf 5v mp28315_f02 figure 2?external bootstrap diode this diode is also recommended for high duty cycle operation (when in out v v >65%) and high output voltage (v out >12v) applications.
mp28315 ? 3a, 16v, 340khz synchrono us rectified step -down converter mp28315 rev. 1.2 www.monolithicpower.com 9 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. tm typical application circuits input 5v to 16v output 2.5v 3a c3 4.7nf c6 (optional) d1 b130 (optional) c5 10nf mp28315 bs in fb sw ss gnd comp en mp28315_f03 1 2 3 5 6 4 8 7 figure 3?mp28315 with avx 47 f, 6.3v ceramic output capacitor input 5v to 16v output 2.5v 3a c3 4.7nf c6 200pf d1 b130 (optional) c5 10nf mp28315 bs in fb sw ss gnd comp en mp28315_f04 1 2 3 5 6 4 8 7 figure 4?mp28315 with panasonic 47 f, 6.3v solid polymer output capacitor
mp28315 ? 3a, 16v, 340khz synchrono us rectified step -down converter mp28315 rev. 1.2 www.monolithicpower.com 10 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. tm input 6v output 5v 3a c3 3.3nf c6 100pf d1 b130 (optional) c5 10nf b130 mp28315 bs in fb sw ss gnd comp en mp28315_f05 1 2 3 5 6 4 8 7 figure 5?mp28315 application circuit with v in = 6v and v o = 5v
mp28315 ? 3a, 16v, 340khz synchrono us rectified step -down converter notice: the information in this document is subject to change wi thout notice. please contact m ps for current specifications. users should warrant and guarantee that third party intellectual property rights ar e not infringed upon when integrating mps products into any application. mps will not assume any legal responsibility for any said applications. mp28315 rev. 1.2 www.monolithicpower.com 11 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. tm package information soic8n (exposed pad) note: 1) control dimension is in inches. dimension in bracket is millimeters. 2) exposed pad option (n-package) ; 2.31mm -2.79mm x 2.79mm - 3.81mm. recommend solder board area: 2.80mm x 3.82mm = 10.7mm 2 (16.6 mil 2 ) 3) the length of the package does not include mold flash. mold flash shall not exceed 0.006in. (0.15mm) per side. with the mold flash included, over-all length of the package is 0.2087in. (5.3mm) max. 4) the width of the package does not include mold flash. mold flash shall not exceed 0.10in. (0.25mm) per side. with the mold flash included, over-all width of the package is 0.177in. (4.5mm) max. 0.016(0.410) 0.050(1.270) 0 o -8 o detail "a" 0.011(0.280) 0.020(0.508) x 45 o see detail "a" 0.0075(0.191) 0.0098(0.249) 0.229(5.820) 0.244(6.200) land pattern .028 .050 0.140 (3.55mm) 0.200 (5.07 mm) 0.060 0.150(3.810) 0.157(4.000) pin 1 ident. 0.050(1.270)bsc 0.013(0.330) 0.020(0.508) note 2 note 4 seating plane 0.001(0.030) 0.004(0.101) 0.189(4.800) 0.197(5.000) 0.053(1.350) 0.068(1.730) 0.049(1.250) 0.060(1.524) note 3

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