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mp28373 3a, 28v, 1.4mhz step-down converter mp28373 rev. 1.0 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 description the mp28373 is a 1.4mhz step-down regulator with a built-in power mosfet. it achieves 3a continuous output current over a wide input supply range with excellent load and line regulation. current mode operation provides fast transient response and eases loop stabilization. fault condition protection includes cycle-by- cycle current limiting and thermal shutdown. adjustable soft-start reduces the stress on the input source at turn-on. in shutdown mode, the regulator draws 20 a of supply current. the mp28373 is available in an 8-pin soic package with an exposed pad, and requires a minimum number of readily available external components to complete a 3a step-down dc to dc converter solution. evaluation board reference board number dimensions ev28373dn-00a 2.0? x 1.9? x 0.4? features ? 3a continuous output current, 4a peak output current ? programmable soft-start ? 100m ? internal power mosfet switch ? stable with low esr output ceramic capacitors ? up to 91% efficiency ? 20 a shutdown mode ? fixed 1.4mhz frequency ? thermal shutdown ? cycle-by-cycle over current protection ? wide 4.75v to 28v operating input range ? output is adjustable from 0.92v to 21v ? under voltage lockout applications ? distributed power systems ? battery chargers ? pre-regulator for linear regulators ?mps? and ?the future of analog ic technology? are registered trademarks of monolithic power systems, inc. typical application input output 3.3v 3a mp28373 bs in fb sw ss gnd comp en 1 3 5 6 4 8 7 2 c6 open open = automatic startup c4 c3 r3 c5 c1 ceramic c2 ceramic r2 r1 l1 d1 efficiency curve 100 90 80 70 60 efficiency (%) 0 0.5 1.0 1.5 2.0 2.5 3.0 load current (a) v in =12v v out =3.3v v out =5v
mp28373 ? 3a, 28v, 1.4mhz step-down converter mp28373 rev. 1.0 www.monolithicpower.com 2 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. package reference bs in sw gnd ss en comp fb 1 2 3 4 8 7 6 5 top view exposed pad connect to pin 4 part number* package temperature MP28373DN soic8n ?40 c to +85 c * for tape & reel, add suffix ?z (eg. MP28373DN?z) for rohs compliant packaging, add suffix ?lf (eg. MP28373DN?lf?z) absolute maxi mum ratings (1) supply voltage v in ....................... ?0.3v to +30v switch voltage v sw .............. ?0.5v 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...............................150 c lead temperature ....................................260 c storage temperature .............?65 c to +150 c recommended operating conditions (2) input voltage v in ............................ 4.75v to 28v ambient operating temp ..........?40 c to +85 c thermal resistance (3) ja jc soic8n .................................. 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 = +25 c, unless otherwise noted. parameters symbol condition min typ max units shutdown supply current v en = 0v 20 30 a supply current v en = 3v, v fb =1.4v 1.3 1.5 ma feedback voltage v fb 4.75v v in 28v, v comp < 2v 0.901 0.920 0.939 v error amplifier voltage gain a vea 400 v/v error amplifier transconductance g ea i comp = 10 a 330 530 730 a/v high-side switch-on resistance r ds(on)1 100 m ? low-side switch-on resistance r ds(on)2 10 ? high-side switch leakage current v en = 0v, v sw = 0v 0.1 10 a short circuit current limit 6.5 a current sense to comp transconductance g cs 6.0 a/v oscillation frequency f s 1.4 mhz short circuit oscillation frequency v fb = 0v 180 khz maximum duty cycle d max v fb = 0.8v 65 % minimum on time t on 130 ns en threshold voltage 0.9 1.2 1.5 v enable pull up current v en = 0v 0.9 1.6 2.3 a under voltage lockout threshold rising 2.3 2.6 2.9 v under voltage lockout threshold hysteresis 210 mv soft-start period c ss = 0.1f 10 ms thermal shutdown 160 c
mp28373 ? 3a, 28v, 1.4mhz step-down converter mp28373 rev. 1.0 www.monolithicpower.com 3 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. 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 10nf 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 4.75v to 28v power source. by pass 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 the exposed pad on backside to pin 4. 5 fb feedback input. fb senses the output voltage to regulate said voltage. drive fb with a resistive voltage divider from the output volt age. the feedback threshold is 0.92v. 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 7 en enable input. en is a digital input that turns t he regulator on or off. drive en higher than 2.9v to turn on the regulator, lower than 0.9v to turn it off. for automatic startup, leave en unconnected. 8 ss soft-start control input. ss controls the soft star t period. connect a capacitor from ss to gnd to set the soft-start period. a 0.1 f capacitor sets the soft-start period to 10ms. electrical characteristics v in = 12v, c1 = 10f, c2 = 47f, l = 4.7h and t a = +25 c, unless otherwise noted. 8.0 7.5 7.0 6.5 6.0 5.5 5.0 peak current (a) 0 20406080 duty cycle (%) peak current vs duty cycle 0.95 0.94 0.93 0.92 0.91 0.90 0.89 feedback voltage (v) -40 0 -20 40 20 80 60 120 100 temperature ( o c) feedback voltage vs die temperature 90 80 70 60 50 40 30 efficiency (%) 0 0.5 1.0 2.0 3.0 1.5 2.5 3.5 load current (a) efficiency vs load current v out = 5v v out = 2.5v v in = 24v
mp28373 ? 3a, 28v, 1.4mhz step-down converter mp28373 rev. 1.0 www.monolithicpower.com 4 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. electrical characteristics (continued) v in = 12v, c1 = 10f, c2 = 47f, l = 4.7h and t a = +25 c, unless otherwise noted. v sw 10v/div. v out ac coupled 20mv/div. i inductor 2a/div. 400ns/div. steady state operation v out = 1.8v, i out = 1.5a 400ns/div. steady state operation v out = 1.8v, i out = 3a 4ms/div. startup through enable v out = 3.3v, i out = 1.5a (resistance load) 4ms/div. startup through enable v out = 3.3v, i out = 3a (resistance load) shutdown through enable v out = 3.3v, i out = 1.5a (resistance load) shutdown through enable v out = 3.3v, i out = 3a (resistance load) v out ac coupled 50mv/div. i inductor 1a/div. i load 1a/div. v sw 10v/div. v out ac coupled 20mv/div. i inductor 2a/div. v out 1v/div. v en 5v/div. i inductor 2a/div. v sw 10v/div. v out 1v/div. v en 5v/div. i inductor 2a/div. v sw 10v/div. v out 1v/div. v en 5v/div. i inductor 2a/div. v sw 10v/div. v out 1v/div. v en 5v/div. i inductor 2a/div. v sw 10v/div.
mp28373 ? 3a, 28v, 1.4mhz step-down converter mp28373 rev. 1.0 www.monolithicpower.com 5 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. operation the mp28373 is a current-mode step-down regulator. it regulates input voltages from 4.75v to 28v down to an output voltage as low as 0.92v, and is able to supply up to 3a of load current. the mp28373 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 error amplifier. the output current of the transconductance error amplifier is presented at comp where a network compensates the regulation control system. the voltage at comp is compared to the switch current measured internally to control the output voltage. the converter uses an internal n-channel mosfet switch to step-down the input voltage to the regulated output voltage. since the mosfet requires a gate voltage greater than the input voltage, a boost capacitor connected between sw and bs drives the gate. the capacitor is internally charged while sw is low. an internal 10 ? switch from sw to gnd is used to insure that sw is pulled to gnd when sw is low to fully charge the bs . capacitor. lockout comparator error amplifier frequency foldback comparator internal regulators 1.8v slope comp clk current comparator current sense amplifier shutdown comparator comp 6 in 2 en 7 gnd 4 ss 8 oscillator 180khz/ 1.4mhz s r q sw 3 bs 1 5v + q 1.2v + + 2.60v/ 2.39v + 0.92v 0.6v + + fb 5 -- -- -- -- -- -- figure 1?functional block diagram
mp28373 ? 3a, 28v, 1.4mhz step-down converter mp28373 rev. 1.0 www.monolithicpower.com 6 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. application information component selection (refer to figure 3) 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 + = where v fb is the feedback voltage and v out is the output voltage. thus the output voltage is: 2 r 2 r 1 r 92 . 0 v out + = a typical value for r2 can be as high as 100k ? , but a typical value is 10k ? . using that value, r1 is determined by: ) k )( 92 . 0 v ( 87 . 10 1 r out ? = 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 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 for determining the inductance to use is to allow the peak-to-peak ripple current in the inductor 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 1 l where v in is the input voltage, f s is the 1.4mhz 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. table 1 lists a number of suitable inductors from various manufacturers. the choice of which style inductor to use mainly depends on the price vs. size requirements and any emi requirement. table 1?inductor selection guide package dimensions (mm) vendor/ model core type core material wl h sumida cr75 open ferrite 7.0 7.8 5.5 cdh74 open ferrite 7.3 8.0 5.2 cdrh5d28 shielded ferrite 5.5 5.7 5.5 cdrh5d28 shielded ferrite 5.5 5.7 5.5 cdrh6d28 shielded ferrite 6.7 6.7 3.0 cdrh104r shielded ferrite 10.1 10.0 3.0 toko d53lc type a shielded ferrite 5.0 5.0 3.0 d75c shielded ferrite 7.6 7.6 5.1 d104c shielded ferrite 10.0 10.0 4.3 d10fl open ferrite 9.7 1.5 4.0 coilcraft do3308 open ferrite 9.4 13.0 3.0 do3316 open ferrite 9.4 13.0 5.1
mp28373 ? 3a, 28v, 1.4mhz step-down converter mp28373 rev. 1.0 www.monolithicpower.com 7 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. output rectifier diode the output rectifier diode supplies the current to the inductor when the high-side switch is off. to reduce losses due to the diode forward voltage and recovery times, use a schottky diode. choose a diode whose maximum reverse voltage rating is greater than the maximum input voltage, and whose current rating is greater than the maximum load current. table 2 lists example schottky diodes and manufacturers. table 2?diode selection guide diode v oltage/current rating manufacture sk33 30v, 3a diodes inc. sk34 40v, 3a diodes inc. b330 30v, 3a diodes inc. b340 40v, 3a diodes inc. mbrs330 30v, 3a on semiconductor mbrs340 40v, 3a on semiconductor 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. 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: 2 i i load 1 c = 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 caused by capacitance can be estimated by: ? ? ? ? ? ? ? ? ? = in out in out s load in v v 1 v v 1 c f i v 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 l is the inductor 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 mp28373 can be optimized for a wide range of capacitance and esr values.
mp28373 ? 3a, 28v, 1.4mhz step-down converter mp28373 rev. 1.0 www.monolithicpower.com 8 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. compensation components mp28373 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 vea cs load vdc v v a g r a = where 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 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, 530 a/v. 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 unstable. a good rule of thumb is to set the crossover frequency to approximately one-tenth of the switching frequency or lower. the switching frequency for the mp28373 is 1.4mhz, so the desired crossover frequency is equal to or less than 140khz. table 3 lists the typical values of compensation components for some standard output voltages with various output capacitors and inductors. the values of the compensation components have been optimized for fast transient responses and good stability at given conditions. table 3?compensation values for typical output voltage/capacitor combinations v out (v) l (h) c2 (f, ceramic) r3 (k ? ) c3 (nf) c6 1.8 2.2 47 7.5 3.3 none 2.5 2.2 - 4.7 47 10 4.7 none 3.3 2.2 - 4.7 47 15 5.6 none 5 4.7 ? 6.8 2 x 22 20 4.7 none 12 6.8 - 10 2 x 22 44.2 2.2 none
mp28373 ? 3a, 28v, 1.4mhz step-down converter mp28373 rev. 1.0 www.monolithicpower.com 9 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. to optimize the compensation components for conditions not listed in table 3, 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 c v v g g f 2 c 2 3 r = where f c is the desired crossover 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 > 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 1.4mhz 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. mp28373 sw bs 10nf 5v 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.
mp28373 ? 3a, 28v, 1.4mhz step-down converter mp28373 rev. 1.0 www.monolithicpower.com 10 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. typical application circuits input 4.75v to 28v output 2.5v 3a c3 4.7nf c6 open d1 b330a c5 10nf mp28373 bs in fb sw ss gnd comp en 1 3 5 6 4 8 7 2 open = automatic startup figure 3?2.5v output typical application schematic input 4.75v to 28v output 3.3v 3a c3 5.6nf c6 open d1 b330a c5 10nf mp28373 bs in fb sw ss gnd comp en 1 3 5 6 4 8 7 2 open = automatic startup figure 4?3.3v output typical application schematic
mp28373 ? 3a, 28v, 1.4mhz step-down converter notice: the information in this document is subject to change wi thout notice. users should warra nt and guarantee that third party intellectual property rights are not infringed upon w hen integrating mps products into any application. mps will not assume any legal responsibility for any said applications. mp28373 rev. 1.0 www.monolithicpower.com 11 12/12/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. package information soic8n (exposed pad) see detail "a" 0.0075(0.19) 0.0098(0.25) 0.050(1.27) bsc 0.013(0.33) 0.020(0.51) seating plane 0.000(0.00) 0.006(0.15) 0.051(1.30) 0.067(1.70) top view front view side view bottom view note: 1) control dimension is in inches . dimension in bracket is in millimeters . 2) package length does not include mold flash , protrusions or gate burrs . 3) package width does not include interlead flash or protrusions. 4) lead coplanarity (bottom of leads after forming ) shall be 0.004" inches max. 5) drawing conforms to jedec ms-012, variation ba. 6) drawing is not to scale. 0.089(2.26) 0.101(2.56) 0.124(3.15) 0.136(3.45) recommended land pattern 0.213(5.40) 0.063(1.60) 0.050(1.27) 0.024(0.61) 0.103(2.62) 0.138(3.51) 0.150(3.80) 0.157(4.00) pin 1 id 0.189(4.80) 0.197(5.00) 0.228(5.80) 0.244(6.20) 14 85 0.016(0.41) 0.050(1.27) 0 o -8 o detail "a" 0.010(0.25) 0.020(0.50) x 45 o 0.010(0.25) bsc gauge plane

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