mp2355 3a, 23v, 380khz step-down converter mp2355 rev. 1.5 www.monolithicpower.com 1 5/1/2006 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2006 mps. all rights reserved. the future of analog ic technology tm tm description the mp2355 is a step-down regulator with a built in internal 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 20a of supply current. the mp2355 uses 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 EV2355DN-00A 2.0?x x 1.3?y x 0.5?z features ? programmable soft-start ? 100m ? internal power mosfet switch ? stable with low esr output ceramic capacitors ? up to 95% efficiency ? 20a shutdown mode ? 3a output current ? wide 4.75v to 23v operating input range ? fixed 380khz frequency ? thermal shutdown ? cycle-by-cycle over current protection ? under voltage lockout applications ? distributed power systems ? battery chargers ? pre-regulator for linear regulators ?mps? and ?the future of analog ic technology? are trademarks of monolithic power systems, inc. typical application mp2355 lx vin bst fb 7 5 32 4 6 1 8 ss comp gnd run output 3.3v / 3a input 4.75v to 23v open automatic startup mp2355_tac_s01 10nf 4.7nf 10nf d1 b330a efficiency (%) 95 90 85 80 75 70 65 60 load current (ma) mp2355_ec01 efficiency vs load current 0 1500 1000 500 2000 3500 3000 2500 v out =2.5v v out =3.3v v out =5.0v
mp2355 ? 3a, 23v, 380khz step-down converter mp2355 rev. 1.5 www.monolithicpower.com 2 5/1/2006 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2006 mps. all rights reserved. tm package reference ss bst vin lx run comp fb gnd 1 2 3 4 8 7 6 5 top view mp2355_pd01-soic8n exposed pad on backside connect to pin 5 part number* package temperature mp2355dn soic8n (exposed pad) ?40 c to +85 c * for tape & reel, add suffix ?z (eg. mp2355dn?z) for rohs compliant packaging, add suffix ?lf (eg. mp2355dn ?lf?z) absolute maxi mum ratings (1) supply voltage v in ....................... ?0.3v to +25v switch voltage v lx ....................... ?0.3v to +26v boost voltage v bst ..........v lx ? 0.3v to v lx + 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 23v operating temperature .............?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. parameter symbol condition min typ max units shutdown supply current v run = 0v 20 30 a supply current v run = 2.8, v fb = 1.5v 1.0 1.2 ma feedback voltage v fb 4.75v v in 23v, v comp < 2v 1.194 1.222 1.250 v error amplifier voltage gain a vea 400 v/v error amplifier transconductance g ea ? i comp = 10a 500 800 1120 a/v high-side switch-on resistance r ds(on)1 95 m ? low-side switch-on resistance r ds(on)2 10 ? high-side switch leakage current v run = 0v, v lx = 0v 0 10 a current limit (4) 3.7 4.3 a current sense to comp transconductance g cs 3.8 a/v oscillation frequency f s 330 380 430 khz short circuit oscillation frequency v fb = 0v 20 35 50 khz maximum duty cycle d max v fb = 1.0v 90 % minimum duty cycle d min v fb = 1.5v 0 % en shutdown threshold voltage 0.9 1.2 1.5 v
mp2355 ? 3a, 23v, 380khz step-down converter mp2355 rev. 1.5 www.monolithicpower.com 3 5/1/2006 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2006 mps. all rights reserved. tm electrical characteristics (continued) v in = 12v, t a = +25 c, unless otherwise noted. parameter symbol condition min typ max units enable pull up current v run = 0v 1.1 1.8 2.5 a en uvlo threshold v en rising 2.37 2.54 2.71 v en uvlo threshold hysteresis 210 mv soft-start period c ss = 0.1f 10 ms thermal shutdown 150 c note: 4) equivalent output current = 1.5a 50% duty cycle 2.0a 50% duty cycle assumes ripple current = 30% of load current. slope compensation changes current limit above 40% duty cycle. typical performanc e characteristics circuit of figure 2, v in = 12v, v o = 3.3v, l1 = 15h, c1 = 10f, c2 = 22f, t a = +25 c, unless otherwise noted. v sw 10v/div. v in, ac 200mv/div. v o, ac 20mv/div. i l 1a/div. v sw 10v/div. v in, ac 20mv/div. v o, ac 20mv/div. i l 1a/div. mp2355-tpc01 heavy load operation 3a load mp2355-tpc02 light load operation no load v en 5v/div. v out 1v/div. i l 1a/div. mp2355-tpc03 startup from shutdown no c4 3a resistive load v en 5v/div. v out 1v/div. i l 1a/div. startup from shutdown c4 = 10nf 3a resistive load v en 5v/div. v out 1v/div. i l 1a/div. startup from shutdown c4 = 10nf no load mp2355-tpc04 mp2355-tpc05
mp2355 ? 3a, 23v, 380khz step-down converter mp2355 rev. 1.5 www.monolithicpower.com 4 5/1/2006 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2006 mps. all rights reserved. tm typical performanc e characteristics (continued) circuit of figure 2, v in = 12v, v o = 3.3v, l1 = 15h, c1 = 10f, c2 = 22f, t a = +25 c, unless otherwise noted. v o, ac 200mv/div. i l 1a/div. i load 1a/div. mp2355-tpc06 load transient v out 2v/div. i l 2a/div. v out 2v/div. i l 2a/div. mp2355-tpc07 short circuit protection mp2355-tpc08 short circuit recovery pin functions pin # name description 1 ss soft-start control input. ss cont rols the soft-start period. co nnect a capacitor from ss to gnd to set the soft-start period. a 0.1f capa citor sets the soft-start period to 10ms. to disable the soft-start feature, leave ss unconnected. 2 bst high-side gate drive boost input. bst supp lies the drive for the high-side n-channel mosfet switch. connect a 10nf or greater capacitor from lx to bst to power the high side switch. 3 vin power input. vin supplies the power to the ic, as well as the step-down converter switches. drive vin with a 4.75v to 23v power source . bypass vin to gnd with a suitably large capacitor to eliminate noise on the input to the ic. see input capacitor 4 lx power switching output. lx is the switching node that supplies power to the output. connect the output lc filter from lx to the output load. note that a capacitor is required from lx to bst to power the high-side switch. 5 gnd ground. (note: connect the exposed pad on backside to pin 5.) 6 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 1.222v. see setting the output voltage 7 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 8 run enable/uvlo. a voltage greater than 2.71v enabl es operation. for complete low current shutdown the en pin voltage needs to be less than 900mv.
mp2355 ? 3a, 23v, 380khz step-down converter mp2355 rev. 1.5 www.monolithicpower.com 5 5/1/2006 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2006 mps. all rights reserved. tm operation the mp2355 is a current-mode step-down regulator. it regulates input voltages from 4.75v to 23v down to an output voltage as low as 1.222v, and is able to supply up to 3a of load current. the mp2355 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 lx and bst drives the gate. the capacitor is internally charged while lx is low. an internal 10 ? switch from lx to gnd is used to insure that lx is pulled to gnd when lx is low to fully charge the bst.capacitor. mp2355_bd01 lockout comparator error amplifier frequency foldback comparator internal regulators 1.8v slope comp clk current comparator current sense amplifier shutdown comparator comp 7 ss 1 vin 3 run 8 gnd 5 oscillator 42/380khz s r q lx 4 bst 2 5v + q 0.7v + + 2.37v/ 2.62v + 1.222v 0.7v + + fb 6 -- -- -- -- -- -- figure 1?functional block diagram
mp2355 ? 3a, 23v, 380khz step-down converter mp2355 rev. 1.5 www.monolithicpower.com 6 5/1/2006 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2006 mps. all rights reserved. tm applications information mp2355_tac_f02 mp2355 lx vin bst 32 4 6 7 5 1 8 fb ss comp gnd run output 3.3v / 3a input 4.75v to 23v open automatic startup c6 open c4 10nf c3 4.7nf c5 10nf d1 b330a figure 2?mp2355 with murata 22 f, 10v ceramic output capacitor 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 22 . 1 v out + = where v fb is the feedback voltage and v out is the output voltage. 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 )( 22 . 1 v ( 18 . 8 1 r out ? ? = for example, for a 3.3v output voltage, r2 is 10k ? , and r1 is 17k ? . 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 l where v in is the input voltage, f s is the 380khz 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.
mp2355 ? 3a, 23v, 380khz step-down converter mp2355 rev. 1.5 www.monolithicpower.com 7 5/1/2006 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2006 mps. all rights reserved. tm table 1?inductor selection guide package dimensions (mm) vendor/ model core type core material w l 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 sumida (continued) 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 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 which has a maximum reverse voltage rating is greater than the maximum input voltage, and who?s 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. 0.1f, 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
mp2355 ? 3a, 23v, 380khz step-down converter mp2355 rev. 1.5 www.monolithicpower.com 8 5/1/2006 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2006 mps. all rights reserved. tm output capacitor the output capacitor 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, 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 mp2355 can be optimized for a wide range of capacitance and esr values. compensation components mp2355 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, 400v/v; g cs is the current sense transconductance, 3.8a/v; 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, 800a/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 (as shown in figure 2), 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 =
mp2355 ? 3a, 23v, 380khz step-down converter mp2355 rev. 1.5 www.monolithicpower.com 9 5/1/2006 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2006 mps. all rights reserved. tm 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. switching frequency for the mp2355 is 380khz, so the desired crossover frequency is around 38khz. 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 l1 c2 r3 c3 c6 2.5v 10h min. 22f ceramic 3.9k ? 5.6nf none 3.3v 15h min. 22f ceramic 4.7k ? 4.7nf none 5v 15h min. 22f ceramic 7.5k ? 2.7nf none 12v 22h min. 22f ceramic 15k ? 1.5nf none 2.5v 10h min. 560f al. 30m ? esr 100k ? 1nf 150pf 3.3v 15h min. 560f al 30m ? esr 120k ? 1nf 120pf 5v 15h min. 470f al. 30m ? esr 150k ? 1nf 82pf 12v 22h min. 220f al. 30m ? esr 169k ? 1nf 39pf to optimize the compensation components for conditions not listed in table 2, 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 = 2) choose the compensation capacitor (c3) to achieve the desired phase margin. for applications with typical inductor values, setting the compensation zero, f z1 , to less than 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 value and f c is the desired crossover frequency, 38khz. 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 380khz 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. mp2355 sw bs 10nf 5v mp2355_f03 figure 3?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.
mp2355 ? 3a, 23v, 380khz step-down converter notice: the information in this document is subject to change wi thout notice. please contact mps 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. mp2355 rev. 1.5 www.monolithicpower.com 10 5/1/2006 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2006 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 sh all 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 flas h. 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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