mp2354 2a, 23v, 380khz step-down converter mp2354 rev. 1.4 www.monolithicpower.com 1 4/19/2011 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2011 mps. all rights reserved. the future of analog ic technology description the mp2354 is a monolithic step down switch mode converter with a built in internal power mosfet. it achieves 2a 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. in shutdown mode the regulator draws 20a of supply current. evaluation board reference board number dimensions ev2354ds-00a 2.3?x x 1.4?y x 0.5?z features ? 0.18 ? internal power mosfet switch ? stable with low esr output ceramic capacitors ? up to 95% efficiency ? 2a output current ? wide 4.75v to 23v operating input range ? fixed 380khz frequency ? thermal shutdown ? cycle-by-cycle over current protection ? programmable under voltage lockout ? frequency synchronization input ? operating temperature: ?40 c to +85 c ? available in an 8-pin so package 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 mp2354 lx vin bst 32 fb 4 6 sync comp gnd 7 5 run 1 8 output 3.3v / 2a input 4.75v to 23v open if not used open automatic startup mp2354_tac_s01 3.3nf b230a 10nf efficiency (%) 95 90 85 80 75 70 65 60 output current (a) mp2354_tac_ec01 efficiency vs output current 0 1.0 0.5 1.5 2.0 2.5 5.0v 3.3v 2.5v
mp2354 ? 2a, 23v, 380khz step-down converter mp2354 rev. 1.4 www.monolithicpower.com 2 4/19/2011 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2011 mps. all rights reserved. package reference sync bst vin lx run comp fb gnd 1 2 3 4 8 7 6 5 top view mp2354_pd01-soic8 part number* package temperature mp2354ds soic8 ?40 c to +85 c * for tape & reel, add suffix ?z (eg. mp2354ds?z) for lead free, add suffix ?lf (eg. mp2354ds?lf?z) absolute maxi mum ratings (1) supply voltage (v in ) .................................... 25v switch voltage (v lx ) ....................... ?1v to +26v bootstrap voltage (v bst ) ....................... v lx + 6v feedback voltage (v fb ) ................... ?0.3 to +6v enable/uvlo voltage (v run ) ........... ?0.3 to +6v comp voltage (v comp ) ..................... ?0.3 to +6v sync voltage (v sync ) ....................... ?0.3 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 soic8 .................................... 105 ..... 50 ... 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 feedback voltage v fb 4.75v ? lower switch on resistance r ds ( on ) 2 10 ? upper switch leakage v run = 0v, v lx = 0v 0 10 a current limit (4) 2.7 3.4 a current sense transconductance output current to comp pin voltage g cs 1.95 a/v error amplifier voltage gain a vea 400 v/v error amplifier transconductance g ea ? i c = 10a 500 700 1000 a/v oscillator frequency f s 342 380 418 khz short circuit frequency v fb = 0v 35 khz sync frequency sync drive 0v to 2.7v 445 600 khz maximum duty cycle d max v fb = 1.0v 90 % minimum duty cycle d min v fb = 1.5v 0 %
mp2354 ? 2a, 23v, 380khz step-down converter mp2354 rev. 1.4 www.monolithicpower.com 3 4/19/2011 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2011 mps. all rights reserved. electrical characteristics (continued) v in = 12v, t a = +25 c, unless otherwise noted. parameter symbol condition min typ max units run shutdown threshold i cc > 100a 0.7 1.0 1.3 v run pull up current v run = 0v 1.0 1.3 a en uvlo threshold rising v en rising 2.37 2.5 2.62 v en uvlo threshold hysteresis 210 mv supply current (shutdown) v run 2.8v, v fb = 1.5v 1.0 1.2 ma thermal shutdown 155 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. pin functions pin # name description 1 sync synchronization input. this pin is used to synchr onize the internal osc illator frequency to an external source. there is an internal 11k ? pull down resistor to gnd, therefore leave sync unconnected if unused. 2 bst bootstrap (c5). this capacitor is needed to dr ive the power switch?s gate above the supply voltage. it is connected between lx and bst pins to form a floating supply across the power switch driver. the voltage across c5 is about 5v and is supplied by the internal +5v supply when the lx pin voltage is low. 3 vin supply voltage. the mp2354 operates from a +4.75v to +23v unregulated input. c1 is needed to prevent large voltage spikes from appearing at the input. 4 lx switch. this connects the inductor to either vin through m1 or to gnd through m2. 5 gnd ground. this pin is the voltage reference for t he regulated output voltage. for this reason care must be taken in its layout. this node should be pl aced outside of the d1 to c1 ground path to prevent switching current spikes from inducing voltage noise into the part. 6 fb feedback. an external resistor divider from t he output to gnd, tapped to the fb pin sets the output voltage. to prevent current limit run aw ay during a short circuit fault condition the frequency foldback comparator lowers the osc illator frequency when the fb voltage is below 700mv. 7 comp compensation. this node is the output of the tr ansconductance error amplifier and the input to the current comparator. frequency compensation is done at this node by connecting a series r-c to ground. see the compensation section for exact details. 8 run enable/uvlo. a voltage greater than 2.62v enabl es operation. leave run unconnected for automatic startup. an under voltage lockout (uvlo) function can be implemented by the addition of a resistor divider from v in to gnd. for complete low cu rrent shutdown it?s the run pin voltage needs to be less than 700mv.
mp2354 ? 2a, 23v, 380khz step-down converter mp2354 rev. 1.4 www.monolithicpower.com 4 4/19/2011 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2011 mps. all rights reserved. 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 o, ac 50mv/div. v in, ac 200mv/div. v lx 10v/div. i l 1a/div. v o, ac 20mv/div. v in, ac 20mv/div. v lx 10v/div. i l 1a/div. mp2354-tpc01 heavy load operation 2a load mp2354-tpc02 light load operation no load v run 2v/div. v out 1v/div. i l 1a/div. mp2354-tpc03 v o, ac 200mv/div. i load 1a/div. i l 1a/div. mp2354-tpc04 load transient v out 2v/div. i l 1a/div. v out 2v/div. i l 1a/div. mp2354-tpc05 short circuit protection startup from shutdown 2a resistive load mp2354-tpc06 short circuit recovery
mp2354 ? 2a, 23v, 380khz step-down converter mp2354 rev. 1.4 www.monolithicpower.com 5 4/19/2011 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2011 mps. all rights reserved. operation the mp2354 is a current mode regulator. the comp pin voltage is proportional to the peak inductor current. at the beginning of a cycle: the upper transistor m1 is off; the lower transistor m2 is on (refer to figure 1), the comp pin voltage is higher than the current sense amplifier output; and the current comparator?s output is low. the rising edge of the 380khz clk signal sets the rs flip-flop. its output turns off m2 and turns on m1 thus connecting the sw pin and inductor to the input supply. the increasing inductor current is sensed and amplified by the current sense amplifier. ramp compensation is summed to current sense amplifier output and compared to the error amplifier output by the current comparator. when the current sense amplifier plus slope compensation signal exceeds the comp pin voltage, the rs flip-flop is reset and the mp2354 reverts to its initial m1 off, m2 on state. if the current sense amplifier plus slope compensation signal does not exceed the comp voltage, then the falling edge of the clk resets the flip-flop. the output of the error amplifier integrates the voltage difference between the feedback and the 1.23v bandgap reference. the polarity is such that an fb pin voltage lower than 1.222v increases the comp pin voltage. since the comp pin voltage is proportional to the peak inductor current an increase in its voltage increases current delivered to the output. the lower 10 ? switch ensures that the bootstrap capacitor voltage is charged during light load conditions. external schottky diode d1 carries the inductor current when m1 is off. mp2354_bd01 lockout comparator error amplifier frequency foldback comparator internal regulators 1.8v slope comp clk current comparator current sense amplifier shutdown comparator comp 7 vin 3 sync 1 run 8 gnd 5 oscillator 35/380khz s r q lx 4 bst 2 5v + q 0.7v + + 2.50v/ 2.29v + 1.22v 0.7v + + fb 6 -- -- -- -- -- -- figure 1?functional block diagram
mp2354 ? 2a, 23v, 380khz step-down converter mp2354 rev. 1.4 www.monolithicpower.com 6 4/19/2011 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2011 mps. all rights reserved. application information mp2354_tac_f02 mp2354 lx vin bst fb sync comp gnd run output 2.5v / 2a input 4.75v to 23v open if not used open automatic startup c6 open c3 3.3nf c5 10nf d1 b230a 32 4 6 7 5 1 8 figure 2?typical application circuit sync pin operation the sync pin driving waveform should be a square wave with a rise time less than 20ns. minimum high voltage level is 2.7v. low level is less than 0.8v. the frequency of the external sync signal needs to be greater than 445khz. a rising edge on the sync pin forces a reset of the oscillator. the upper transistor m1 is switched off immediately if it is not already off. 250ns later m1 turns on connecting lx to v in . setting the output voltage the output voltage is set using a resistive voltage divider from the output to fb (see figure 2). the voltage divider divides the output voltage down 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 23 . 1 v out + = r2 can be as high as 100k ? , but a typical value is 10k ? . using that value, r1 is determined by: ()() ? ? = k 23 . 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.
mp2354 ? 2a, 23v, 380khz step-down converter mp2354 rev. 1.4 www.monolithicpower.com 7 4/19/2011 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2011 mps. all rights reserved. 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 vendor/ model core type core material package dimensions (mm) 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 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 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. 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 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 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
mp2354 ? 2a, 23v, 380khz step-down converter mp2354 can be optimized for a wide range of capacitance and esr values. output rectifier diode the output rectifier diode supplies the current to the inductor when the upper transistor m1 is off. use a schottky diode to reduce losses due to the diode forward voltage and recovery times. 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 provides the schottky diode part numbers based on the maximum input voltage and current rating. table 2?schottky rectifier selection guide v in (max) 2a load current part numbe r v endor ( 5 ) 15v 30bq015 4 20v b220 1 sk23 6 sr22 6 26v 20bq030 4 b230 1 sk23 6 sr23 3, 6 ss23 2, 3 note: 5) refer to table 3 for rectifier manufacturers table 3?schottky diode manufacturers # v endo r web site 1 diodes, inc. www.diodes.com 2 fairchild semiconductor www.fairchildsemi.com 3 general semiconductor www.gensemi.com 4 international rectifier www.irf.com 5 on semiconductor www.onsemi.com 6 pan jit international www.panjit.com.tw compensation mp2354 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, 1.95a/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, 770a/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 = smaller f z1 provides more phase margin, but longer transient settling time. a trade-off has to be made between the stability and the transient response. a typical value is less than one-fourth of the crossover frequency. 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 =
mp2354 ? 2a, 23v, 380khz step-down converter mp2354 rev. 1.4 www.monolithicpower.com 9 4/19/2011 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2011 mps. all rights reserved. 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. switching frequency for the mp2354 is 380khz, so the desired crossover frequency is around 38khz. table 4 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 4?compensation values for typical output voltage/capacitor combinations v out l1 c2 r3 c3 c6 2.5v 10h min. 22f ceramic 5.6k ? 4.7nf none 3.3v 15h min. 22f ceramic 7.5k ? 3.3nf none 5v 15h min. 22f ceramic 11k ? 2.2nf none 12v 22h min. 22f ceramic 27k ? 1nf none 2.5v 10h min. 560f al. 30m ? esr 140k ? 1nf 120pf 3.3v 15h min. 560f al 30m ? esr 187k ? 1nf 82pf 5v 15h min. 470f al. 30m ? esr 237k ? 1nf 56pf 12v 22h min. 220f al. 30m ? esr 267k ? 1nf 22pf to optimize the compensation components for conditions not listed in table 4, 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. 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 =
mp2354 ? 2a, 23v, 380khz 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. mp2354 rev. 1.4 www.monolithicpower.com 10 4/19/2011 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2011 mps. all rights reserved. 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. mp2354 sw bs 10nf 5v mp2354_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. package information soic8 note: 1) control dimension is in inches. dimension in bracket is millimeters. 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) seating plane 0.001(0.030) 0.004(0.101) 0.189(4.800) 0.197(5.004) 0.053(1.350) 0.068(1.730) 0.049(1.250) 0.060(1.524) 0.150(3.810) 0.157(4.000) pin 1 ident. 0.050(1.270)bsc 0.013(0.330) 0.020(0.508)
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