MP28369 2a, 16v, 1.4mhz step-down converter MP28369 rev. 1.0 www.monolithicpower.com 1 12/13/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. the future of analog ic technology tm tm description the MP28369 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 protections include cycle-by-cycle current limiting and thermal shutdown. in shutdown mode the regulator draws 25 a of supply current. programmable soft-start minimizes the inrush supply current and the output overshoot at initial startup. the MP28369 requires a minimum number of readily available standard external components. features ? 2a output current ? 0.18 ? internal power mosfet switch ? stable with low esr output ceramic capacitors ? 90% efficiency ? 25 a shutdown mode ? fixed 1.4mhz frequency ? thermal shutdown ? cycle-by-cycle over current protection ? wide 4.75v to 16v operating input range ? output adjustable from 0.92v to 16v ? programmable under voltage lockout ? available in a tiny msop package ? evaluation board available applications ? distributed power systems ? battery charger ? dsl modems ? pre-regulator for linear regulators ?mps? and ?the future of analog ic technology? are trademarks of monolithic power systems, inc. typical application MP28369 sw in bs 42 fb 5 7 ss comp gnd 8 6 en 10 9 v out 2.5v/2a input 4.75v to 16v MP28369_tac_s01 c5 10nf c3 1.8nf c4 10nf d1 b220a c6 open v out =5v v out =2.5v v out =3.3v 100 90 80 70 60 50 efficiency (%) 0 2.0 0.5 1.0 1.5 load current (a) MP28369_ec01 efficiency vs load current
MP28369 ? 2a, 16v, 1.4mhz step-down converter MP28369 rev. 1.0 www.monolithicpower.com 2 12/13/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. tm package reference nc bs nc in sw 1 2 3 4 5 10 9 8 7 6 ss en comp fb gnd top view MP28369_pd01_msop10 part number* package temperature MP28369dk msop10 ?40 c to +85 c * for tape & reel, add suffix ?z (eg. MP28369dk?z) for lead free, add suffix ?lf (eg. MP28369dk?lf?z) absolute maxi mum ratings (1) supply voltage (v in ) .................................... 18v switch node voltage (v sw ).......................... 16v bootstrap voltage (v bs ) ....................... v sw + 6v feedback voltage (v fb ) ................. ?0.3v to +6v enable/uvlo voltage (v en )........... ?0.3v to +6v comp voltage (v comp ) ................... ?0.3v to +6v junction temperature.............................+150 c lead temperature ..................................+260 c storage temperature ..............?65c to +150 c recommended operating conditions (2) supply voltage (v in ) ...................... 4.75v to 16v operating temperature .............?40 c to +85 c thermal resistance (3) ja jc msop10 ................................ 150 ..... 65... 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 0.892 0.920 0.948 v upper switch on resistance r ds(on)1 0.18 ? upper switch leakage v en = 0v, v sw = 0v 0 50 a current limit (4) 2.5 3.5 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 = 10 a 630 930 1230 a/v oscillator frequency f s 1.4 mhz short circuit frequency v fb = 0v 210 khz soft-start pin equivalent output resistance 9 k ? maximum duty cycle d max v fb = 0.8v 70 % en shutdown threshold voltage v en i cc > 100 a 0.7 1.0 1.3 v enable pull-up current i en v en = 0v 1.0 a en uvlo threshold rising v uvlo v en rising 2.37 2.50 2.62 v en uvlo threshold hysteresis 210 mv supply current (shutdown) i off v en 0.4v 25 50 a supply current (quiescent) i on v en 3v 1.2 1.4 ma thermal shutdown 160 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.
MP28369 ? 2a, 16v, 1.4mhz step-down converter MP28369 rev. 1.0 www.monolithicpower.com 3 12/13/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. tm pin functions pin # name description 1 nc no connect. 2 bs bootstrap (c5). this capacitor is needed to drive the power switch?s gate above the supply voltage. it is connected between sw and bs 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 sw pin voltage is low. 3 nc no connect. 4 in supply voltage. the MP28369 operates from a +4.75v to +16v unregulated input. c1 is needed to prevent large voltage spikes from appearing at the input. 5 sw switch. this connects the inductor to either in through m1 or to gnd through m2. 6 gnd ground. this pin is the voltage reference for the 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. 7 fb feedback. an external resistor divider from the 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 400mv. 8 comp compensation. this node is the output of the transconductance error amplifier and the input to the current comparator. frequency compensation is do ne at this node by connecting a series r-c to ground. see the compensation section for exact details. 9 en enable/uvlo. a voltage greater than 2.62v enables operation. leave en 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 en pin voltage needs to be less than 700mv. 10 ss soft-start pin. connect ss to an external capa citor to program the soft-start. if unused, leave it open.
MP28369 ? 2a, 16v, 1.4mhz step-down converter MP28369 rev. 1.0 www.monolithicpower.com 4 12/13/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. tm operation the MP28369 is a current mode regulator. that is, 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 (see 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 1.4mhz 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 MP28369 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 0.92v bandgap reference. the polarity is such that the fb pin voltage lower than 0.92v 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. MP28369_bd01 lockout comparator error amplifier frequency foldback comparator internal regulators 1.8v slope comp 5v clk current comparator current sense amplifier shutdown comparator comp 8 in 4 en 9 gnd 6 oscillator 210khz/ 1.4mhz s r q sw 5 bs 2 5v + q 0.7v + + 2.29v/ 2.50v + 0.92v 0.4v + + fb 7 -- -- -- -- -- -- ss 10 figure 1?functional block diagram
MP28369 ? 2a, 16v, 1.4mhz step-down converter MP28369 rev. 1.0 www.monolithicpower.com 5 12/13/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. tm application 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 92 . 0 v out + = where v out is the output voltage and v fb is the feedback 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: ) 92 . 0 v ( 87 . 10 1 r out ? = for example, for a 3.3v output voltage, r2 is 10k ? , and r1 is 25.8k ? . 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 f s is the switching frequency, i l is the peak-to-peak inductor ripple current and v in is the input voltage. 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. 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. 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.
MP28369 ? 2a, 16v, 1.4mhz step-down converter MP28369 rev. 1.0 www.monolithicpower.com 6 12/13/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. tm 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 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, r esr is the equivalent series resistance (esr) value of the output capacitor and c2 is the output capacitance value. 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 MP28369 can be optimized for a wide range of capacitance and esr values. compensation components the MP28369 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 r load is the load resistor value, g cs is the current sense transconductance and a vea is the error amplifier voltage gain. 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. 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 =
MP28369 ? 2a, 16v, 1.4mhz step-down converter MP28369 rev. 1.0 www.monolithicpower.com 7 12/13/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. tm 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 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 c v v g g f 2 c 2 3 r = where f c is the desired crossover frequency, which is typically less than 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 , to below one forth of the crossover frequency provides sufficient phase margin. determine the c3 value by the following equation: c f 3 r 2 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 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 boost diode for 5v input or 5v output applications, it is recommended that an external boost 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 MP28369 regulator. the boost diode can be a low cost one such as in4148 or bat54. MP28369 sw bs 5 2 10nf 5v boost diode MP28369_f02 figure 2?external boost diode
MP28369 ? 2a, 16v, 1.4mhz 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. MP28369 rev. 1.0 www.monolithicpower.com 8 12/13/2007 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2007 mps. all rights reserved. tm package information msop10
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