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mp4470/4470a high-efficiency, fast-transient, 5a, 36v synchronous, step-down converter mp4470/4470a rev. 1.1 www.monolithicpower.com 1 8/26/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. the future of analog ic technology description the mp4470/4470a is a fully-integrated, high- frequency, synchronous, rectified, step-down, switch-mode converter. it offers a very compact solution to achieve a 5a, continuous-output current over a wide input-supply range with excellent load and line regulation. it also provides fast transient response and good stability for wide input-supply and load range. the mp4470/4470a operates at high efficiency over a wide output current load range. mp4470 has full protection features include scp, ocp, ovp latch, uvp, and thermal shutdown. MP4470A has the same protection features to mp4470 except has no ovp latch function. the mp4470/4470a requires a minimal number of readily-available, standard, external components, and is available in a space-saving 3mm4mm, 20-pin, qfn package. features ? wide 4.5v-to-36v operating input range ? guaranteed 5a, continuous output current ? internal 40m ? high-side, 20m ? low-side power mosfets ? proprietary switching-loss-reduction technology ? 1% reference voltage ? programmable soft-start time ? low drop-out mode ? 200khz-to-1mhz switching frequency ? scp, ocp, ovp latch(mp4470 only), uvp, and thermal shutdown ? output adjustable from 0.8v to 0.9 v in ? available in a 34mm 20-pin qfn package applications ? notebook systems and i/o power ? automotive systems ? networking systems ? industrial supplies ? optical communications systems ? distributed power and pol systems all mps parts are lead-free and adhere to the rohs directive. for mps green status, please visit mps website under quality assurance. ?mps? and ?the future of analog ic technology? are registered trademarks of monolithi c power systems, inc. typical application 0 10 20 30 40 50 60 70 80 90 100 0.01 0.1 1 10 output current (a)
mp4470/4470a?high-efficiency, fast-transient, synchronous, step-down converter mp4470/4470a rev. 1.1 www.monolithicpower.com 2 8/26/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. ordering information part number* package top marking mp4470gl qfn20 (34mm) 4470 MP4470Agl qfn20 (34mm) 4470a * for tape & reel, add suffix ?z (e.g. mp4470/4470agl?z) package reference 78 20 19 18 17 910 21 in 22 in 23 in 25 sw 24 sw 16 15 14 13 12 11 1 2 3 4 5 6 qfn20 (34mm) absolute maxi mum ratings (1) supply voltage v in ....................................... 40v v sw ........................................-0.3v to v in + 0.3v v bst ...................................................... v sw + 6v v pgood ....................................-0.3v to v cc +0.6v all other pins ..................................-0.3v to +6v continuous power dissipation (t a = +25c) (2) ????????????? ?????.2.6w operating junction temperature ..............150c lead temperature ....................................260c storage temperature............... -65c to +150c recommended operating conditions (3) supply voltage v in ...........................4.5v to 36v output voltage v out ................... 0.8v to 0.9 v in operating junction temp. (t j ). -40c to +125c thermal resistance (4) ja jc qfn20 (34mm) .....................48 ...... 10 ... c/w notes: 1) exceeding these ratings may damage the device. 2) the maximum allowable power dissipation is a function of the maximum junction temperature t j (max), the junction-to- ambient thermal resistance ja , and the ambient temperature t a . the maximum allowable continuous power dissipation at any ambient temperature is calculated by p d (max)=(t j (max)- t a )/ ja . exceeding the maximum allowable power dissipation will cause excessive die temperature, and the regulator will go into thermal shutdown. internal thermal shutdown circuitry protects the device from permanent damage. 3) the device is not guaranteed to function outside of its operating conditions. 4) measured on jesd51-7, 4-layer pcb.
mp4470/4470a?high-efficiency, fast-transient, synchronous, step-down converter mp4470/4470a rev. 1.1 www.monolithicpower.com 3 8/26/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. electrical characteristics v in = 24v, v en = 2v, t a = +25 c, unless otherwise noted. parameters symbol condition min typ max units supply current (shutdown) i in v en = 0v 10 200 na supply current (quiescent) i in v fb = 0.95v 500 600 a hs switch on resistance hs rds-on 40 55 m ? ls switch on resistance (5) ls rds-on 20 m ? switch leakage sw lkg v en = 0v v sw = 0v or 36v 10 200 na current limit i limit 6 8 a one-shot on time t on v in =12v, r freq =30k ? 230 280 330 ns minimum off time (5) t off 100 ns foldback off time (5) t fb i lim =1(high), v fb >50%v ref 4.8 s foldback off time (5) t fb i lim =1(high), v fb <50%v ref 16.8 s ocp hold-off time (5) t oc i lim =1(high) 100 s feedback voltage v fb 807 815 823 mv feedback current i fb v fb = 815mv 10 50 na soft-start charging current i ss v ss =0v 6 8.5 11 a power-good rising threshold pg vth-hi 0.87 0.9 0.93 v fb power-good falling threshold pg vth-lo 0.82 0.85 0.88 v fb power-good threshold hysteresis pg vth-hys 0.05 v fb power-good rising delay t pg 500 700 900 s en rising threshold en vth-hi 1.1 1.25 1.4 v en falling threshold en vth-lo 0.8 0.86 0.92 v en threshold hysteresis en vth-hys 390 mv en input current i en v en = 2v 1.5 2 a v in under-voltage lockout threshold rising inuv vth 3.7 4.0 4.3 v v in under-voltage lockout threshold hysteresis inuv hys 880 mv v cc regulator v cc i cc =0 4.5 4.85 5.2 v v cc load regulation i cc =10ma 1 % vo over-voltage protection threshold (6) v ovp 1.25 v fb thermal shutdown (5) t sd 175 c thermal shutdown hysteresis (5) t sd-hys 45 c note: 5) derived from bench characteri zation, not tested in production. 6) for mp4470 only. MP4470A has no ovp function.
mp4470/4470a?high-efficiency, fast-transient, synchronous, step-down converter mp4470/4470a rev. 1.1 www.monolithicpower.com 4 8/26/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. typical performanc e characteristics v in = 24v, v out = 3.3v, l = 10h, t a = +25c, unless otherwise noted. efficiency vs. load current 0 10 20 30 40 50 60 70 80 90 100 0.01 0.1 1 10 450 500 550 0 5 10 15 20 25 30 35 40 vin (v) case temperature rise vs. output current 0 5 10 15 20 25 30 35 40 012345 line regulation -1.50 -1.00 -0.50 0.00 0.50 1.00 1.50 010203040 load regulation -1.00 -0.50 0.00 0.50 1.00 012345 200 300 400 500 600 700 800 1 1.5 2 2.5 3 3.5 4 4.5 5
mp4470/4470a?high-efficiency, fast-transient, synchronous, step-down converter mp4470/4470a rev. 1.1 www.monolithicpower.com 5 8/26/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. typical performanc e characteristics (continued) v in = 24v, v out = 3.3v, l = 10h, t a = +25c, unless otherwise noted. v o (ac) 10mv/div. sw 10v/div. i l 500ma/div. v in 10v/div. v o 2v/div. sw 20v/div. i l 2a/div. v in 10v/div. v o 2v/div. sw 20v/div. i l 5a/div. v in 10v/div. v o 2v/div. sw 10v/div. i l 200ma/div. v in 10v/div. v o 2v/div. sw 10v/div. i l 2a/div. en 2v/div. v o 2v/div. sw 20v/div. i l 500ma/div. en 2v/div. v o 2v/div. sw 20v/div. i l 5a/div. en 2v/div. v o 2v/div. sw 20v/div. i l 2a/div. v o (ac) 10mv/div. sw 20v/div. i l 1a/div.
mp4470/4470a?high-efficiency, fast-transient, synchronous, step-down converter mp4470/4470a rev. 1.1 www.monolithicpower.com 6 8/26/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. typical performanc e characteristics (continued) v in = 24v, v out = 3.3v, l = 10h, t a = +25c, unless otherwise noted.
mp4470/4470a?high-efficiency, fast-transient, synchronous, step-down converter mp4470/4470a rev. 1.1 www.monolithicpower.com 7 8/26/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. pin functions pin # name description 1 agnd analog ground. 2 freq frequency set (for ccm). the input voltage and the frequency-set resistor connected to gnd determine the on peri od. decouple with a 1nf capacitor. 3 fb feedback. the tap of external resistor divider from the output to gnd sets the output voltage. 4 ss soft-start. connect an external capacitor to program the soft-start time for the switch-mode regulator. when the en pin goes high, an internal current source (8.5a) charges up the capacitor and the ss voltage slowly and smoothly ramps up from 0 to v fb . when the en pin goes low, the internal current source discharges the capacitor and the ss voltage slowly ramps down. 5 en enable. en=1 to enable the mp4470/4470a. for automatic start-up, connect en pin to in with a 100k ? resistor. it includes an internal 1m ? pull-down resistor. 6 pgood power good output. the output of this pin is an open drain and goes high if the output voltage exceeds 90% of the nominal voltage. there is delay of ~700s from fb 90% to pgood high. 7 bst bootstrap. requires a 0.1f-to-1f c apacitor connected between sw and bs pins to form a floating supply across the high-side switch driver. 8, 19, exposed pads 21, 22, 23 in supply voltage. the mp4470/4470a operates from a 4.5v-to-36v input rail. requires c in to decouple the input rail. connect using wide pcb traces and multiple vias. 9, 10, 17, 18, exposed pads 24, 25 sw switch output. connect using wi de pcb traces and multiple vias. 11-16 pgnd system ground. this pin is the referenc e ground of the regulated output voltage. for this reason care must be taken in pcb layout. 20 vcc internal bias supply. decouple with a 1f capacitor as close to the pin as possible.
mp4470/4470a?high-efficiency, fast-transient, synchronous, step-down converter mp4470/4470a rev. 1.1 www.monolithicpower.com 8 8/26/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. block diagram figure 1?functional block diagram
mp4470/4470a?high-efficiency, fast-transient, synchronous, step-down converter mp4470/4470a rev. 1.1 www.monolithicpower.com 9 8/26/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. operation pwm operation the mp4470/4470a is a fully-integrated, synchronous, rectified, step-down, switch-mode converter. at the beginning of each cycle, the high-side mosfet (hs-fet) turns on when the feedback voltage (v fb ) drops below the reference voltage (v ref ), which indicates an insufficient output voltage. the on period is determined by the input voltage and the frequency-set resistor as: () ( ) freq on delay in 96 r k tns t (ns) v =+ (1) after the on period elapses, the hs-fet turns off. it is turned on again when v fb drops below v ref . by repeating this operation, the converter regulates the output voltage. the integrated low- side mosfet (ls-fet) turns on when the hs- fet is off to minimize conduction loss. a dead short occurs between input and gnd if both the hs-fet and the ls-fet turn on at the same time (shoot-through). an internal dead-time (dt) generated between hs-fet off and ls-fet on, or ls-fet off and hs-fet on prevents shoot-through. heavy-load operation t on figure 2: heavy-load operation in continuous-conduction mode (ccm), when the output current is high, the hs-fet and ls-fet repeatedly turn on/off as shown in mps. all rights reserved. the inductor current never goes to zero. in ccm, the switching frequency (f sw ) is fairly constant. light-load operation at light-load or no-load conditions, the output drops very slowly and the mp4470/4470a reduces the switching frequency automatically to maintain high efficiency. figure 3 shows the light- load operation. v fb does not reach v ref as the inductor current approaches zero. the ls-fet driver enters a tri-state (high z) whenever the inductor current reaches zero. a current modulator takes control of the ls-fet and limits the inductor current to less than -1ma. hence, the output capacitors discharge slowly to gnd through the ls-fet to greatly improve the light- load efficiency. at light loads, the hs-fet does not turn on as frequently as at heavy loads. this is called skip mode. t on figure 3: light-load operation as the output current increases from light-load condition, the current modulator?s regulatory time period becomes shorter. the hs-fet turns on more frequently, thus increasing the switching frequency increases. the output current reaches its critical level when the current modulator time is zero. the critical output current level is: in out out out sw in (v v ) v i 2lf v ? = (2) it enters pwm mode once the output current exceeds the critical level. after that, the switching frequency stays fairly constant over the output current range. switching frequency the input voltage is feed-forwarded to the on- time one-shot timer through the resistor, r freq . the duty ratio remains at v out /v in . hence, the switching frequency is fairly constant over the input voltage range. the switching frequency can be set as:
mp4470/4470a?high-efficiency, fast-transient, synchronous, step-down converter mp4470/4470a rev. 1.1 www.monolithicpower.com 10 8/26/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. 6 sw freq in delay in out 10 f(khz) 96 r (k ) v [t(ns)] vv = + (3) where t delay is the comparator delay (~20ns). the mp4470/4470a is optimized for 200khz-to- 1mhz applications to operate at high switching frequencies with high efficiency. the high- switching frequency allows for smaller lc-filter components to reduce pcb space requirements. ramp compensation figure 4 and figure 5 show jitter occurring in both pwm mode and skip mode. noise on v fb ?s downward slope causes the hs-fet on time to deviate from its intended position and produces jitter. there is a relationship between system stability and the steepness of the v fb ripple: the slope steepness of the v fb ripple dominates noise immunity. the magnitude of the v fb ripple doesn?t affect the noise immunity directly. figure 4: jitter in pwm mode figure 3: jitter in skip mode ceramic output capacitors lack enough esr ripple to stabilize the system, and requires an external compensation ramp. i c4 i fb i i r4 i fb figure 6: simplified circuit in pwm mode with external ramp compensation in pwm mode has an equivalent circuit with hs- fet off and uses a external ramp compensation circuit (r 4 , c 4 ), shown as a simplified circuit in figure 6. derive the external ramp from the inductor-ripple current. choose c 4 , r 1 , and r 2 to meet the following condition: 12 sw 4 1 2 rr 11 2f c 5rr ?? < ?? + ?? (4) then: r4 c4 fb c4 iiii = + (5) the v fb downward slope ripple is then estimated as: out slope1 44 v v rc ? = (6) from equation 6, reduce r 4 or c 4 to reduce instability in pwm mode. if c4 cannot be reduced further due to equation 4?s limitations, then only reduce r 4 . based on bench experiments, v slope1 is around 20v/ms-40v/ms. in the case of poscap or other types of capacitors with higher esr, an external ramp is not necessary. figure 7: simplified circuit in pwm mode without external ramp compensation
mp4470/4470a?high-efficiency, fast-transient, synchronous, step-down converter mp4470/4470a rev. 1.1 www.monolithicpower.com 11 8/26/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. figure 7 shows an equivalent circuit in pwm mode with the hs-fet off and without an external ramp circuit. the esr ripple dominates the output ripple. the v fb downward slope is: ref slope1 esr v v l ? = (7) from equation 7, the v fb downward slope is proportional to esr/l. therefore, it?s necessary to know the minimum esr value of the output capacitors without an external ramp. there is also an inductance limit: a smaller inductance leads to more stability. based on bench experiments, keep v slope1 around 15v/ms to 30v/ms. in skip mode, the external ramp does not affect the downward slope, and v fb ripple?s downward slope is the same with or without the external ramp. figure 8 shows an equivalent circuit with the hs-fet off and the current modulator regulating the ls-fet. i mod figure 8: simplified circuit in skip mode the v fb ripple?s downward slope is: () ? = + ref slope2 12 out v v rr c (8) to keep the system stable during light loads, avoid large v fb resistors. also, keep the v slope2 value around 0.4v/ms to 0.8mv/ms. note that i mod is excluded from the equation because it does not impact the system?s light-load stability. soft-start the mp4470/4470a employs soft start (ss) to ensure a smooth output during power-up. when the en pin goes high, an internal current source (8.5 a) charges up the ss capacitor (c ss ). the c ss voltage takes over the ref voltage to the pwm comparator. the output voltage smoothly ramps up with v ss . once v ss reaches the same level as v ref , it continues ramping up while v ref takes over the pwm comparator. at this point, soft-start finishes and the mp4470/4470a enters steady-state. c ss is then: () ( ) ( ) () ss ss ss ref tmsi a cnf vv = (9) if the output capacitors have large capacitance values, avoid setting a short ss or risk hitting the current limit during ss. select a minimum value of 4.7nf if the output capacitance value exceeds 330 f. power good (pgood) the mp4470/4470a has power-good (pgood) output. the pgood pin is the open drain of a mosfet. it should connect to v cc or some other voltage source through a resistor (e.g. 100k ? ). in the presence of an input voltage, the mosfet turns on so that the pgood pin is pulled to gnd before ss is ready. after v fb reaches 90%v ref , the pgood pin is pulled high after a delay; typically 700 s. when the fb voltage drops to 85%v ref , the pgood pin is pulled low. over-current protection (ocp) and short- circuit protection (scp) the mp4470/4470a has cycle-by-cycle over- current limit control. the inductor current is monitored during the on state. once the inductor current exceeds the current limit, the hs-fet turns off. at the same time, the ocp timer starts. the ocp timer is set at 100 s. hitting the current limit during each cycle during this 100 s time frame will trigger hiccup scp. if a short circuit occurs, the mp4470/4470a will immediately hit its current limit and v fb will drop below 50%v ref (0.815v). the device considers this an output dead short and will trigger hiccup scp immediately. over/under-voltage protection (ovp/uvp) the mp4470 monitors the output voltage through the tap of a resistor divider to the fb pin to detect output over-voltage conditions. a v fb that exceeds 125%v ref (0.815v) triggers ovp latch- off. once ovp triggers, the ls-fet turns on to
mp4470/4470a?high-efficiency, fast-transient, synchronous, step-down converter mp4470/4470a rev.0.8 www.monolithicpower.com 12 8/26/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. preliminary specifications subject to change ? 2013 mps. all rights reserved. discharge v o until the inductor current drops to zero while the hs-fet remains off. the mp4470 needs to power cycle to restart. note that MP4470A has no this ovp function. the mp4470/4470a also monitors fb pin voltage to detect output under-voltage condition. a v fb drop below 50% v ref triggers uvp as well as a current-limit that triggers scp. uvlo protection the mp4470/4470a has under-voltage lock-out protection (uvlo). when the input voltage is higher than the uvlo rising threshold voltage, the mp4470/4470a will be powered up. it shuts off when the input voltage is lower than the uvlo falling threshold voltage. this is non-latch protection. floating driver and bootstrap charging an external bootstrap capacitor power the floating-power-mosfet driver. a dedicated internal regulator charges and regulates the bootstrap capacitor voltage to ~5v. when the voltage between the bst and sw nodes drops below regulation, a pmos pass transistor connected from vin to bst turns on. the charging current path is from vin, bst and then to sw. the external circuit should provide enough voltage headroom to facilitate charging. as long as v in is significantly higher than sw, the bootstrap capacitor remains charged. when the hs-fet is on, v in v sw so the bootstrap capacitor cannot charge. when the ls-fet is on, v in ? v sw reaches its maximum for fast charging. when there is no inductor current, v sw =v out so the difference between v in and v out can charge the bootstrap capacitor. at higher duty cycles, the bootstrap-charging time is shorter so the bootstrap capacitor may not charge sufficiently. in case the internal circuit has insufficient voltage and time to charge the bootstrap capacitor, the bootstrap capacitor voltage will drop low. when v bst ? v sw drops below 2.3v, the hs-fet turns off. a uvlo circuit allows the ls-fet to conduct and refresh the charge on the bootstrap capacitor. once bootstrap capacitor voltage is charged, the hs- fet can turn on again and the part resumes normal switching. with this bootstrap refreshing function, mp4470/4470a is able to work on the low drop-out mode. thermal shutdown the mp4470/4470a uses thermal shutdown. the junction temperature of the ic is internally monitored. if the junction temperature exceeds the threshold value (typically 175c), the converter shuts off. this is a non-latched protection. there is about 45c hysteresis. once the junction temperature drops to about 130c, it initiates a ss.
mp4470/4470a?high-efficiency, fast-transient, synchronous, step-down converter mp4470/4470a rev. 1.1 www.monolithicpower.com 13 8/26/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. application information setting the output voltage a resistor divider from the output voltage to the fb pin set v out . without an external ramp employed, the feedback resistors (r 1 and r 2 ) set the output voltage. to determine the values for the resistors, first, choose r 2 (typically 5k ? -40k ? ). then r 1 is: out ref ref vv r1 r2 v ? = (10) when using a low-esr ceramic capacitor on the output, add an external voltage ramp to the fb pin through r 4 and c 4 . the ramp voltage (v ramp ) affects output voltage. calculate v ramp as per equation 19. choose r 2 between 5k ? and 40k ? . determine r 1 as: 1 4 2 1 1 2 1 2 1 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? + = r ) v v v ( r v v r ramp ref out ramp ref (11) using equation 11 to calculate the output voltage can be complicated. furthermore, as v ramp changes due to changes in v out and v in , v fb also varies. to improve the output voltage accuracy and simplify the r 2 calculation from equation 11, add a dc-blocking capacitor (c dc ). figure 9 shows a simplified circuit with external ramp compensation and a dc-blocking capacitor. equation 10 can then estimate r 1 ) select a c dc value between 1f and 4.7 f to improve dc-blocking performance. cdc figure 9: simplified circuit with external ramp compensation and dc blocking capacitor input capacitor the input current to the step-down converter is discontinuous, and therefore requires a capacitor to supply the ac current to the step- down converter while maintaining the dc input voltage. ceramic capacitors are recommended for best performance. be sure to place the input capacitors as close to the in pin as possible. the capacitance varies significantly with temperature. capacitors with x5r and x7r ceramic dielectrics are are fairly stable over temperature fluctuations. the capacitors must also have a ripple-current rating greater than the converter?s maximum input-ripple current. the input ripple current can be estimated as follows: out out cin out in in vv ii (1 ) vv = ? (12) the worst-case condition occurs at v in = 2v out , where: out cin i i 2 = (13) for simplification, choose an input capacitor whose rms current rating is greater than half of the maximum load current.the input capacitance value determines the input voltage ripple of the converter. if there is an input-voltage-ripple requirement in the system design, choose an input capacitor that meets the specification the input voltage ripple can be estimated as follows: out out out in sw in in in iv v v(1) fc v v = ? (14) the worst-case condition occurs at vin = 2vout, where: out in sw in i 1 v 4f c = (15)
mp4470/4470a?high-efficiency, fast-transient, synchronous, step-down converter mp4470/4470a rev. 1.1 www.monolithicpower.com 14 8/26/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. output capacitor the output capacitor maintains the dc output voltage. use ceramic or poscap capacitors. the output voltage ripple can be estimated as: out out out esr sw in sw out vv 1 v(1)(r ) fl v 8fc = ? + (16) where r esr is the equivalent series resistance of the output capacitor. for ceramic capacitors, capacitance dominates the impedance at the switching frequency, can is the primary cause of the output-voltage ripple. for simplification, estimate the output voltage ripple as: out out out 2 sw out in vv v(1) 8f lc v = ? (17) the output voltage ripple caused by esr is very small and therefore requires an external ramp to stabilize the system. the voltage ramp is ~30mv. the external ramp can be generated through r 4 and c 4 using the following equation: in out on ramp (v v ) t v r4 c4 ? = (18) select c 4 to meet the following condition: sw r1 r2 11 () 2f c45r1r2 < + (19) for poscap capacitors, the esr dominates the impedance at the switching frequency. the ramp voltage generated from the esr is high enough to stabilize the system. therefore, an external ramp is not needed. a minimum esr value of 12m ? is required to ensure stable operation of the converter. for simplification, the output ripple can be approximated as: out out out esr sw in vv v(1)r fl v = ? (20) inductor the inductor is required to supply constant current to the output load while being driven by the switching input voltage. a larger inductance will result in less ripple current and a lower output ripple voltage. however, a larger inductance resultsin a larger inductor, which will physically larger, and have a higher series resistance and/or lower saturation current. a good rule for determining the inductor value is to allow the peak-to-peak ripple current in the inductor to be approximately 30% to 40% of the maximum switch current limit. ensure that the peak inductor current is below the maximum switch current limit. the inductance value can be calculated as: out out sw l in vv l(1) fi v =? (21) where 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 as: out out lp out sw in vv ii (1 ) 2f l v =+ ? (22)
mp4470/4470a?high-efficiency, fast-transient, synchronous, step-down converter mp4470/4470a rev. 1.1 www.monolithicpower.com 15 8/26/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. typical design parameter tables the following tables include recommended component values for typical output voltages (3.3v, 5v) and switching frequencies (300khz, 500khz, and 700khz). refer to tables 1 through 3 for design cases without external ramp compensation, and tables 4 through 6 for design cases with external ramp compensation. an external ramp is not needed when using high- esr capacitors, such as electrolytic or poscaps. an external ramp is needed when using low-esr capacitors, such as ceramic capacitors. for cases not listed in this datasheet, an excel spreadsheet available through your local sales representative can calculate approximate component values. table 1?300khz, 24v in v out (v) l ( h) r1 (k ? ) r2 (k ? ) r freq (k ? ) 3.3 10 30.1 10 110 5 10 51.1 10 169 table 2?500khz, 24v in v out (v) l ( h) r1 (k ? ) r2 (k ? ) r freq (k ? ) 3.3 10 30.1 10 63.4 5 10 51.1 10 100 table 3?700khz, 24v in v out (v) l ( h) r1 (k ? ) r2 (k ? ) r freq (k ? ) 3.3 10 30.1 10 44.2 5 10 51.1 10 69.8 table 4?300khz, 24v in v out (v) l ( h) r1 (k ? ) r2 (k ? ) r4 (k ? ) c4 (pf) r freq (k ? ) 3.3 10 30.9 10 953 390 110 5 10 53.6 10 845 560 169 table 5?500khz, 24v in v out (v) l ( h) r1 (k ? ) r2 (k ? ) r4 (k ? ) c4 (pf) r freq (k ? ) 3.3 10 31.6 10 620 390 63.4 5 10 53.6 10 845 390 100 table 6?700khz, 24v in v out (v) l ( h) r1 (k ? ) r2 (k ? ) r4 (k ? ) c4 (pf) r freq (k ? ) 3.3 10 31.6 10 560 390 44.2 5 10 54.9 10 620 390 69.8
mp4470/4470a?high-efficiency, fast-transient, synchronous, step-down converter mp4470/4470a rev. 1.1 www.monolithicpower.com 16 8/26/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. layout recommendation 1. place high-current paths (gnd, in, and sw) very close to the device with short, direct, and wide traces. 2. place input capacitors on both vin sides (pin8 and pin19) and as close to the in and gnd pins as possible. 3. place the decoupling capacitor as close to the vcc and gnd pins as possible. 4. keep the switching node sw short and away from the feedback network. 5. place the external feedback resistors next to the fb pin. do not place vias on the fb trace. 6. keep the bst voltage path (bst, c3, and sw) as short as possible. 7. connect the bottom in and sw pads to a large copper area to achieve better thermal performance. 8. a four-layer layout is strongly recommended to achieve better thermal performance. c1a c1b c1c c2a c2b top layer inner1 layer inner2 layer bottom layer figure 10: pcb layout
mp4470/4470a?high-efficiency, fast-transient, synchronous, step-down converter mp4470/4470a rev. 1.1 www.monolithicpower.com 17 8/26/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. typical application circuits figure 11: typical application circuit, 3.3v-output
mp4470/4470a?high-efficiency, fast-transient, synchronous, 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. mp4470/4470a rev. 1.1 www.monolithicpower.com 18 8/26/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. package information 3mm 4mm qfn20

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