MP38894 4.5a, 42v, 420khz step-down converter with synchronizable gate driver MP38894 rev.1.0 www.monolithicpower.com 1 9/28/2009 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2009 mps. all rights reserved. the future of analog ic technology description the MP38894 is a monolithic step-down switch mode converter with a built in internal power mosfet. it achieves 4.5a 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. the MP38894 requires a minimum number of readily available standard external components and is available in an 8-pin soic package with exposed pad. features ? wide 4.5v to 42v operating input range ? 4.5a continuous output current ? 1.5% vfb accuracy ? 100m ? internal power mosfet switch ? synchronizable gate driver delivers up to 95% efficiency ? fixed 420khz frequency ? synchronizable up to 1.5mhz ? cycle-by-cycle over current protection with hiccup retry ? thermal shutdown ? output adjustable from 0.8v to 15v ? stable with low esr output ceramic capacitors ? available in a thermally enhanced 8-pin soic package applications ? digital set top boxes ? personal video recorders ? broadband communications ? flat panel television and monitors ?mps? and ?the future of analog ic technology? are registered trademarks of monolithic power systems, inc. typical application sw bg fb bst 1 4 6 5 2 gnd in vcc en/sync 8 3 7 m2 v in off on
MP38894 ? 4.5a, 42v, 420khz step-dow n with synchronizable gate driver MP38894 rev.1.0 www.monolithicpower.com 2 9/28/2009 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2009 mps. all rights reserved. package reference part number* package temperature MP38894dn soic8e ?40 c to +85 c * for tape & reel, add suffix ?z (eg. MP38894dn?z) for rohs compliant packaging, add suffix ?lf (eg. MP38894dn?lf?z) absolute maxi mum ratings (1) supply voltage v in ....................................... 45v v sw ......................... ?0.3v(-5v for <10ns) to 46v v bst ? v sw ...................................................... 6v all pins........................................... ?0.3v to +6v junction temperature...............................150 c lead temperature ....................................260 c storage temperature ..............?65 c to +150 c recommended operating conditions (2) supply voltage v in ........................... 4.5v to 42v output voltage v out ........................ 0.8v to 15v operating temperature .............?40 c to +85 c thermal resistance (3) ja jc soic8e .................................. 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 jesd51-7, 4-layer pcb. electrical characteristics v in = 12v, t a = +25 c, unless otherwise noted. parameters symbol condition min typ max units feedback voltage v fb 4.5v v in 42v 0.796 0.808 0.820 v feedback current i fb v fb = 0.8v 10 na switch on resistance (4) r ds(on) 100 m ? switch leakage v en = 0v, v sw = 0v 0.1 10 a current limit (4) i o-max duty=40% 6.5 a oscillator frequency f sw v fb = 0.6v 240 420 600 khz fold-back frequency v fb = 0v 25 115 205 khz maximum duty cycle v fb = 0.6v 85 90 % minimum on time (4) t on 100 ns under voltage lockout threshold rising 3.90 4.10 4.30 v under voltage lockout threshold hysteresis 880 mv en input low voltage 0.4 v en input high voltage 2 v v en = 2v 2 en input current v en = 0v 0.1 a sync frequency range (low) f syncl 300 khz sync frequency range (high) f synch 1.5 mhz enable turnoff delay t off 5.0 s supply current (shutdown) v en = 0v 1 10 a supply current (quiescent) v en = 2v, v fb = 1v 0.9 1.1 ma thermal shutdown 150 c bg driver bias supply voltage v cc i cc = 5ma 4.5 5.0 v gate driver sink impedance (4) r sink 1 ? gate driver source impedance (4) r source 4 ? gate drive current sense trip threshold v sw 20 mv note: 4) guaranteed by design. sw bst vcc bg in en/sync fb gnd 1 2 3 4 8 7 6 5 top view exposed pad on backside
MP38894 ? 4.5a, 42v, 420khz step-dow n with synchronizable gate driver MP38894 rev.1.0 www.monolithicpower.com 3 9/28/2009 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2009 mps. all rights reserved. pin functions pin # name description 1 sw switch output. 2 bst bootstrap. this capacitor is needed to driv e 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. 3 vcc bg driver bias supply. decouple with a 1f ceramic capacitor. 4 bg gate driver output. connect this pi n to the gate of the synchronous mosfet. 5 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 shoul d be placed outside of the m2 to c1 ground path to prevent switching current spikes from inducing voltage noise into the part. 6 fb feedback. an external resistor divider from the output to gnd, tapped to the fb pin sets the output voltage. to prevent cu rrent limit run away during a sh ort circuit fault condition the frequency foldback comparator lowers the os cillator frequency when the fb voltage is below 250mv. 7 en/sync on/off control and external frequency synchronization input. 8 in supply voltage. the MP38894 operates from a +4.5v to +42v unregulated input. c1 is needed to prevent large voltage spikes from appearing at the input.
MP38894 ? 4.5a, 42v, 420khz step-dow n with synchronizable gate driver MP38894 rev.1.0 www.monolithicpower.com 4 9/28/2009 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2009 mps. all rights reserved. typical performanc e characteristics v in = 12v, v out = 3.3v, l = 4.3h, t a = +25oc, unless otherwise noted. load regulation operating range case temperature rise vs. output current 1 10 0
MP38894 ? 4.5a, 42v, 420khz step-dow n with synchronizable gate driver MP38894 rev.0.9 www.monolithicpower.com 5 9/28/2009 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2009 mps. all rights reserved. typical performanc e characteristics v in = 12v, v out = 3.3v, l =4.3h, t a = +25oc, unless otherwise noted. ( continued )
MP38894 ? 4.5a, 42v, 420khz step-dow n with synchronizable gate driver MP38894 rev.1.0 www.monolithicpower.com 6 9/28/2009 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2009 mps. all rights reserved. operation in en/sync fb sw vcc bg gnd bst regulator oscillator 420khz driver current sense amplifier error amplifier comp current limit comparator pwm comparator driver d regulator reference -- + -- + -- + s r r q q -- + v cc v cc figure 1?functional block diagram the MP38894 is a fixed frequency, synchronous, step-down switching regulator with an integrated high-side power mosfet and a gate driver for a low-side external mosfet. it achieves 4.5a continuous output current over a wide input supply range with excellent load and line regulation. it provides a single highly efficient solution with current mode control for fast loop response and easy compensation. the MP38894 operates in a fixed frequency, peak current control mode to regulate the output voltage. a pwm cycle is initiated by the internal clock. the integrated high-side power mosfet is turned on and remains on until its current reaches the value set by the comp voltage. when the power switch is off, it remains off until the next clock cycle starts. if, in 90% of one pwm period, the current in the power mosfet does not reach the comp set current value, the power mosfet will be forced to turn off. error amplifier the error amplifier compares the fb pin voltage with the internal 0.8v reference (ref) and outputs a current proportional to the difference between the two. this output current is then used to charge or discharge the internal compensation network to form the comp voltage, which is used to control the power mosfet current. the optimized internal compensation network minimizes the external component counts and simplifies the control loop design. internal regulator most of the internal circuitries are powered from the 5v internal regulator. this regulator takes the vin input and operates in the full vin range. when vin is greater than 5.0v, the output of the regulator is in full regulation. when vin is lower than 5.0v, the output decreases. since this internal regulator provides the bias current for the bottom gate driver that requires significant amount of current depending upon the external mosfet selection, a 1f ceramic capacitor for decoupling purpose is required.
MP38894 ? 4.5a, 42v, 420khz step-dow n with synchronizable gate driver MP38894 rev.1.0 www.monolithicpower.com 7 9/28/2009 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2009 mps. all rights reserved. enable/synch control the MP38894 has a dedicated enable/synch control pin (en/sync). by pulling it high or low, the ic can be enabled and disabled by en. tie en to vin for automatic start up. to disable the part, en must be pulled low for at least 5s. the MP38894 can be synchronized to external clock range from 300khz up to 1.5mhz through the en/sync pin. the internal clock rising edge is synchronized to the external clock rising edge. under-voltage lockout (uvlo) under-voltage lockout (uvlo) is implemented to protect the chip from operating at insufficient supply voltage. the MP38894 uvlo comparator monitors the output voltage of the internal regulator, vcc. the uvlo rising threshold is about 4.1v while its falling threshold is a consistent 3.2v. internal soft-start the soft-start is implemented to prevent the converter output voltage from overshooting during startup. when the chip starts, the internal circuitry generates a soft-start voltage (ss) ramping up from 0v to 1.2v. when it is lower than the internal reference (ref), ss overrides ref so the error amplifier uses ss as the reference. when ss is higher than ref, ref regains control. over-current-protection (ocp) the MP38894 has cycle-by-cycle over current limit when the inductor current peak value exceeds the set current limit threshold. meanwhile, output voltage starts to drop until fb is below the under-voltage (uv) threshold, typically 30% below the reference. once a output uv is triggered, the MP38894 enters hiccup mode, which is especially useful to ensure system safety under fault condition. the MP38894 exits the hiccup mode once the en or input power is re-cycled. thermal shutdown thermal shutdown is implemented to prevent the chip from operating at exceedingly high temperatures. when the silicon die temperature is higher than 150 c, it shuts down the whole chip. when the temperature is lower than its lower threshold, typically 140 c, the chip is enabled again. floating driver and bootstrap charging the floating power mosfet driver is powered by an external bootstrap capacitor. this floating driver has its own uvlo protection. this uvlo?s rising threshold is 2.2v with a hysteresis of 150mv. the bootstrap capacitor voltage is regulated internally by v in through d1, m3, c4, l1 and c2 (figure 2). if (v in -v sw ) is more than 5v, u2 will regulate m3 to maintain a 5v bst voltage across c4. -- + -- + v in 5v u2 d1 m3 bst sw c4 c2 l1 v out figure 2 ? internal bootstrap charging circuit startup and shutdown if both vin and en are higher than their appropriate thresholds, the chip starts. the reference block starts first, generating stable reference voltage and currents, and then the internal regulator is enabled. the regulator provides stable supply for the remaining circuitries. three events can shut down the chip: en low, vin low and thermal shutdown. in the shutdown procedure, the signaling path is first blocked to avoid any fault triggering. the comp voltage and the internal supply rail are then pulled down. the floating driver is not subject to this shutdown command.
MP38894 ? 4.5a, 42v, 420khz step-dow n with synchronizable gate driver MP38894 rev.1.0 www.monolithicpower.com 8 9/28/2009 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2009 mps. all rights reserved. application information the schematic on the front page shows a typical MP38894 application. the ic can provide up to 4.5a output current at a nominal output voltage of 3.3v. for proper thermal performance, the exposed pad of the device must be soldered down to the printed circuit board. setting the output voltage the external resistor divider is used to set the output voltage (see the schematic on front page). the feedback resistor r1 also sets the feedback loop bandwidth with the internal compensation capacitor (see figure 1). choose r1 to be around 40.2k ? for optimal transient response. r2 is then given by: 1 v 8 . 0 v 1 r 2 r out ? = table 1?resistor selection for common output voltages v out (v) r1 (k ? ) r2 (k ? ) 1.8 40.2 (1%) 32.4 (1%) 2.5 40.2 (1%) 19.1 (1%) 3.3 40.2 (1%) 13 (1%) 5 40.2 (1%) 7.68 (1%) selecting the inductor a 1h to 10h inductor with a dc current rating of at least 25% higher than the maximum load current is recommended for most applications. for highest efficiency, the inductor dc resistance should be less than 15m ? . for most designs, the inductance value can be derived from the following equation. osc l in out in out f i v ) v v ( v l ? = where i l is the inductor ripple current. choose inductor ripple current to be approximately 30% of the maximum load current, 4.5a. the maximum inductor peak current is: 2 i i i l load ) max ( l + = under light load conditions below 100ma, larger inductance is recommended for improving efficiency. synchronous mosfet the external synchronous mosfet is used to freewheel the inductor current when the internal high-side switch is off. it significantly reduces the power loss compared against a schottky rectifier. table 2 lists example synchronous mosfets and manufacturers. table 2?synchronous mosfet selection guide part no. manufacture si7370 vishay si4470 vishay am4417 analog power selecting the input capacitor the input capacitor (c1) reduces the surge current drawn from the input and the switching noise from the device. the input capacitor impedance at the switching frequency should be less than the input source impedance to prevent high frequency switching current from passing to the input. ceramic capacitors with x5r or x7r dielectrics are highly recommended because of their low esr and small temperature coefficients. for 4.5a output applications, a 22f capacitor is sufficient. selecting the output capacitor the output capacitor (c2) keeps output voltage small and ensures regulation loop stability. the output capacitor impedance should be low at the switching frequency. ceramic capacitors with x5r or x7r dielectrics are recommended.
MP38894 ? 4.5a, 42v, 420khz step-dow n with synchronizable gate driver MP38894 rev.1.0 www.monolithicpower.com 9 9/28/2009 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2009 mps. all rights reserved. pcb layout guide pcb layout is very important to achieve stable operation. please follow these guidelines and take figure3 for references. 1) keep the path of switching current short and minimize the loop area formed by input cap, high-side and low-side mosfets. 2) keep the connection of low-side mosfet between sw pin and input power ground as short and wide as possible. 3) ensure all feedback connections are short and direct. place the feedback resistors and compensation components as close to the chip as possible. 4) route sw away from sensitive analog areas such as fb. 5) connect in, sw, and especially gnd respectively to a large copper area to cool the chip to improve thermal performance and long-term reliability. r3 cb 8 7 6 5 4 3 2 1 r2 top layer bottom layer figure 3?pcb layout external bootstrap diode an external bootstrap diode may enhance the efficiency of the regulator, the applicable conditions of external bst diode are: z v out =5v or 3.3v; and z duty cycle is high: d= in out v v >65% in these cases, an external bst diode is recommended from the output of the voltage regulator to bst pin, as shown in fig.4 MP38894 sw bst c l bst c out 5v or 3.3v external bst diode in4148 figure 4?add optional external bootstrap diode to enhance efficiency the recommended external bst diode is in4148, and the bst cap is 0.1~1f.
MP38894 ? 4.5a, 42v, 420khz step-dow n with synchronizable gate driver 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. MP38894 rev. 1.0 www.monolithicpower.com 10 9/28/2009 mps proprietary information. unaut horized photocopy and duplication prohibited. ? 2009 mps. all rights reserved. package information soic8e (exposed pad) see detail "a" 0.0075(0.19) 0.0098(0.25) 0.050(1.27) bsc 0.013(0.33) 0.020(0.51) seating plane 0.000(0.00) 0.006(0.15) 0.051(1.30) 0.067(1.70) top view front view side view bottom view note: 1) control dimension is in inches. dimension in bracket is in millimeters. 2) package length does not include mold flash, protrusions or gate burrs. 3) package width does not include interlead flash or protrusions. 4) lead coplanarity (bottom of leads after forming) shall be 0.004" inches max. 5) drawing conforms to jedec ms-012, variation ba. 6) drawing is not to scale. 0.089(2.26) 0.101(2.56) 0.124(3.15) 0.136(3.45) recommended land pattern 0.213(5.40) 0.063(1.60) 0.050(1.27) 0.024(0.61) 0.103(2.62) 0.138(3.51) 0.150(3.80) 0.157(4.00) pin 1 id 0.189(4.80) 0.197(5.00) 0.228(5.80) 0.244(6.20) 14 85 0.016(0.41) 0.050(1.27) 0 o -8 o detail "a" 0.010(0.25) 0.020(0.50) x 45 o 0.010(0.25) bsc gauge plane
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