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mp3425 3a, 55v boost converter with programmable switching frequency and uvlo mp3425 rev.1.02 www.monolithicpower.com 1 6/28/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. the future of analog ic technology description the mp3425 is a current mode step-up converter with a 3.5a, 90m ? internal switch to provide a highly efficient regulator with fast response. the mp3425 features a programmed frequency up to 2mhz allowing for easy filtering and low noise. an external compensation pin gives the user flexibility in setting loop dynamics, and operates with small, low-esr ceramic output capacitors. soft-start results in small inrush current and can be programmed with an external capacitor. the mp3425 operates from an input voltage as low as 3.1v and can generate 48v at up to 350ma from a 12v supply. the mp3425 includes under-voltage lockout, current limiting and thermal overload protection to prevent damage in the event of an output overload. the mp3425 is available in a low profile 14-pin 3x4mm qfn package with exposed pad. features ? 3.5a, 90m ? , 55v power mosfet ? uses tiny capacitors and inductors ? wide input range: 3.1v to 22v ? output voltage as high as 55v ? programmable fsw: 300khz ? 2 mhz ? programmable uvlo, soft-start, uvlo hysteresis ? micropower shutdown <1 a ? thermal shutdown 160 c ? available in 14-pin 3x4mm qfn package applications ? telecom ? power supply ? audio - microphone and tuner bias ? automotive 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 vin en fset ss mp3425 pgnd pgnd pgnd ep agnd sw sw sw vin comp fb vdd u1 2 14 12 8 9 10 15 11 1 13 7 4 5 6 3 r1 100k 0805 100v c2 33nf r2 383k r3 10k r6 20k c7 3.3nf d1 50v, 3a r4 51k r5 150k c5 33nf vout
mp3425- 3a, 55v boost converter with programmable switching frequency and uvlo mp3425 rev.1.02 www.monolithicpower.com 2 6/28/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. ordering information part number* package top marking free air temperature (t a ) MP3425DL 3x4mm qfn 14 3425 -40 c to +85 c * for tape & reel, add suffix ?z (e.g. MP3425DL?z); for rohs compliant packaging, add suffix ?lf (e.g. MP3425DL?lf?z) package reference 14 13 12 11 10 9 8 1 2 3 4 5 6 7 top view exposed pad comp en vin sw sw sw vdd fset fb ss agnd pgnd pgnd pgnd 3x4mm qfn 14 absolute maxi mum ratings (1) sw ................................................-0.5v to +55v in ................................................-0.5v to +22v all other pins ...............................-0.3v to +6.5v continuous power dissipation (t a = +25c) (2) qfn14 ....................................................... 2.5w junction temperature ...............................150 c lead temperature ....................................260 c storage temperature............... -65 c to +150 c recommended operating conditions (3) supply voltage v in ...........................3.1v to 22v output voltage v out .........................3.1v to 55v operating junct. temp (t j )..... - 40 c to +125 c thermal resistance ja jc qfn14 ....................................50 ...... 12 ... 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.
mp3425- 3a, 55v boost converter with programmable switching frequency and uvlo mp3425 rev.1.02 www.monolithicpower.com 3 6/28/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. electrical characteristics v in = v en = 5v, t a = +25 c, unless otherwise noted. parameter symbol condition min typ max units operating input voltage v in 3.1 22 v undervoltage lockout v in rising 2.8 3.1 v undervoltage lockout hysteresis 250 mv vdd voltage gate drive voltage supply v vdd c = 10nf 6 v supply current (shutdown) v en = 0v 1 a supply current (quiescent) v fb = 1.35v 650 900 a switching frequency fset = 84.5k ? 0.44 0.55 0.66 mhz minimum off time v fb = 0v 100 ns minimum on time v fb = 1.35v 100 ns en high threshold v en rising (switching) 1.45 1.5 1.55 v en high threshold v en rising (micro power) 1.0 v en low threshold 0.5 v en input bias current v en = 0v, 5v 0.1 1 a uvlo hysteresis current into en pin 1.0 < en < 1.4 4 a soft-start current 5 a fb voltage 1.200 1.225 1.250 v fb input bias current -200 -100 na error amp voltage gain a vea 300 v/v error amp transconductance ( a v ) g ea 160 mho error amp output current 20 a gcs : i(sw) / vcomp g cs 9 a/v sw on-resistance r on 90 m ? sw current limit duty cycle = 0% 3.5 5 a thermal shutdown 160 c
mp3425- 3a, 55v boost converter with programmable switching frequency and uvlo mp3425 rev.1.02 www.monolithicpower.com 4 6/28/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. pin functions qfn14 pin # name description 1 comp compensation pin. connect a capacitor and resistor in series to analog ground for loop stability. 2 en regulator on/off control input. a high input at en turns on the converter, and a low input turns it off. when not used, connect en to the input so urce (through a 100k ? pull- up resistor if v in > 6v) for automatic startup. en pi n can also be used to program vin uvlo. en cannot be left floating. 3 vin input supply pin. in must be locally bypassed. 4, 5, 6 sw power switch output. sw is the drain of the internal mosfet switch. connect the power inductor and output rectifier to sw. 7 vdd ldo output 8, 9, 10, exposed pad pgnd the bottom exposed pad is the power ground. for best thermal resistance, solder the exposed pad to underlying copper backplane 11 agnd analog ground. connect to ground plane through exposed pad. 12 ss soft-start control pin. connect a soft-start capa citor to this pin. the soft-start capacitor is charged with a constant current of 5 a. leave ss disconnected if the soft-start is not used. 13 fb feedback input. reference voltage is 1.25v. connect a resistor divider this pin. 14 fset frequency programming pin. connect a resistor from this pin to agnd. fset pin voltage is internally regulated to 0.5v. the curr ent flowing out of this pin linearly sets the operation frequency. 15 ep exposed pad. the bottom exposed pad is the power ground. for best thermal resistance, solder the exposed pad to underlying cooper backplane.
mp3425- 3a, 55v boost converter with programmable switching frequency and uvlo mp3425 rev.1.02 www.monolithicpower.com 5 6/28/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. typical performanc e characteristics v in =12v, v out =48v, l=33 h, c out =4.7 f, f sw =300khz, t a =+25 c, unless otherwise noted.
mp3425- 3a, 55v boost converter with programmable switching frequency and uvlo mp3425 rev.1.02 www.monolithicpower.com 6 6/28/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. typical performanc e characteristics (continued) v in =12v, v out =48v, l=33 h, c out =4.7 f, f sw =300khz, t a =+25 c, unless otherwise noted.
mp3425- 3a, 55v boost converter with programmable switching frequency and uvlo mp3425 rev.1.02 www.monolithicpower.com 7 6/28/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. typical performanc e characteristics (continued) v in =12v, v out =48v, l=33 h, c out =4.7 f, f sw =300khz, t a =+25 c, unless otherwise noted.
mp3425- 3a, 55v boost converter with programmable switching frequency and uvlo mp3425 rev.1.02 www.monolithicpower.com 8 6/28/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. block diagram pwm control logic current sense amp oscillator internal regulator and enable circuitry -- + gm + -- gnd fb comp ss 1.225v sw in -- + fset en figure 1?function block diagram
mp3425- 3a, 55v boost converter with programmable switching frequency and uvlo mp3425 rev.1.02 www.monolithicpower.com 9 6/28/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. application information components referenced below apply to the ? typical application circuit ?. theory of operation the mp3425 uses a constant frequency, peak current mode boost regulation architecture to regulate the feedback voltage. the operation of the mp3425 can be understood by referring to the block diagram of functional. at the beginning of each cycle, the n-channel mosfet switch is turned on, forcing the inductor current to rise. the current at the source of the switch is internally measured and converted to a voltage by the current sense amplifier. that voltage is compared to the error voltage at comp. the voltage at the output of the error amplifier is an amplified version of the difference between the 1.225v reference voltage and the feedback voltage. when theses two voltages are equal, the pwm comparator turns off the switch forcing the inductor current to the output capacitor through the external rectifier. this causes the inductor current to decrease. the peak inductor current is controlled by the voltage at comp, which in turn is controlled by the output voltage. thus the output voltage is regulated by the inductor current to satisfy the load. the use of current mode regulation improves the transient response and control loop stability. selecting the switching frequency the switching frequency is set by r5. the equation is: () ? = 0.86 set f23r5 where r5 is in k ? uvlo hysteresis the mp3425 features a programmable uvlo hysteresis. upon power up a 4a current sink is applied to the resistor divider attached to the en pin. this means that on power up vin must increase by an extra amount to overcome the current sink. that extra amount is the current sink times the resistor from vin to en. once the en pin reaches about 1.5v the current sink will turn off to create the reverse hysteresis for vin falling: t o p u v l o h y s t e r e s i s 4 a r =? table1?frequency selection r5 (k ? ) freq (mhz) 180 0.26 160 0.29 150 0.31 143 0.32 66.5 0.62 35.7 1.06 25 1.44 18 1.91 16 2.12 14 2.37 selecting the soft-start capacitor the mp3425 includes a soft-start timer that limits the voltage at comp during startup to prevent excessive current at the input. this prevents premature termination of the source voltage at startup due to input current overshoot. when power is applied to the mp3425, and enable is asserted, a 5a internal current source charges the external capacitor at ss. as the ss capacitor is charged, the voltage at ss rises. when the ss voltage reaches 250mv, the mp3425 starts switching at ? of the programmed frequency (frequency fold back mode). at 800mv the switching frequency becomes the programmed value. the soft-start ends when the voltage at ss reaches 2.5v. this limits the inductor current at start-up, forcing the input current to rise slowly to the current required to regulate the output voltage.
mp3425- 3a, 55v boost converter with programmable switching frequency and uvlo mp3425 rev.1.02 www.monolithicpower.com 10 6/28/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. the soft-start period is determined by the equation: 9 ss ss c102.5v t 5a ? = where c ss (nf) is the soft-start capacitor from ss to gnd, and t ss is the soft-start period. setting the output voltage this is the actual output voltage. it is fed back through two sense resistors in series. the feedback voltage is 1.225v typical. the equation to the output voltage is: out ref r2 vv(1) r3 =+ where: r2 is the top feedback resistor r3 in the bottom feedback resistor v ref is the reference voltage (1.225v typical) choose the feedback resistors to be in the 10k or higher range for good efficiency. selecting the input capacitor an input capacitor is required to supply the ac ripple current to the inductor, while limiting noise at the input source. a low esr capacitor is required to keep the noise at the ic to a minimum. ceramic capacitors are preferred, but tantalum or low-esr electrolytic capacitors may also suffice. use an input capacitor value greater than 4.7f. the capacitor can be electrolytic, tantalum or ceramic. however since it absorbs the input switching current it requires an adequate ripple current rating. use a capacitor with a rms current rating greater than the inductor ripple current (see ?selecting the inductor? to determine the inductor ripple current). to insure stable operation place the input capacitor as close to the ic as possible. alternately a smaller high quality ceramic 0.1 f capacitor may be placed closer to the ic with the larger capacitor placed further away. if using this technique, it is recommended that the larger capacitor be a tantalum or electrolytic type. all ceramic capacitors should be placed close to the mp3425. selecting the output capacitor the output capacitor is required to maintain the dc output voltage. low esr capacitors are preferred to keep the output voltage ripple to a minimum. the characteristic of the output capacitor also affects the stability of the regulation control system. ceramic, tantalum, or low esr electrolytic capacitors are recommended. in the case of ceramic capacitors, the impedance of the capacitor at the switching frequency is dominated by the capacitance, and so the output voltage ripple is mostly independent of the esr. the output voltage ripple is estimated to be: in load out ripple out sw v (1 ) i v v cf ? ? where vripple is the output ripple voltage, vin and vout are the dc input and output voltages respectively, iload is the load current, fsw is the switching frequency, and c out is the capacitance of the output capacitor. in the case of tantalum or low-esr electrolytic capacitors, the esr dominates the impedance at the switching frequency, and so the output ripple is calculated as: in load out load esr out ripple out sw in v (1 ) i virv v cf v ? ?+ where r esr is the equivalent series resistance of the output capacitors. choose an output capacitor to satisfy the output ripple and load transient requirements of the design. a 4.7f ? 22f ceramic capacitor is suitable for most applications. selecting the inductor the inductor is required to force the higher output voltage while being driven by the input voltage. a larger value inductor results in less ripple current that results in lower peak inductor current, reducing stress on the internal n-channel switch. however, the larger value inductor has a larger physical size, higher series resistance, and/or lower saturations current.
mp3425- 3a, 55v boost converter with programmable switching frequency and uvlo mp3425 rev.1.02 www.monolithicpower.com 11 6/28/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. a good rule of thumb is to allow the peak-to-peak ripple current to be approximately 30-50% of the maximum input current. make sure that the peak inductor current is below 75% of the current limit at the operating duty cycle to prevent loss of regulation due to the current limit. also make sure that the inductor does not saturate under the worst-case load transient and startup conditions. calculate the required inductance value by the equation: in out in out sw v(v v) l vf i ? = out load(max) in(max) in vi i v = where : i load (max) is the maximum load current i is the peak-to-peak inductor ripple current i = (30% - 50%) x i load (max) ? is efficiency. selecting the diode the output rectifier diode supplies current to the inductor when the internal mosfet is off. to reduce losses due to diode forward voltage and recovery time, use a schottky diode with the mp3425. the diode should be treated for a reverse voltage equal to or greater than the output voltage used. the average current rating must be greater than the maximum load current expected, and the peak current rating must be greater than the peak inductor current. compensation the output of the transconductance error amplifier (comp) is used to compensate the regulation control system. the system uses two poles and one zero to stabilize the control loop. the poles are f p1 set by the output capacitor c out and the load resistance and f p2 set by the compensation capacitor c comp and the compensation resistor r comp . these are determined by the equations: p1 load out 1 f 2rc = (hz) ea p2 vea comp g f 2ac = (hz) z1 comp comp 1 f 2r c = (hz) where r load is the load resistance, g ea is the error amplifier transconductance, and a vea is the error amplifier voltage gain. the dc loop gain is vea in load fb cs vdc 2 out a vr v g a 0.5 v = (v/v) where g cs is the compensation voltage to inductor current gain, and the v fb is the feedback regulation threshold. there is also a right-half-plane zero (f rhpz ) that exists in continuous conduction mode (inductor current does not drop to zero on each cycle) step-up converters. the frequency of the right half plane zero is: 2 load in rhp out r v f() 2lv = (hz) the ?component selection? table lists generally recommended compensation components for different input voltages, output voltages and capacitances of most frequently used output ceramic capacitors. ceramic capacitors have extremely low esr, therefore the second compensation capacitor (from comp to gnd) is not required.
mp3425- 3a, 55v boost converter with programmable switching frequency and uvlo mp3425 rev.1.02 www.monolithicpower.com 12 6/28/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. table 2?component selection v in (v) v out (v) c out (f) rcomp (k ? ) ccomp (nf) switching frequency (khz) inductor (h) 3 12 4.7 10 6.8 600 8.2 3 12 10 15 6.8 600 8.2 3 12 22 30 6.8 600 8.2 5 12 10 12 4.9 600 6.8 5 12 22 25 4.9 600 6.8 5 18 4.7 12 4.9 600 10 5 18 10 25 4.9 600 10 5 18 22 50 4.9 600 10 12 24 4.7 10 6.8 600 10 12 24 10 20 6.8 600 10 12 24 22 40 6.8 600 10 12 48 4.7 30 4.7 600 33 12 48 10 60 4.7 600 33 12 48 22 60 10 600 33 for faster control loop and better transient response, set the capacitor c7 to the recommended value in the table. then slowly increase the resistor r6 and check the load step response on a bench to make sure the ringing and overshoot on the output voltage at the edge of the load steps is minimal. finally, the compensation needs to be checked by calculating the dc loop gain and the crossover frequency. the crossover frequency where the loop gain drops to 0db (a gain of 1) can be obtained visually by placing a -20db/decade slope at each pole, and a +20db/decade slope at each zero. the crossover frequency should be at least one decade below the frequency of the right-half-plane zero at maximum output load current to obtain high enough phase margin for stability. layout considerations high frequency switching regulators require very careful layout for stable operation and low noise. all components must be placed as close to the ic as possible. keep the path between l1, d1, and c out extremely short for minimal noise and ringing. c in must be placed close to the in pin for best decoupling. all feedback components must be kept close to the fb pin to prevent noise injection on the fb pin trace. the ground return of c in and c out should be tied close to the gnd pin. see the mp3425 demo board layout for reference.
mp3425- 3a, 55v boost converter with programmable switching frequency and uvlo 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. mp3425 rev.1.02 www.monolithicpower.com 13 6/28/2013 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2013 mps. all rights reserved. package information qfn 14 side view top view 1 14 8 7 bottom view 2. 90 3. 10 1.60 1.80 3.90 4.10 3.20 3.40 0.50 bsc 0.18 0.30 0.80 1.00 0.00 0.05 0.20 ref pin 1 id marking 1.70 0.50 0.25 recommended land pattern 2.90 note: 1) all dimensions are in millimeters . 2) exposed paddle size does not include mold flash . 3) lead coplanarity shall be 0.10 millimeter max. 4) drawing conforms to jedec mo-229, variation veed-5. 5) drawing is not to scale. pin 1 id see detail a 3.30 0.70 pin 1 id option b r0.20 typ. pin 1 id option a 0.30x45 o typ. detail a 0.30 0.50 pin 1 id index area

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