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mp2481 36v, 1.2a, 1.4mhz white led driver mp2481 rev. 1.42 www.monolithicpower.com 1 4/28/2012 mps proprietary information. patent protected. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. the future of analog ic technology description the mp2481 is a 36v, 1.2a, white led driver suitable for either step-down or inverting step-up/down applications. it achieves 1.2a peak 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 thermal shutdown, cycle-by-cycle peak current limiting, input over voltage protection, open strings protection and output short circuit protection. the mp2481 incorporates both dc and pwm dimming onto a single control pin. the separate input reference ground pin allows for direct enable and/or dimming control for a positive to negative power conversion. the mp2481 requires a minimum number of readily available standard external components and is available in 8-pin msop8e package. evaluation board reference board number mode ev2481-h-00a step-down ev2481-h-01a step-up/down features ? 1.2a maximum output current ? unique step-up/down operation (buck- boost mode) ? wide 4.5v to 36v operating input range for step-down applications (buck mode) ? 0.30 ? internal power mosfet switch ? fixed 1.4mhz frequency ? analog and pwm dimming ? 0.2v reference voltage ? up to 95% efficiency ? 5 a shutdown mode ? no minimum led required ? stable with low esr output ceramic capacitors ? cycle-by-cycle over current protection ? thermal shutdown protection ? open strings protection ? input over voltage protection ? output short circuit protection ? available in 8-pin msop8e package applications ? general led illuminations ? lcd backlight panels ? handheld computers ? automotive internal lighting ? portable multimedia players ? portable gps devices 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 trademarks of monolithic powe r systems, inc. typical application (st ep-up/down application) mp2481 sw fb bst vss vdd ovp en/dim ingnd cbs 1 5 6 2 4 8 7 3 d1 3 series, 9 paralleled c1 vin 4.5v-36v l1 c2 r1 r2 rs off dc or pwm input on efficiency vs. input step-up/down application voltage 60 65 0 5 10 15 20 25 30 70 75 80 85 90 efficiency (%) input voltage (v) 4 series,400ma 3 series,400ma 3 series,9 paralleded,180ma total
mp2481 ?36v, 1.2a, 1.4mhz white led driver mp2481 rev. 1.42 www.monolithicpower.com 2 4/28/2012 mps proprietary information. patent protected. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. ordering information part number* package top marking MP2481DH msop8e 2481d * for tape & reel, add suffix ?z (e.g. MP2481DH?z). for rohs compliant packaging, add suffix ?lf (e.g. MP2481DH?lf?z) package reference vdd vss ovp fb exposed pad on back side * * do not connect to system ground in buck-boost mode 1 2 3 4 8 7 6 5 sw bst ingnd en/dim top view absolute maximum ratings (1) supply voltage v dd ? v ss .............................40v v sw ? v ss .............................?0.3v to v in + 0.3v v bst .................................................... v sw + 6v v ovp ? v ss ..................................... ?0.3v to +6v v en ? v ingnd .................................. ?0.3v to +6v v dim ? v ingnd ................................. ?0.3v to +6v v ingnd ? v ss .................................. ?0.3v to 40v continuous power dissipation (t a = +25c) (2) ??????????????????....2.3w junction temperature.............................. 150 c lead temperat ure ................................... 260 c storage temperature.............. ?65 c to +150 c recommended operating conditions (3) supply voltage v dd ? v ss ................ 4.5v to 36v operating junct. temp (t j )..... ?40 c to +125 c thermal resistance (4) ja jc msop8e (exposed pad) ........55 ...... 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 powe r dissipation will cause excessive die temperature, and the regulator will go into thermal shutdown. internal thermal shutdown circuitry protects the device from permanen t damage. 3) the device is not guaranteed to function outside of its operating conditions. 4) measured on jesd51-7, 4-layer pcb.
mp2481 ?36v, 1.2a, 1.4mhz white led driver mp2481 rev. 1.42 www.monolithicpower.com 3 4/28/2012 mps proprietary information. patent protected. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. electrical characteristics v in = 12v, t a = +25 c, all voltages with respect to v ss , unless otherwise noted. parameters symbol condition min typ max units feedback voltage v fb 4.5v v in 36v 0.187 0.203 0.219 v feedback current i fb v fb = 0.2v 0.2 a switch-on resistance (5) r ds(on) 0.30 ? switch leakage v en = 0v, v sw = 0v 10 a switch current limit (5) 2.0 a oscillator frequency f sw v fb = 0.1v 1.0 1.4 1.8 mhz fold-back frequency v ovp < 0.4v 120 khz maximum duty cycle v fb = 0.1v 89 % minimum on-time (5) t on 100 ns under voltage lockout threshold rising 3.1 3.4 3.7 v under voltage lockout threshold hysteresis 480 mv v en = 2v 2.0 en input current v en = 0v 0.1 a en off threshold (w/respect to ingnd) v en falling 0.4 v en on threshold (w/respect to ingnd) v en rising 0.7 v supply current (quiescent) i q v en = 2v, v fb = 1v 0.6 1.1 ma thermal shutdown (5) 150 c open led ov threshold v ovp, th 1.23 v open led ov hysteresis v ovp, hys 0.1 v note: 5) guaranteed by design.
mp2481 ?36v, 1.2a, 1.4mhz white led driver mp2481 rev. 1.42 www.monolithicpower.com 4 4/28/2012 mps proprietary information. patent protected. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. pin functions pin # name description 1 vdd supply voltage. the mp2481 operates from a +4.5v to +36v unregulated input (with respect to vss). c1 is needed to prevent large voltage spikes from appearing at the input. 2 vss power return pin. connect to the lowest potential in the circuit, which is typically the an ode of the schottky rectifier. 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 placed outside of the d1 to c1 ground path to prevent switching current spikes from inducing voltage noise into the part. the exposed pad is also connected to this pin. 3 ovp over voltage protection pin. use one external resistor voltage divider to program ovp threshold. when the ovp pin voltage (with respect to vss) is lower than 0.4v, the chip frequency will be folded back. program the ovp pin voltage from 0.4v to 1.23v for normal operation. when the ovp pin voltage reaches the shutdown threshold 1.23v, the switch will be turned off 4 fb led current feedback input. mp2481 regulates the voltage across the current sensing resistor between fb and vss. connect the current sensing resistor from the bottom of the led strings to vss. the fb pin is connected to the bottom of the led strings. the regulation voltage is 0.2v. 5 en/dim on/off control input and dimming command input. a voltage greater than 0.7v will turn on the chip. when the en/dim pin voltage (with respect to ingnd) rises from 0.7v to 1.4v, the led current will change from 0% to 100% of the maximum led current. to use pwm dimming, apply a 100hz to 1khz square wave signal with amplitude greater than 1.4v to this pin. 6 ingnd input ground reference. this pin is the reference for the en/dim signal. 7 bst bootstrap. a capacitor is connected between sw and bst pins to form a floating supply across the power switch driver. this capacitor is needed to drive the power switch?s gate above the supply voltage. 8 sw switch output. sw is the source of the internal mosfet switch. connect to the power inductor and cathode of the schottky rectifier.
mp2481 ?36v, 1.2a, 1.4mhz white led driver mp2481 rev. 1.42 www.monolithicpower.com 5 4/28/2012 mps proprietary information. patent protected. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. typical performance characteristics v in =12v, i led =1a, one 5w wled, step-down application (refer to figure 3), unless otherwise noted. steady state operation short circuit protection pwm dimming open load protection pwm dimming duty short led+ to ingnd f pwm =1khz,d pwm =50% f pwm =1khz v sw 10v/div. v pwm 5v/div. v out 2v/div. v out 2v/div. v sw 10v/div. v sw 10v/div. v fb 200mv/div. i inductor 500ma/div. i led 500ma/div. i led 1a/div. v out ac coupled 200mv/div. 4 400ns/div 4 2 v sw 10v/div. i inductor 500ma/div. efficiency vs. input voltage led current vs. analog dimming voltage led current vs. pwm dimming duty 60 0 0.4 0 0 0.2 0.4 0.6 0.8 1.0 1.2 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0 1.2 0.6 0.8 1.0 1.2 1.4 1.6 1.8 10 20 30 40 65 70 75 80 85 90 95 100 efficiency (%) led current (a) led current (a) input voltage (v) analog dimming voltage (v) 3 wleds in series,500ma 2 wleds in series,500ma 1 wled,1a 1 wled,500ma
mp2481 ?36v, 1.2a, 1.4mhz white led driver mp2481 rev. 1.42 www.monolithicpower.com 6 4/28/2012 mps proprietary information. patent protected. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. typical performance characteristics (continued) v in =12v, i led =180ma, 3 wleds in series, 9 strings, step-up/down application (refer to figure 4), unless otherwise noted. steady state operation short circuit protection pwm dimming open load protection pwm dimming duty short led+ to vss f pwm =1khz,d pwm =50% v sw 10v/div. v pwm 5v/div. v ss 2v/div. v ss 5v/div. v sw 20v/div. v sw 20v/div. i inductor 500ma/div. i led 200ma/div. i led 200ma/div. v ss ac coupled 200mv/div. 4 400ns/div 4 2 v sw 10v/div. i inductor 500ma/div. led current vs.analog dimming voltage led current vs. pwm dimming duty 0.4 0 0.2 0.4 0.6 0.8 1.0 0 20 40 60 80 100 120 140 160 180 200 0.6 0.8 1.0 1.2 1.4 1.6 1.8 led current (ma) analog dimming voltage (v) efficiency vs. input voltage 60 65 0 5 10 15 20 25 30 70 75 80 85 90 efficiency (%) input voltage (v) 0 20 40 60 80 100 120 140 160 180 200 led current (ma) f pwm =1khz 4 series,400ma 3 series,400ma 3 series,9 paralleded,180ma total
mp2481 ?36v, 1.2a, 1.4mhz white led driver mp2481 rev. 1.42 www.monolithicpower.com 7 4/28/2012 mps proprietary information. patent protected. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. function block diagram vdd en/dim ingnd fb vss sw vdd ovp bst enable 4.0v en por& regulator dimming control oscillator 1.4mhz/120khz ramp generator driver current sense amplifier error amplifier current limit comparator pwm comparator 1.2v d x20 m1 regulator reference (ven-0.7)/3.5 ea softstart -- + + -- + -- + s r rr q -- + 27pf 1pf 0.2v ovp -- + + figure 1?function block diagram
mp2481 ?36v, 1.2a, 1.4mhz white led driver mp2481 rev. 1.42 www.monolithicpower.com 8 4/28/2012 mps proprietary information. patent protected. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. operation the mp2481 is a current mode regulator. the ea output voltage is proportional to the peak inductor current. at the beginning of a cycle, m1 is off. the ea output 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 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 the current sense amplifier output and compared to the error amplifier output by the pwm comparator. when the sum of the current sense amplifier output and the slope compensation signal exceeds the ea output voltage, the rs flip-flop is reset and m1 is turned off. the external schottky rectifier diode (d1) conducts the inductor current. if the sum of the current sense amplifier output and the slope compensation signal does not exceed the ea output for a whole cycle, 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 0.2v reference. the polarity is such that a fb pin voltage lower than 0.2v increases the ea output voltage. since the ea output voltage is proportional to the peak inductor current, an increase in its voltage also increases current delivered to the output. open strings protection if led strings are open, there is no voltage on the fb pin. the duty cycle increases until v(vdd) - v(vss) reaches to 38v or v(ovp) - v(vss) reaches the shutdown threshold 1.23v. then the top switch is kept off. it will be switching again when the voltage v(vdd) - v(vss) and v(ovp) - v(vss) decreases sufficiently. dimming control the mp2481 allows both dc and pwm dimming. when v(en/dim) ? v(ingnd) is less than 0.4v, the chip is turned off. for analog dimming, when v(en/dim) ? v(ingnd) rises from 0.7v to 1.4v, the led current will change from 0% to 100% of the maximum led current. if v(en/dim) ? v(ingnd) is higher than 1.4v, maximum led current is generated. if a pwm signal is used, its amplitude v (en/dim) ? v (ingnd) must exceed 1.4v. the mp2481 is designed with an internal en shut-down delay (see figure 2). if en is pulled down, the fb reference drops to zero immediately and the led current decreases. but the ic keeps internal logic on for about 2.5ms. in this period, the ic may switch with very short pulses in pulse-skipping mode, while the led current is zero since the fb reference is zero. it has benefit that the ic will start up more quickly without delay when next pwm duty comes. this function offers the possibility to extend the pwm dimming frequency and also the minimum pwm on time for further dimming depth. figure 2? internal en shut-down delay function waveform diagram
mp2481 ?36v, 1.2a, 1.4mhz white led driver mp2481 rev. 1.42 www.monolithicpower.com 9 4/28/2012 mps proprietary information. patent protected. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. application information setting the led current the external resistor is used to set the maximum led current (see the schematic on front page) through the use of the equation: sense led 0.200v r= i setting the ovp point to make sure the chip functions properly, the over voltage protection (ovp) resistor divider must be set with proper value, the recommended ovp point is about 1.3~1.5 times higher than the output voltage for normal operation and over voltage protection. selecting the inductor (step-down applications, see figure 3) a 3.3h 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?s dc resistance should be less than 200m ? . refer to table 1 for suggested surface mount inductors. for most designs, the required inductance value can be derived from the following equation. sw l in out in out f i v ) v v ( v l ? = where v in is the lowest input voltage, v out is the output voltage, i l is the inductor ripple current. choose the inductor ripple current to be 30% of the maximum load current. the maximum inductor peak current is calculated from: 2 i i i l load ) max ( l + = if the calculation of above equations results in an inductance greater than 10uh, a 10uh inductor is still preferred. under light load conditions below 100ma, a larger inductance is recommended for improved efficiency. also note that the maximum recommended load current is 1a if the duty cycle exceeds 35%. (step up/down applications, see figure 4) operation at step-up/down mode, the recommended inductance value can be derived from the following equation: out in in out l sw -v v l= (v - v ) when v in is the lowest input voltage, v out is the output voltage (negative), i l is the inductor ripple current. for most step-up/down applications, a 3.3uh to 22uh with suitable dc current rating is recommended. see table 1 for suggested inductors. selecting the input capacitor the input capacitor reduces the surge current drawn from the input supply 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 through the input. ceramic capacitors with x5r or x7r dielectrics are highly recommended because of their low esr and small temperature coefficients. for most applications, a 4.7f capacitor is sufficient. table 1?suggested surface mount inductors manufacturer part number inductance(h) max dcr( ? ) current rating (a) dimensions l x w x h (mm 3 ) toko a921cy-4r7m 4.7 0.027 1.66 6 x 6.3 x 3 sumida cdrh4d28c/ld 4.7 0.036 1.5 5.1 x 5.1 x 3 wurth electronics 7440530047 4.7 0.038 2.0 5.8 x 5.8 x 2.8
mp2481 ?36v, 1.2a, 1.4mhz white led driver mp2481 rev. 1.42 www.monolithicpower.com 10 4/28/2012 mps proprietary information. patent protected. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. selecting the output capacitor the output capacitor keeps the output voltage ripple small and ensures feedback loop stability. the output capacitor impedance should be low at the switching frequency. ceramic capacitors with x5r or x7r dielectrics are recommended for their low esr characteristics. for most applications, a 2.2f ceramic capacitor will be sufficient. pc board layout considerations the high current paths (vss, vdd and sw) should be placed very close to the device with short, direct and wide traces. the input capacitor needs to be as close as possible to the vdd and vss pins. the external feedback resistors should be placed next to the fb pin. keep the switch node traces short and away from the feedback network.
mp2481 ?36v, 1.2a, 1.4mhz white led driver mp2481 rev. 1.42 www.monolithicpower.com 11 4/28/2012 mps proprietary information. patent protected. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. typical application circuits mp2481 sw fb bst vss vdd ovp en/dim ingnd c3 10nf 1 5 6 2 4 8 7 3 d1 b140 vin 6v-36v off dc or pwm input on 39 led1 10 0.2 1% figure 3?step-down white led driver application mp2481 sw fb bst vss vdd ovp en/dim ingnd c3 10nf 1 5 6 2 4 8 7 3 d1 10 1.1 b140 vin 4.5v-36v off dc or pwm input on 3 series, 9 paralleled figure 4?step-up/down white led driver application mp2481 sw fb bst vss vdd ovp en/dim ingnd c3 10nf 1 5 6 2 4 8 7 3 d1 r1 r2 b140 1.1 vin 4.5v-7.5v 100 10 off dc or pwm input on 3 series, 9 paralleled figure 5?optional step-up white led driver application
mp2481 ?36v, 1.2a, 1.4mhz white led driver notice: the information in this document is subject to change without notice. users should warrant 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. mp2481 rev. 1.42 www.monolithicpower.com 12 4/28/2012 mps proprietary information. patent protected. unauthorized photocopy and duplication prohibited. ? 2012 mps. all rights reserved. package information msop8e w/ exposed pad

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