data sheet 26185.300b A8430 constant current led driver boost converter use the following complete part number when ordering: ab so lute max i mum rat ings sw pin ................................................ ?0.3 v to 36 v remaining pins .................................. ?0.3 v to 10 v package thermal impedance: two-sided pcb 1 ........................... ja tbaoc / w ?high-k? pcb 2 ............................ ja tbaoc / w operating temperature range ambient temperature, t a ............. ?40c to +85c junction temperature, t j .................. +150c max. storage temperature, t s .......... ?55c to +150c 1 measured on a typical two-sided pcb with 2 in. 2 copper ground plane. 3 measured on a jedec-standard ?high-k? 4-layer pcb. A8430eek mlpd advance information data sheet subject to change without notice january 20, 2004 output voltage up to 36 v 2.5 v to 10 v input drives up to 4 leds from a 3.2 v supply drives up to 6 leds from a 5 v supply 1.2 mhz switching frequency 200 ma switch current limit 1 a shutdown current features led backlights portable battery-powered equipment cellular phones pdas (personal digital assistant) camcorders, personal stereos, mp3 players, cameras mobile gps systems applications the A8430 is a noninverting boost converter that steps-up the input voltage, to provide a programmable constant current output at up to 36 v for driving white leds in series. driving leds in series ensures identical currents and uniform brightness. up to four white leds can be driven from a single cell li-ion or a multicell nimh power source. up to seven white leds can be driven by increasing the supply voltage up to 10 v. the A8430 incorporates a power switch and feedback sense ampli er to provide a solution with minimum external components. the output current can be set by adjusting a single external sense resistor and can be varied with a voltage or ltered pwm signal when dimming control is required. the high switching frequency of 1.2 mhz allows the use of small inductor and capacitor values. the A8430 is available in a low pro le 5-lead mlp package (part num- ber suf x ek ). 5 1 2 3 4 sw gnd fb vin en part number package description A8430eek 5-pin, mlpd surface mount same pad footprint as sot-23-5.
2 worcester, massachusetts 01615-0036 (508) 853-5000 115 northeast cutoff, box 15036 www.allegromicro.com data sheet advance information 26185.300b A8430 constant current led driver boost converter functional block diagram vin fb enable sw v ref 95 mv switch control osc en gnd electrical characteristics at t a = 25c, v in = 3 v (unless otherwise noted) characteristics symbol test conditions min. typ. max. units input voltage range v in ? 2.5 ? 10 v supply current i sup active: i load = 15 ma, v load = 12 v ? 2.5 3.5 ma shutdown (en = 0 v) ? 0.1 1 a feedback reference voltage v ref i sw = 100 ma, d = 66% 86 95 104 mv feedback input current i fb ? ? 45 100 na switch current limit i swlim ? ? 200 ? ma switch frequency f sw ? 0.8 1.2 1.6 mhz switch maximum duty cycle d ? 85 90 ? % switch saturation voltage v cesat ? ? 350 ? mv switch leakage current i sl ???5a enable input input threshold low v il ? ? ? 0.4 v input threshold high v ih ? 1.5 ? ? v input leakage i il ? ? 65 ? a
3 worcester, massachusetts 01615-0036 (508) 853-5000 115 northeast cutoff, box 15036 www.allegromicro.com data sheet advance information 26185.300b A8430 constant current led driver boost converter pin functions (refer to schematic diagram 1.) vin. supply to the control circuit. a bypass capacitor must be connected from close to this pin to gnd. gnd. ground reference connected directly to the ground plane. the sense resistor (r1) should have a separate connection directly to this point. en. setting lower than 0.4 v disables the A8430 and puts the control circuit into the low-power sleep mode. greater than 1.5 v fully enables the A8430. sw. low-side switch connection between the inductor (l1) and ground. because rapid changes of current occur at this pin, the traces on the pcb that are connected to this pin should be minimized. in addition, the inductor (l1) and diode (d1) should be connected as close to this pin as possible. fb. feedback pin for led current control. the reference voltage is 95 mv. the top of the sense resistor (r1) is typically connected to this pin. device operation the A8430 uses a constant-frequency, current-mode control scheme to regulate the current through the load. the load current produces a voltage across the external sense resistor (r1) and the input at the fb pin. this voltage is then compared to the internal 95 mv reference to produce an error signal. the switch current is sensed by the internal sense resistor and compared to the load current error signal. as the load current increases, the error signal diminishes, reducing the maximum switch current and thus the current delivered to the load. as the load current decreases, the error signal rises, increasing the maximum switch current and thus increasing the current delivered to the load. to set the load current, ensure that the required internal reference value of 95 mv is produced at the desired load. to do so, select a resistance value for the sense resistor, r1 ( ? ), such that: r1 = 95 mv / i load where i load is the target load current (ma). the following table shows typical values for r1. note that the resistance value is from the standard e96 series. target load current ( i load ) (ma) sense resistor (r1) ( ? ) 5 19.1 10 9.53 12 7.87 15 6.34 20 4.75 as load current is reduced, the energy required in the inductor diminishes, resulting in the inductor current dropping to zero for low load current levels. this is known as discontinuous mode operation, and results in some low-frequency ripple. the average load current, however, remains regulated down to zero. in discontinuous mode, when the inductor current drops to zero, the voltage at the sw pin rings, due to the capacitance in the resonant lc circuit formed by the inductor and the capacitance of the switch and the diode. this ringing is low-frequency and is not harmful. it can be damped with a resistor across the inductor, but this reduces ef ciency and is not recommended. functional description schematic 1. typical application A8430 vin sw en gnd fb li-ion 2.5v to 4.2v c1 1f l1 22h d1 c2 0.22f r1 6r3 enable ?
4 worcester, massachusetts 01615-0036 (508) 853-5000 115 northeast cutoff, box 15036 www.allegromicro.com data sheet advance information 26185.300b A8430 constant current led driver boost converter component selection the component values shown in schematic 1 are suf cient for most applications. to reduce the output ripple the inductor may be increased, but in most cases this results in excessive board area and cost. inductor selection. with an internal pwm frequency of 1.2 mhz, the optimal inductor value for most cases is 22 h. the inductor should have low winding resistance, typically < 1 ? , and the core should have low losses when operating at 1.2 mhz. for worst case conditions, high output voltage and current and low input voltage, the inductor should be rated at the switch current limit, i swlim . if high temperature operation is required a derating factor will have to be considered. in some cases, where lower inductor currents are expected, the current rating can be decreased. several inductor manufacturers have and are developing suitable small-size inductors, including: murata, panasonic, sumida, taiyo yuden, and tdk. diode selection. the diode should have a low forward voltage to reduce conduction losses. in addition, it should have a low capacitance to reduce switching losses. schottky diodes can provide both these features, if carefully selected. the forward voltage drop is a natural advantage for schottky diodes, and it reduces as the current rating increases. however, as the current rating increases, the diode capacitance also increases. as a result, the optimal selection is usually the lowest current rating above the circuit maximum. with the A8430, a current rating in the range from 100 ma to 200 ma is usually suf cient. capacitor selection. because the capacitor values are low, ceramic capacitors are the best choice for use with the A8430. to reduce performance variation as temperature changes, low drift capacitor types, such as x7r and x5r, should be used. suitable capacitors are available from: taiyo yuden, murata, kemet, and avx. dimming control led brightness can be controlled either by modifying the voltage at the top of the sense resistor (r1) to control the application information schematic 2. dimming control with dc voltage feedback modulation A8430 vin sw en gnd fb li-ion 2.5v to 4.2v c1 1f l1 22h d1 c2 0.22f r1 6r3 v c r3 90k r2 5k enable ? ? ? led current, i load , directly, or by using a pwm signal on the en pin to chop the output. feedback modulation. by adding a voltage drop between the fb pin and r1 (the sense resistor), as shown in sche- matic 2, the led current, i load , can be made to decrease. as v c (control voltage) increases, the voltage drop across r2 also increases. this causes the voltage at fb to increase, and the A8430 reduces i load to compensate. as v c increases further, the current drops to zero, and r2 maintains the full 95 mv on fb. reducing v c diminishes the voltage across r2 until, at 95 mv on v c , there is no drop across r2 and the cur- rent level is de ned by r1. reducing v c below 95 mv causes i load to increase further, due to the voltage drop across r2 in the reverse direction. this continues until, at zero volts on v c , there is approximately 5 mv across r2. at that point, i load (ma), is de ned as: i load = 100 mv / r1 where r1 is the resistance of the sense resister ( ? ). pwm control . led dimming control can also be gener- ated by a ltered pwm signal as shown in schematic 3. in this case, a 0% duty cycle (pwm = 0 v) corresponds to full brightness and a 100% duty cycle causes the led current, i load , to go to zero.
5 worcester, massachusetts 01615-0036 (508) 853-5000 115 northeast cutoff, box 15036 www.allegromicro.com data sheet advance information 26185.300b A8430 constant current led driver boost converter schematic 3. dimming control with ltered pwm A8430 vin sw en gnd fb li-ion 2.5v to 4.2v c1 1f l1 22h d1 c2 0.22f r1 6r3 v c (pmw) r3 90k r2 5k enable r4 10k c3 100 nf ? ? ? ? schematic 4. soft start operation A8430 vin sw en gnd fb li-ion 2.5v to 4.2v c1 1f l1 22h d1 c2 0.22f r1 6r3 r3 5k r2 1k enable c3 2.2 nf ? ? ? by applying a pwm signal directly to the en pin, the A8430 is turned on or off, and i load is either full (as de ned by r1) or zero. by varying the duty cycle of the pwm signal, the led brightness can be controlled from off (0% duty cycle) to full (100% duty cycle). the pwm frequency should be in the range from 1 khz to 10 khz. several other schemes are possible, for example, digitally switching additional resistors across r1 to increase i load . in this case, r1 would be selected for the minimum desired brightness. soft start-up to provide fast start-up operation, no soft start is implemented in the control circuit. at power-on, the input capacitor (c1) is discharged, which means that the supply must provide the in-rush current through the inductor. this can be reduced by modulating the feedback with a soft-start circuit as shown in schematic 4. when power is rst applied, the capacitor c3 is discharged and pulls the fb pin high, reducing the output drive to minimum. as c3 charges, when the bottom drops below about 0.8 v, the feedback from the sense resistor (r1) takes over full control of the output current.
6 worcester, massachusetts 01615-0036 (508) 853-5000 115 northeast cutoff, box 15036 www.allegromicro.com data sheet advance information 26185.300b A8430 constant current led driver boost converter package ek terminal list table pin name function 1 sw internal power fet 2 gnd ground 3 fb feedback input 4 en enable input 5 vin input supply 3.15 2.85 0.124 0.112 .50 .30 0.020 0.012 .50 .30 0.020 0.012 .95 bsc 0.037 .80 .70 0.031 0.028 .05 max 0.002 .20 ref 0.008 2 5 1 2 5 1 dimensions in millimeters u.s. customary dimensions (in.) in brackets, for reference only a exposed thermal pad a
7 worcester, massachusetts 01615-0036 (508) 853-5000 115 northeast cutoff, box 15036 www.allegromicro.com data sheet advance information 26185.300b A8430 constant current led driver boost converter the products described here are manufactured under one or more u.s. patents or u.s. patents pending. allegro microsystems, inc. reserves the right to make, from time to time, such de par tures from the detail spec i ca tions as may be required to permit improvements in the per for mance, reliability, or manufacturability of its products. before placing an order, the user is cautioned to verify that the information being relied upon is current. allegro products are not authorized for use as critical compo- nents in life-support devices or sys tems without express written approval. the in for ma tion in clud ed herein is believed to be ac cu rate and reliable. how ev er, allegro microsystems, inc. assumes no re spon - si bil i ty for its use; nor for any in fringe ment of patents or other rights of third parties which may result from its use. copyright?2003, 2004 allegromicrosystems, inc.
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