AN30888A High Brightness LED Driver IC - Panasonic

Established : 2012-02-28Revised : 2013-02-18Doc No. TA4-EA-06057Revision. 3Product StandardsAN30888AOPERATION (continued)Functions and properties descriptions (continued)(7) Setting output LED current and choosing current sense resistor R CSThe LED current in this LSI can be set easily by selecting the appropriate R CS resistor to be used at CS pin of thischip.For Boost Mode and Buck-Boost mode :R CS resistor can be set in the following way :First is to calculate input current I IN at the required operating condition :I IN = (V OUT + V D ) (I LED / V IN ) …………………………… Eq[6] (Boost mode, Buck-Boost mode)V OUT = Output voltageV D = Schottky diode forward drop voltageI LED = Required LED currentV IN = Input supply voltageAfter which the peak input current, I PK can be determine by adding I IN with half the peak to peak ripple current atthe inductor.(V OUT – V IN ) T OFFI PK = I IN + ——————————— …………………… Eq[7] (Boost mode, Buck-Boost mode)2LT OFF = Fixed off time = 1 µsL = Inductor value found in part (6) inductor selectionV OUT = Output voltageV IN = Input supply voltageI IN = Input current found in Eq[6]Lastly, R CS resistor can be determine by using :V FBR CS = ——— ……………………………………………… Eq[8] (Boost mode, Buck-Boost mode)I PKV FB = Voltage at CS pin. Refer to data graphs for the V FB voltageat different input voltage condition.I PK = Peak current found in Eq[7]Using numeric example of operating condition :V IN =6V, V OUT =10V, I LED = 500 mA, T OFF = 1 µs, L = 16 µH, V D = 0.4 V, V FB =0.1V@V IN =6VFrom Eq[6] :From Eq[7] :From Eq[8] :I IN = (10 + 0.4) (0.5 / 6) = 0.8667 A(10 – 6) 1 µI PK = 0.8667 + ——————— = 0.9971 A2 16 µ0.1R CS = ———— = 100.8 m0.9917Page 15 of 21