MP24895 – STEP-DOWN WHITE-LED DRIVER WITH 6-36V INPUT VOLTAGE

MP24895 Rev. 1.0

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11

8/6/2015

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t0

Tsw

PWM Signal

t1t2

t0-t1: Delay time caused by transistor turning-off.

For MMBT3904, t1 is about 1us-2us

t1-t2: Delay time caused by signal transmission

(less than 1us).

Tsw: one switching period

The average LED current is proportional to duty

cycle of PWM signal. For good PWM dimming

linearity, inductor current has to achieve the peak

threshold during PWM on time. The minimum

PWM duty cycle can be estimated as below:

Dmin/fpwm=t1+t2+4*D*Tsw

Circuit Layout Consideration

Pay careful attention to the PCB board layout

placed close to the IN pin and RS pin to minimize

set current error. The input loop—including the

input capacitor, Schottky diode, and internal

MOSFET—should be as short as possible.

TYPICAL APPLICATION CIRCUITS

Figure 4: Drive 3 LEDs in Series, 1A Current Output