AL8807B 查看數據表（PDF） - Diodes Incorporated.

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AL8807B

HIGH EFFICIENCY 36V 1.3A PWM DIMMABLE BUCK LED DRIVER

Diodes Incorporated. 

AL8807B

Applications Information (cont.)

PWM Dimming

LED current can be adjusted digitally, by applying a low frequency Pulse Width Modulated (PWM) logic signal to the CTRL pin to turn the device

on and off.

This will produce an average output current proportional to the duty cycle of the control signal. In particular, a PWM signal with a max resolution of

10bit can be applied to the CTRL pin to change the output current to a value below the nominal average value set by resistor RSET.

Figure 33 PWM Dimming waveforms (fPWM = 500Hz, 25% Duty Cycle fSW(NOM) = 530kHz)

While the PWM pin is high, the AL8807B switches as normal. When the PWM pin is brought low the output switch is turned off causing the SW pin

to go high (one Schottky voltage drop above VIN). It remains high (one Schottky voltage drop above VIN) until the current through the inductor falls

to zero. The time taken for the inductor current is dependent on the LED current, inductor value and LED chain voltage.

As the duty cycle gets smaller or PWM dimming frequency increases then fewer normal hysteretic switching cycles occur which will affect the

overall average LED current.

Figure 34 PWM Dimming waveforms (fPWM = 500Hz, 2% Duty Cycle fSW(NOM) = 530kHz)

To achieve high resolution the PWM frequency has to be much lower than the nominal switching frequency and the LED current output filter

capacitor across the LEDs must not be used. The figures above have an LED current output filter present.

AL88070B

Document number: DS36191 Rev. 1 - 2

10 of 18

www.diodes.com

March 2014

© Diodes Incorporated

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