AL8805

HIGH EFFICIENCY 30V 1A BUCK LED DRIVER

AL8805

Document number: DS35030 Rev. 1 - 2

11 of 16

www.diodes.com

September 2010

© Diodes Incorporated

PWM Dimming (cont.)

The PWM pin is designed to be driven by both 3.3V and

5V logic levels directly from a logic output with either an

open drain output or push-pull output stage.

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Soft Start

The AL8805 does not have in-built soft-start action – this

provides very fast turn off of the output the stage improving

PWM dimming accuracy; nonetheless, adding an external

capacitor from the CTRL pin to ground will provide a soft-

start delay. This is achieved by increasing the time taken for

the CTRL voltage to rise to the turn-on threshold and by

slowing down the rate of rise of the control voltage at the

input of the comparator. Adding a capacitor increases the

time taken for the output to reach 90% of its final value, this

delay is 0.1ms/nF, but will impact on the PWM dimming

accuracy depending on the delay introduced.