AL8806

Document number: DS35144 Rev. 5 - 2

9 of 15

www.diodes.com

July 2013

© Diodes Incorporated

AL8806

Soft Start

The AL8806 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.

Reducing Output Ripple

Peak to peak ripple current in the LED(s) can be reduced, if required, by shunting a capacitor C2 across the LED(s) as shown already in the circuit

schematic.

A value of 1

μF will reduce the supply ripple current by a factor three (approx.). Proportionally lower ripple can be achieved with higher capacitor

values. Note that the capacitor will not affect operating frequency or efficiency, but it will increase

start-up delay, by reducing the rate of rise of

LED voltage. By adding this capacitor the current waveform through the LED(s) changes from a triangular ramp to a more sinusoidal version

without altering the mean current value.