AL8807

HIGH EFFICIENCY LOW EMI 36V 1A BUCK LED DRIVER

AL8807

Document number: DS35281 Rev. 4 - 2

11 of 21

www.diodes.com

June 2012

© Diodes Incorporated

Soft Start

The AL8807 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.

Capacitor Selection

The small size of ceramic capacitors makes them ideal for AL8807 applications. X5R and X7R types are recommended

because they retain their capacitance over wider voltage and temperature ranges than other types such as Z5U.

A 2.2

μF input capacitor is sufficient for most intended applications of AL8807; however a 4.7μF input capacitor is suggested

for input voltages approaching 36V.