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AL8805
Application Information (cont.)
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.
Figure 33 Soft Start with 22nF Capacitor on CTRL Pin (V IN = 36V, I LED = 667mA, 1 LED)
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 μ F 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 AL8805 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 μ F input capacitor is sufficient for most intended applications of AL8805; however a 4.7 μ F input capacitor is suggested for input voltages
approaching 36V.
AL8805
Document number: DS35030 Rev. 4 - 2
11 of 16
www.diodes.com
July 2012
? Diodes Incorporated
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