Diodes AL8807 Manuel d'utilisateur télécharger pdf (Page 11)

AL8807

Document number: DS35281 Rev. 5 - 2

11 of 20

www.diodes.com

March 2013

L8807

Application Information (cont.)

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.

Figure 33 Soft start with 22nF capacitor on CTRL pin (V

= 30V, 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 1μF 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 2.2μF input capacitor is sufficient for most intended applications of AL8807; however a 4.7μF input capacitor is suggested for input voltages

approaching 36V.

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Diodes AL8807 Manuel d'utilisateur Page 11

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