al8807 Diodes, Inc., al8807 Datasheet - Page 11

• Manufacturer Part Number: al8807
• Description: High Efficiency Low Emi 30v 1a Buck Led Driver
• Manufacturer: Diodes, Inc.

AL8807
HIGH EFFICIENCY LOW EMI 30V 1A BUCK LED DRIVER
Applications 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.
Fig. 33 Soft start with 22nF capacitor on CTRL pin (V
= 30V, I
= 667mA, 1 LED)
IN
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 30V.
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AL8807
November 2011
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© Diodes Incorporated
Document Number: DS35281 Rev. 3 - 2