Diodes AL8807B - Manual (Page 12)

AL88070B
Document number: DS36191 Rev. 1 - 2
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AL8807B
Applications Information
(cont.)
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 AL8807B 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 AL8807B; however a 4.7μF input capacitor is suggested for input voltages
approaching 36V.
Diode Selection
For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance Schottky diode with low reverse leakage at the
maximum operating voltage and temperature. The Schottky diode also provides better efficiency than silicon PN diodes, due to a combination of
lower forward voltage and reduced recovery time.
It is important to select parts with a peak current rating above the peak coil current and a continuous current rating higher than the maximum
output load current. In particular, it is recommended to have a diode voltage rating at least 15% higher than the operating voltage to ensure safe
operation during the switching and a current rating at least 10% higher than the average diode current. The power rating is verified by calculating
the power loss through the diode.
Schottky diodes, e.g. B240 or B140, with their low forward voltage drop and fast reverse recovery, are the ideal choice for AL8807B applications.
Inductor Selection
Recommended inductor values for the AL8807B are in the range 33μH to 100μH.
Higher values of inductance are recommended at higher supply voltages in order to minimize errors due to switching delays, which result in
increased ripple and lower efficiency. Higher values of inductance also result in a smaller change in output current over the supply voltage range.
(See graphs).
Figure 37 Inductor value with input voltage and number of LEDs
The inductor should be mounted as close to the device as possible with low resistance/stray inductance connections to the SW pin.