Diodes AL8808 - Manual (Page 14)

AL8808
Document number: DS35648 Rev. 2 - 2
14 of 20
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January 2013
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AL8808
Application Information
(cont.)
Capacitor Selection
The small size of ceramic capacitors makes them ideal for AL8808 applications. X7R type is recommended because it retains capacitance value
over wider voltage and temperature ranges than other types such as Y5V or Z5U. X5R is a useful compromise over a restricted temperature
range. Note that even X7R capacitance reduces significantly with increased DC bias voltage. At 50% of rated voltage, the capacitance loss is
between about 10% to 50% of nominal. Therefore it is often necessary to select a voltage rating which is at least twice the operating voltage.
Input Capacitor
In Figure 32, the input capacitor C1 is required as a reservoir. Diode D1 switches at a rate of typically up to 400kHz. The power supply has a
finite impedance, often including a wiring inductance value of the order of 100nH to 1uH or more depending upon the system design. C1 is
required to limit the power supply voltage and current ripple both to allow stable regulation of the LED current, and also to meet EMC
requirements.
A 2.2μF input capacitor is sufficient for most DC powered applications of AL8808. This depends upon the operating voltage and current and the
maximum level of ripple required. Additional capacitors may be required in parallel for EMC purposes. This is described below in a separate
section.
However, if operated from a rectified low voltage AC source, such as MR16, then the input capacitance will need to be significantly increased to
provide enough reservoir charge when the input voltage falls below the minimum operating voltage of the AL8808 or the LED chain voltage
Output Capacitor
In Figure 32, the output capacitor C2 is normally required to limit the load voltage and current ripple, in order to meet EMC requirements. A value
of 0.1µF to 1µF is sufficient for many requirements, depending on voltage and current conditions. Additional capacitors may be required in parallel
for EMC purposes. This is described below in a separate section.
Diode Selection
For maximum efficiency and performance, the flywheel rectifier (D1) should be a fast low capacitance Schottky diode with low reverse leakage at
the maximum operating voltage and temperature. The silicon PN diode is not suitable because of its increased power loss, due to a combination
of lower forward voltage and reduced recovery time. The use of a Super-Barrier-Rectifier (SBR) is not recommended for use as a flywheel diode
in this application. (However the SBR provides significant advantages when used with an AC power input as a bridge rectifier driving V
IN
.)
It is important to select D1 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 V
IN
to ensure
safe operation during the switching and a mean current rating at least 10% higher than the peak diode current. The power rating is verified by
calculating the power loss through the diode. In practice, the voltage rating selection is often increased by up to about 50% to obtain a better
compromise with loss due to reverse leakage current at higher temperature. Also the current rating is typically selected to provide a margin of up
to about 50%.
Schottky diodes, e.g. DFLS240L or DFLS140, with their low forward voltage drop and fast reverse recovery, are the ideal choice for AL8808
applications. Leakage current is sufficiently limited for the application.