Al8807a, Application information – Diodes AL8807A User Manual
Page 13
AL8807A
Document number: DS35990 Rev. 2 - 2
13 of 20
March 2013
© Diodes Incorporated
AL8807A
Application Information
(cont.)
Input Bulk Capacitor Selection
The small size of ceramic capacitors makes them ideal for AL8807A 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
μF input capacitor is sufficient for most intended applications of AL8807A; 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 AL8807A applications.
Inductor Selection
Recommended inductor values for the AL8807A are in the range 33
μH to 100μH.
Higher values of inductance are recommended at higher supply voltages as they result in lower switching frequencies which in turn reduce the
errors due to switching delays. Higher values of inductance also result in a smaller change in output current over the supply voltage range. (See
graphs).
Figure 38 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.
The chosen coil should have a saturation current higher than the peak output current and a continuous current rating above the required mean
output current.
Suitable coils for use with the AL8807A are listed in the table below:
Part No.
L
(µH)
DCR
(V)
I
SAT
(A)
Manufacturer
MSS1038-333 33
0.093
2.3
MSS1038-683 68
0.213
1.5
NPIS64D330MTRF 33 0.124 1.1