Al8807, Application information – Diodes AL8807 User Manual
Page 12
AL8807
Document number: DS35281 Rev. 5 - 2
12 of 20
March 2013
© Diodes Incorporated
AL8807
Application Information
(cont.)
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 AL8807 applications.
Inductor Selection
Recommended inductor values for the AL8807 are in the range 33
μH 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 34 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 AL8807 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
The inductor value should be chosen to maintain operating duty cycle and switch 'on'/'off' times over the supply voltage and load current range.