Diodes AP8800A - Manual (Page 7)

AP8800A
Document number: DS35100 Rev. 3 - 2
7 of 13
www.diodes.com
August 2012
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AP8800A
Application Information
(cont.) (@T
A
= +25°C, V
IN
= 12V, unless otherwise specified.)
Inductor Selection
A 68 μH inductor or higher, is recommended for most AP8800A applications with input voltage at 12V.
Figure 3 displays the resulting switching frequency with various main circuit parameters: Supply voltage, Inductor value and number of LEDs to
be driven.
Switching Frequency
47
68
100
470
100
150
470
150
220
220
0
100
200
300
400
500
600
700
0 50 100 150 200 250 300 350 400 450 500
Inductor Value [uH]
Fr
e
q
ue
n
c
y
[
k
H
z
]
12V - 1LED
24V - 3 LEDs
28V - 4LEDs
Figure 3 Switching Frequency vs. Supply Voltage, Inductor, and Number of LEDs
Capacitor Selection
A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor appears in series with the supply source impedance and
lowers overall efficiency. This capacitor has to supply the relatively high peak current to the coil and smooth the current ripple on the input
supply. A minimum value of 1 μF is acceptable if the DC input source is close to the device, but higher values will improve performance at lower
input voltages, especially when the source impedance is high. The input capacitor should be placed as close as possible to the IC.
For AC input sources a bigger capacitor ( 300uF in the case of 12C AC) to guarantee the accuracy of the LED current.
For maximum stability over temperature and voltage, capacitors with X7R, X5R, or better dielectric are recommended.
Capacitors with Y5V dielectric are not suitable for decoupling in this application and should NOT be used.
Diode Selection
For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance SBR diode with low reverse leakage at the
maximum operating voltage and temperature. The recommended diode for use with this part is the SBR1A40, with a current rating above the
peak coil current and a continuous current rating higher than the maximum output load current. It is very important to consider the reverse
leakage of the diode when operating above 85°C. Excess leakage will increase the power dissipation in the device.