Diodes AP5726 - Manual (Page 9)

AP5726
WHITE LED STEP-UP CONVERTER
AP5726
Document number: DS31845 Rev. 3 - 2
9 of 17
www.diodes.com
July 2010
© Diodes Incorporated
Applications Information
Inductor Selection
A 10
μH~22μH inductor is recommended for most AP5726 applications. For high efficiency the inductor should
have low core losses at 1.2MHz and low DCR (copper wire resistance). The inductor saturation current rating
should also exceed the peak input current, especially for high load current applications (like 3S8P).
Capacitor Selection
The small size of ceramic capacitors are ideal for AP5726 applications. X5R and X7R types are recommended
because they retain their capacitance over wider voltage and temperature ranges than other types such as Y5V
or Z5U. A 1
μF input capacitor and a 1μF output capacitor are sufficient for most AP5726 applications. For high
output current applications like 3S8P, larger output capacitor of 2.2uF ~ 4.7uF is recommended to minimize
output ripple.
Diode Selection
Schottky diodes, with their low forward voltage drop and fast reverse recovery, are the ideal choices for AP5726
applications. The forward voltage drop of a Schottky diode represents the conduction losses in the diode, while
the diode capacitance (C
T
or C
D
) represents the switching losses. For diode selection, both forward voltage drop
and diode capacitance need to be considered. Schottky diodes with higher current ratings usually have lower
forward voltage drop and larger diode capacitance. Larger Schottky diode capacitance can cause significant
switching losses at the 1.2MHz switching frequency of the AP5726. A Schottky diode rated at 100mA to 200mA is
sufficient for most AP5726 applications.
LED Current Control
The LED current is controlled by the feedback resistor (R
SET
in Figure 1). The feedback reference is 0.31V. The
LED current is 0.31V/ R
SET
. In order to have accurate LED current, precision resistors are preferred (1% is
recommended). The formula and table for R
SET
selection are shown below.
R
SET
= 0.31V/I
LED
(See Table 1)
Table 1. R
SET
Resistor Value Selection
I
LED
(mA) R
SET
( Ω)
5 62
10 31
15 20.6
20 15.5
30 10.3