AP5724 Datasheet, PDF (9/16 Pages) Diodes Incorporated – WHITE LED STEP-UP CONVERTER

AP5724
WHITE LED STEP-UP CONVERTER
Application Information
Inductor Selection
A 10μH~22μH inductor is recommended for most AP5724 applications. Although small size and high
efficiency are major concerns, the inductor should have low core losses at 1.2MHz and low DCR (copper wire
resistance).
Capacitor Selection
The small size of ceramic capacitors are ideal for AP5724 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 1μF input capacitor and a 1μF output capacitor are sufficient for most AP5724
applications.
Diode Selection
Schottky diodes, with their low forward voltage drop and fast reverse recovery, are the ideal choices for
AP5724 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, which can cause significant
switching losses at the 1.2MHz switching frequency of the AP5724. A Schottky diode rated at 100mA to
200mA is sufficient for most AP5724 applications.
LED Current Control
The LED current is controlled by the feedback resistor (R_SET in Figure 1). The feedback reference is 0.1V.
The LED current is 0.1V/ 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 = 0.1V/I_LED (See Table 1)
Table 1. R_SET Resistor Value Selection
I_LED (mA)
5
10
15
20
30
R_SET (Ω)
20
10
6.6
5
3.3
AP5724 Rev. 1
9 of 16
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JUNE 2009
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