Diodes AP5724 - Manual (Page 8)

AP5724
Document number: DS31843 Rev. 4 - 2
8 of 15
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December 2013
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AP5724
Applications Information
Inductor Selection
A 10μH to 22μH inductor is recommended for most AP5724 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 the 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. For high output current applications like 3S8P, larger output capacitor of 2.2µF to 4.7µF 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 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. 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 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) R
SET
(Ω)
5 20
10 10
15 6.6
20 5
30 3.3
40 2.5
Open-Circuit Protection
In the cases of output open circuit, when the LEDs are disconnected from the circuit or the LEDs fail, the feedback voltage will be zero. The AP5724
will then switch at a high duty cycle resulting in a high output voltage, which may cause the SW and OVP pin voltage to exceed the voltage rating of
these pins. The OVP pin monitors the output voltage. If the output voltage reaches the over voltage protection threshold at the OVP pin (Figure 2),
the over voltage protection is activated and SW pin stops switching.
V
IN
SW
EN
GND
FB
AP5724
C
OUT
1uF
R
SET
5
L1
22uH
D1
V
IN
C
IN
1uF
OVP
x
x
Figure 2 LED Driver with Open-Circuit Protection