Diodes AP8802 - Manual (Page 7)

AP8802
1A LED STEP-DOWN CONVERTER
AP8802
Document number: DS31766 Rev. 9 - 2
7 of 19
www.diodes.com
May 2012
© Diodes Incorporated
Application Information
LED Current Control
The LED current is controlled by the resistor R
SET
in Figure 14 connected between V
IN
and SET. The nominal average output current in the
LED(s) with the CTRL pin open circuit is defined as:
SET
THD
LED
R
V
I
=
where V
THD
is the voltage between the V
IN
and SET pins and is nominally 200mV.
Figure. 14 Typical Application Circuit for I
LED
= 1A
Inductor Selection
This section highlights how to select the inductor suitable for the application requirements in terms of switching frequency, LED current
accuracy and temperature.
0 50 100 150 200 250 300 350 400 450 500
INDUCTOR VALUE (µH)
Figure. 15 Switching Frequency vs. Inductor Value
68
47
68
68
100
47
100
150
220
470
100
150
220
470
150
220
470
Switching Frequency @ I = 700mA
LED
12V - 3 LEDs
24V - 5 LEDs
48V - 10 LEDs
F
REQUENC
Y
(
k
Hz
)
500
450
400
350
300
250
200
150
100
0
50
The inductor influences the LED current accuracy that the system is able to provide. The following section highlights how to select the
inductor in relation to the device packages and the LED current, while maintaining the chip temperature below 70°C.