AP8800 Datasheet, PDF (5/11 Pages) Diodes Incorporated – 350mA LED Step-Down Converter

AP8800
350mA LED Step-Down Converter
Application Information
LED Current Control
The LED current is controlled by the resistor RSET in Figure 1.
If the CTRL pin is floating and at a nominal voltage of VREF, the external current sense resistor RSET
(greater than 0.3 Ω) is connected between VIN and SET and defines the nominal average output current in
the LED(s) as:
I_LED
=
V_THD
R_SET
If the CTRL pin is driven by an external voltage (lower than 2.5V), the average LED current in this case is:
I_LED
= ^VCTRL
V_REF
V_THD
R_SET
The graph in figure 2 gives values of nominal average output current for several values of current setting
resistor (RSET) in the typical application circuit shown on Figure 1, for different voltages applied on the CTRL
pin.
LED current Versus RSET and VCTRL
400
350
0.56
0.3
300
0.33
0.68
250
0.39
0.75
0.82
0.47
200
1
0.56
150
0.3
0.68
1.2
0.39
0.82
1.5
100
0.47
0.56
1
1.2
2
50
0.68
0.82 ₁
1.2
1.5
1.5
2
2
3
3
3
0
0 0.15 0.3 0.45 0.6 0.75 0.9 1.05 1.2 1.35 1.5 1.65 1.8 1.95 2.1 2.25 2.4 2.55 2.7 2.85 3
RSET value [Ohms]
Figure 2: LED Current setting Vs R_SET and V_CTRL
LED current @
VCTRL = 1.25V
LED current @
VCTRL = 2.5V
LED current @
VCTRL = 0.625V
AP8800 Rev. 1
5 of 11
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MARCH 2009
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