Ap8803, Application information – Diodes AP8803 User Manual
Page 6
AP8803
Document number: DS31970 Rev. 5 - 2
6 of 10
August 2012
© Diodes Incorporated
AP8803
Application Information
(cont.)
LED Current Dimming
The LED current can be dimmed in two ways;
1.
Analog Dimming: Where a DC voltage is applied to the CTRL pin.
or
2.
PWM Dimming: Where a Pulse Width Modulated (PWM) signal is applied to the CTRL pin.
Analog Dimming
If the CTRL pin is driven by an external voltage (lower than 2.5V), the average LED current is:
SET
THD
REF
CTRL
LED
R
V
V
V
I
=
A DC signal from 0.3V to 2.5V applied to the CTRL pin will vary the LED current from 24% to 200% of nominal LED current. This gives an
approximate 8:1 dimming range; care, should be exercised when overdriving the CTRL pin to 200% of nominal LED current not to exceed the
power dissipation of the package.
The graph in Figure 4 shows values of nominal average output current for 3 values of current setting resistor (R
SET
) in the typical application
circuit shown on Figure 1, for different voltages applied on the CTRL pin.
It can be used to determine the R
SET
value based on the desired LED current and the condition of the CTRL pin (floating or driven with an
external DC voltage lower than 2.5V and higher than 0.2V).
LED current Versus RSET and VCTRL
0.2
0.3
0.39
0.47
3
0.82
1
1.5
2
3
0.82
0.75
2
1.5
1
0.75
0.47
0.3
0.1
0.15
0
200
400
600
800
1000
1200
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]
L
E
D Cu
rr
e
n
t
[m
A
]
LED current @
VCTRL = 1.25V
LED current @
VCTRL = 2.5V
LED current @
VCTRL = 0.625V
Figure 4 LED Current Setting vs. R
SET
and V
CTRL