Diodes AP8802 - Manual (Page 10)

AP8802
1A LED STEP-DOWN CONVERTER
AP8802
Document number: DS31766 Rev. 9 - 2
10 of 19
www.diodes.com
May 2012
© Diodes Incorporated
Application Information (cont.)
LED Current Adjustment/Dimming
The LED current for the AP8802 can be adjusted by driving the CTRL with a digital signal (PWM dimming) or by driving the CTRL with a DC
voltage between 0.3V and 2.5V (dc 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
× =
where V
REF
is nominally 1.25V
1200
0.2
0.33
0.39
0.47
0.56
0.39
0.47
0.56
0.68
0.82
1
3
0.2
0.33
0.47
0.68
0.82
1
1.2
1.5
2
3
1.2
1.5
2
3
1
0.82
0.68
2
1.5
1.2
0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0
1000
800
600
200
0
400 L
E
D CURR
ENT
(
m
A)
LED Current @
V
= 1.25V
CTRL
LED Current @
V
= 2.5V
LED Current @
V
= 0.625V
CTRL
CTRL
R VALUE ( )
SET
Ω
Figure. 19 LED Current Setting vs. R and V
SET CTRL
Figure 19 shows that reducing the CTRL voltage by a factor of 2 also reduces the LED current by a factor of 2. The AP8802 has the ability
vary the LED current by a factor of 2 above the default value set by R
SET down to a factor of 0.24 of the nominal LED current. This provides
an 8.33:1 dynamic range of the dc dimming.
A low pass filter on the CTRL pin of the AP8802 automatically provides some soft-start function of the LED current on initial start-up (this
phenomenon can be seen in figure 7); the built in soft-start period can be increased by the addition of an external capacitor onto the CTRL
pin.
The AP8802’s dimming range can be increased above this DC dimming factor by applying a PWM signal to the CTRL pin using this method
dimming dynamic ranges above 100 can be achieved.