Diodes AP8801 - Manual (Page 8)

AP8801
500mA LED STEP-DOWN CONVERTER
AP8801
Document number: DS31765 Rev. 7 - 2
8 of 14
www.diodes.com
June 2012
© Diodes Incorporated
Application Information (cont.)
LED Current Adjustment/Dimming.
The LED current for the AP8801 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
LED Current @
V
= 1.25V
CTRL
LED Current @
V
= 2.5V
LED Current @
V
= 0.625V
CTRL
CTRL
0 0.3 0.6 0.9 1.2 1.5 1.8 2.4 2.7 3 2.1
R VALUE ( )
SET
Ω
Figure. 17 LED Current vs. R and V
SET CTRL
LE
D
C
U
R
R
EN
T
(mA
)
600
500
100
0
400
300
200
0.82
0.82
0.82
1
1
1
1.2
1.2
1.2
1.5
1.5
1.5
2
2
2
3
3
3
0.4
0.47
0.68
0.47
0.56
0.68
Figure 17 shows that reducing the CTRL voltage by a factor of 2 also reduces the LED current by a factor of 2. The AP8801 has the ability
vary the LED current by a factor of 2 above the default value set by RSET 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 AP8801 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 AP8801’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.