Diodes AP8802H - Manual (Page 8)

AP8802H
60V 1A LED STEP-DOWN CONVERTER
AP8802H
Document number: DS32227 Rev. 6 - 2
8 of 14
www.diodes.com
May 2012
© Diodes Incorporated
PRELIMINARY
NEW PROD
UC
T
Applications Information (cont.)
LED Current Adjustment/Dimming
The LED current for the AP8802H 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
1000
800
600
400
200
0
0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3
R VALUE ( )
SET
Ω
Figure. 17 LED Current vs. R and V
SET CTRL
L
E
D
C
U
R
R
E
N
T (
m
A
)
Figure 17 shows that reducing the CTRL voltage by a factor of 2 also reduces the LED current by a factor of 2. The AP8802H 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 (pole frequency ~ 500kHz) on the CTRL pin of the AP8802H 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 AP8802H’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.