Datasheet

ADM8845
Rev. C | Page 16 of 20
DRIVING FOUR BACKLIGHT WHITE LEDs AND
FLASH LEDs
The ADM8845 can be configured to power four backlight white
LEDs and a camera flash, bundled into one package. FB1 to FB4
power the backlight light, FB5 and FB6 power two of the flash
LEDs, while the third is powered by an external current mirror
to control the current flowing through the third flash LED (see
Figure 33). All the backlight white LEDs have 15 mA/LED, and
the flash current is 20 mA/LED. The total load on the
ADM8845 charge pump is 120 mA; the maximum load on
the ADM8845 charge pump is 180 mA (see Figure 21).
CTRL1 controls the flash on/off, and CTRL2 controls the back-
light on/off and brightness control. Because the R
SET
resistor sets
the current that each of the six current control blocks can sink,
a PWM signal is used to change the current in the backlight
from 20 mA to 5 mA/LED. The CTRL2 duty cycle is 15/20 to
give 15 mA/backlight LED.
04867-0-034
CURRENT
CONTROL 1
CURRENT
CONTROL 2
CURRENT
CONTROL 3
CURRENT
CONTROL 4
CURRENT
CONTROL 5
CURRENT
CONTROL 6
CONTROL
LOGIC
V
REF
CHARGE PUMP
1×/1.5×/2× MODE
OSC
LED
CURRENT
CONTROL
CIRCUIT
FB1
CURRENT CONTROLLED SINKS
FB2
FB3
FB4
FB5
FB6
V
OUT
C4
4.7µF
V
CC
CTRL1
CTRL2
C2
1µF
C1
1µF
R
SET
7.32kΩ
I
SET
GND
C3
2.2µF
ADM8845
15mA/LED
20mA/LED
CURRENT
CONTROL 7
R
BACK-
LIGHT
FLASH
Figure 33. Driving Four Backlight LEDs and Flash LED