AAT2801
Multi-Mode Charge Pump for
White LED Backlight and Flash Applications
C3
1μF
C3+
C3-
4.5V
V
OUT_FLSH
IN
IN
CIN
CF
1μF
1μF
Flash LED
AAT2801
RB
Flash Enable
EN_FLSH
Figure 1: Flash LED Application.
The forward current (IF) through the flash LED may
be determined with the use of a series ballast resis-
tor. The typical forward voltage (VF) for the flash
LED in a given application should be derived from
the LED manufacturer's datasheet for the desired
forward current (IF) of the flash application. Once
the forward current has been determined, the flash
ballast resistor can be calculated using the follow-
ing equation:
applications. The device start-up time into a maxi-
mum load is about 200µs, thus eliminating the
need for pre-flash control synchronization.
If a "light" or "movie" mode is also needed along
with the flash function, this can be accomplished
with the addition of a second ballast resistor with a
flash function gating MOSFET switch as shown in
Figure 2. Refer to the following equations for the
calculation of flash and light resistors, RB and RL.
(VOUT - VF)
(VOUT - VF)
RB =
IF
RL =
IF
Where:
Where:
RB
=
=
Flash ballast resistor value in ohms (Ω)
RL
=
=
=
=
Light mode ballast resistor value in
ohms (Ω)
VOUT
Regulated charge pump output voltage
(typically 4.5V)
VOUT
VF
Regulated charge pump output voltage
(typically 4.5V)
VF
IF
=
=
Flash LED forward voltage at the
desired forward current
Flash LED forward voltage at the
desired forward current
Desired flash LED forward current
The flash LED function can be controlled by the
AAT2801 flash enable (EN_FLSH) pin in most
IF
Desired flash LED forward current in
the "light" mode
18
2801.2006.04.1.3
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
