PRODUCT DATASHEET
AAT2845
TM
ChargePump
Four-Channel Backlight Driver with Dual LDOs
Functional Block Diagram
C1+ C1- C2+ C2-
Tri-Mode
(1X, 1.5X and 2X)
Charge Pump
OUT
IN
Soft-Start
Control
1MHz
Oscillator
Voltage
Reference
D/A
D/A
D/A
D1
D2
D3
2
6x16
Bit ROM
S Cwire
Interface
EN/SET
D/A
D4
IN
LDOA
FBA
LDO A
LDO B
ENLDO
LDOB
FBB
1.2V
Reference
CBP
GND
ceramic input capacitor (CIN), and one 0.33µF to 1µF
ceramic charge pump output capacitor (COUT).
Functional Description
The AAT2845 is an integrated solution for LCD display
applications with a built-in driver for white LED backlight
and two LDO voltage regulators for logic power supplies.
The four constant current sink inputs (D1 to D4) can
drive four individual LEDs with a maximum current of
20mA each. The unused sink inputs must be connected
to the OUT pin; otherwise the part will operate only in 2X
charge pump mode. The S2Cwire serial interface enables
the charge pump and sets the current sink magnitudes.
The backlight driver is a tri-mode load switch (1X) and
high-efficiency (1.5X or 2X) charge pump device. To
maximize power conversion efficiency, an internal sensing
circuit monitors the voltage required on each constant
current sink input and sets the load switch and charge
pump modes based on the input battery voltage and the
current sink input voltage. As the battery discharges over
time, the charge pump is enabled when any of the four
current sink inputs nears dropout. The charge pump ini-
tially starts in 1.5X mode. If the charge pump output
droops enough for any current source output to become
close to dropout, the charge pump will automatically tran-
sition to 2X mode. The charge pump requires only four
external components: two 1µF ceramic capacitors for the
charge pump flying capacitors (C1 and C2), one 1µF
Constant Current Output Level Settings
The constant current sink levels for D1 to D4 are set via
the S2Cwire serial interface according to a logarithmic
scale for the first 12 codes, and a separate low-current
scale for the last four codes. Because the inputs D1 to
D4 are true independent constant current sinks, the volt-
age observed on any single given input will be deter-
mined by the difference between VOUT and the actual
forward voltage (VF) of the LED being driven.
w w w . a n a l o g i c t e c h . c o m
2845.2008.05.1.2
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