AAT2845
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
6x16
Bit ROM
S2Cwire
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 dis-
play 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 cur-
rent 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 initially 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 transition to 2X mode. The
charge pump requires only four external compo-
nents: 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 sep-
arate low-current scale for the last four codes.
Because the inputs D1 to D4 are true independent
constant current sinks, the voltage observed on
any single given input will be determined by the dif-
ference between VOUT and the actual forward volt-
age (VF) of the LED being driven.
Since the current level is programmable, no PWM
(pulse width modulation) or additional control cir-
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2845.2006.12.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
