AAT2847
Four-Channel Backlight Driver with Dual LDOs
Functional Block Diagram
C1+ C1- C2+ C2-
Tri-Mode
AAT2847-EE
(AAT 2847-QG/-QI)
(1x, 1.5x and 2x)
OUT
IN
Charge Pump
1.2V
Reference
1MHz
Oscillator
Voltage
Reference
D/A
D/A
D/A
D1
D2
D3
AS2Cwire
Interface
6 x16 bit
ROM
EN/SET
D/A
D4
IN
LDOA (2.8V/2.8V)
FBA (NC)
LDO A
LDO B
ENA
LDOB (1.5V/1.8V)
FBB (NC)
ENB
(AAT 2847-QG/-QI)
GND
PGND
input battery power source, the LED cathode input
pin voltage will drop below VTH and the AAT2847
will transition into 2X mode. The charge pump
requires only four external capacitors, 1µF ceramic
capacitors for the flying (C1 and C2), input (CIN),
and output (COUT) capacitors.
Functional Description
The AAT2847 is an integrated solution for LCD dis-
play applications with a built-in four channel white
LED driver (charge pump) and dual 200mA LDO
voltage regulators. The AAT2847 incorporates a
tri-mode charge pump with load switch (1X) func-
tionality and high efficiency (1.5X or 2X) perform-
ance. To maximize power conversion efficiency, an
internal sensing circuit monitors the voltage
required at each white LED cathode input pin (D1-
D4) and sets the load switch and charge pump
mode based on the input battery voltage and the
white LED cathode pin voltage. The voltage thresh-
old for 1X to 1.5X and 1.5X to 2X mode transitions
is VTH.
The four LED cathode input pins (D1-D4) can drive
individual LEDs with a maximum current of 30mA
each. The unused LED cathode input pins have to
be connected to the OUT pin, otherwise the
AAT2847 will operate in 2X mode only. Operating
in 2X mode when it is not necessary will result in a
2
significant reduction in efficiency. The AS Cwire
serial interface enables the charge pump and sets
the current flowing into the LED cathode input pins.
The AAT2847 also contains dual LDO voltage reg-
ulators that have separate enable pins from each
other and the charge pump. The separate enable
pins for each DC/DC device in the AAT2847 allow
for every possible device operational combination.
The two LDO regulators require 2.2µF output
capacitors for stable operation.
The charge pump in the AAT2847 starts-up in 1X
mode (default); under this condition, if the LED
cathode input pin voltage is below VTH the
AAT2847 will transition into 1.5X mode. When
1.5X mode does not provide enough current
through the white LEDs for backlight applications,
which occurs during the normal discharge of the
2847.2007.09.1.0
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