AAT2847_08 Datasheet, PDF (11/20 Pages) Advanced Analogic Technologies – Four-Channel Backlight Driver with Dual LDOs

ChargePump^TM
PRODUCT DATASHEET
AAT2847
Four-Channel Backlight Driver with Dual LDOs
Functional Block Diagram
C1+ C1- C2+ C2-
AAT 2847-EE
(AAT 2847-QG/-QI)
IN
Tri-Mode
(1x, 1.5x and 2x)
Charge Pump
OUT
EN /SET
1MHz
Oscillator
Voltage
Reference
AS²Cwire
Interface
6 x16 bit
ROM
1.2V
Reference
D/A
D1
D/A
D2
D/A
D3
D/A
D4
IN
ENA
ENB
LDO A
GND
PGND
LDO B
(AAT 2847-QG/-QI)
LDOA (2.8V/2.8V)
FBA (NC)
LDOB (1.5V/1.8V)
FBB (NC)
Functional Description
The AAT2847 is an integrated solution for LCD display
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) functionality and high efficiency (1.5X
or 2X) performance. To maximize power conversion effi-
ciency, 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 threshold for 1X to 1.5X and
1.5X to 2X mode transitions is V_TH.
The charge pump in the AAT2847 starts-up in 1X mode
(default); under this condition, if the LED cathode input
pin voltage is below V_TH the AAT2847 will transition into
1.5X mode. When 1.5X mode does not provide enough
current through the white LEDs for backlight applica-
tions, which occurs during the normal discharge of the
input battery power source, the LED cathode input pin
voltage will drop below V_TH and the AAT2847 will transi-
tion into 2X mode. The charge pump requires only four
external capacitors, 1µF ceramic capacitors for the flying
(C1 and C2), input (C_IN), and output (C_OUT) capacitors.
The four LED cathode input pins (D1-D4) can drive indi-
vidual 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 neces-
sary will result in a significant reduction in efficiency. The
AS2Cwire serial interface enables the charge pump and
sets the current flowing into the LED cathode input pins.
The AAT2847 also contains dual LDO voltage regulators
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.
2847.2008.05.1.2
www.analogictech.com
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