PRODUCT DATASHEET
AAT2846
ChargePump
TM
High Current Charge Pump with Dual LDO for Backlight and Flash Applications
Functional Block Diagram
IN
IN
C1+
C1-
C2+
C2-
OUTA
1x/1.5x/2x
Charge Pump
LDO A
FBA
OUTB
LDO B
FBB
1.2V
V
REF
ENA
ENB
To LDO A
To LDO B
REF
OUT
BL1
BL2
ENS
FEN
Control
Logic
BL3
BL4
BL5
BL6
FL1
FL2
AGND PGND
CT
FSET
Functional Description
The AAT2846 is a highly integrated backlight and flash
LED driver with two LDO linear regulators. The charge
pump LED driver drives backlight and flash LEDs from a
2.7V to 5.5V input voltage. The LDO regulators are oper-
ated from the same input voltage range and produce
regulated output voltages as low as 1.2V.
more compact solution than typical inductor-based step-
up converter solutions. Each individual LED is driven by a
current sink to AGND, allowing individual current control
with high accuracy over a wide range of input voltages and
LED forward voltages while maintaining high efficiency.
The charge pump is controlled by the voltage across the
LED current sinks. When any one of the active current
sinks begins to dropout, the charge pump goes to the
next higher mode (from 1X to 1.5X or from 1.5X to 2X
mode) to maintain sufficient LED voltage for constant
LED current. The AAT2846 continuously monitors the
LED forward voltages and uses the input voltage to
determine when to reduce the charge pump mode for
better efficiency. There is also a 500mV mode-transition
hysteresis that prevents the charge pump from oscillat-
ing between charge pump modes.
LED Drivers
The LEDs are driven from an internal charge pump that,
depending on the battery voltage and LED forward volt-
age, drives LEDs directly from the supply voltage (1X or
bypass mode) or steps up the supply voltage by a factor
of 1.5 (1.5X mode) or 2 (2X mode). The charge pump
requires only two tiny 1µF ceramic capacitors, providing a
2846.2008.03.1.1
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