AAT2846
High Current Charge Pump with Dual LDO
for Backlight and Flash Applications
Functional Block Diagram
IN
IN
C1+
OUTA
FBA
LDO A
LDO B
C1-
1x/1.5x/2x
Charge Pump
C2+
OUTB
FBB
C2-
ENA
ENB
To LDO A
To LDO B
1.2V
VREF
REF
OUT
BL1
BL2
BL3
BL4
ENS
FEN
Control
Logic
BL5
BL6
CT
FSET
FL1
FL2
AGND PGND
verter 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.
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 operated from the same input
voltage range and produce regulated output volt-
ages as low as 1.2V.
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 con-
tinuously 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 pre-
vents the charge pump from oscillating between
charge pump modes.
LED Drivers
The LEDs are driven from an internal charge pump
that, depending on the battery voltage and LED for-
ward voltage, drives LEDs directly from the supply
voltage (1X or bypass mode) or steps up the sup-
ply 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 more compact
solution than typical inductor-based step-up con-
The backlight and flash LED current levels are
2
dynamically controllable by the AS Cwire single-wire
2846.2007.08.1.0)
11
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
