AAT2842
2
High-Current Charge Pump with S Cwire™
Control and Dual LDO for Backlight and Flash
Functional Block Diagram
IN IN
C1+
OUTA
FBA
LDO A
Tri-Mode
Charge Pump
(1X/1.5X/2X)
C1-
C2+
C2-
OUTB
FBB
LDO B
1.2V
VREF
REF
OUT
To LDO A&B
ENL
BL1
BL2
BL3
BL4
BENS
FENS
Control
Logic
CT
FL1
FL2
FL3
FL4
BSET
FSET
AGND PGND
itors, making a more compact solution than an
inductor-based step-up converter solution. Each
individual LED is driven by a current sink to GND
allowing individual current control with high accura-
cy over a wide range of input voltages and LED for-
ward voltages while maintaining high efficiency.
Functional Description
The AAT2842 is a highly integrated LED driver with
two LDO linear regulators. The charge pump LED
driver simultaneously drives the backlight and flash
LEDs from a 2.7V to 5.5V input voltage. The LDO
regulators operate from the same input voltage
range and produce regulated output voltages 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 starts 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 suffi-
cient LED voltage and keep constant LED current.
The AAT2842 continuously monitors the LED for-
ward voltages, and the input voltage determines
when to reduce the charge pump mode for better
efficiency. There is also a 350mV mode-transition
hysteresis that prevents the charge pump from
oscillating between modes.
LED Drivers
The LEDs are driven from an internal charge pump
that, depending on the battery voltage and LED for-
ward voltage, drives the LED directly from the input
voltage (1X mode) or steps the input voltage up by
a factor of 1.5 (1.5X mode) or 2 (2X mode). The
charge pump requires only two tiny ceramic capac-
10
2842.2007.09.1.2
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
