AAT2856_08 Datasheet, PDF (8/18 Pages) Advanced Analogic Technologies – High Current Charge Pump with Dual LDO for Backlight Applications

PRODUCT DATASHEET
AAT2856
ChargePump^TM High Current Charge Pump with Dual LDO for Backlight Applications
Functional Block Diagram
IN IN
C1+
C1-
1X/1.5X/2X
Tri-mode
C2+
Charge Pump
C2-
ENA
ENB
ENS
To LDO A
To LDO B
V_REF
V_REF
Control
Logic
OUTA
FBA
OUTB
FBB
REF
OUT
BL1
BL2
BL3
BL4
BL5
BL6
AGND PGND
Functional Description
The AAT2856 is a highly integrated backlight LED driver
with two LDO linear regulators. The charge pump LED
driver drives backlight 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.
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
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 AAT2856 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.
The backlight LED current levels are dynamically control-
lable by the AS²Cwire single-wire interface. The backlight
section has multiple current level scales and the maxi-
mum current level is fixed at 20mA or 30mA, depending
on the scale chosen through programming.
If any one of the backlight or flash current sinks is not
used, connect that current sink to OUT. The current con-
troller monitors the sink voltage and, if it is connected to
OUT, then the controller determines that the current sink
is not used or that the LED is shorted. In either case, the
controller turns off the affected current sink.
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