AAT2856 AAT, AAT2856 Datasheet - Page 10

• Manufacturer Part Number: AAT2856
• Description: High Current Charge Pump
• Manufacturer: AAT

Functional Block Diagram
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 voltages as low as 1.2V.
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-
verter solutions. Each individual LED is driven by a
10
ENA
ENB
ENS
C1+
C2+
C2-
C1-
Charge Pump
To LDO A
To LDO B
1X/1.5X/2X
Tri-mode
AGND
IN
Dual LDO for BacklightApplications
PGND
IN
Control
Logic
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 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.
The backlight LED current levels are dynamically
controllable by the AS
The backlight section has multiple current level
High Current Charge Pump with
V
V
REF
REF
2
Cwire single-wire interface.
OUTA
FBA
OUTB
FBB
BL1
BL2
BL3
BL4
BL5
BL6
REF
OUT
AAT2856
2856.2007.06.1.0