AAT2848 Datasheet, PDF (9/19 Pages) Advanced Analogic Technologies – High-Current Charge Pump For Backlight Display and Flash Applications

ChargePump^TM
PRODUCT DATASHEET
AAT2848
High-Current Charge Pump For Backlight Display and Flash Applications
Functional Block Diagram
IN
C1+
C1-
C2+
C2-
BENS
FENS
BSET
FSET
FADE
FLTMR
Tri-Mode
Charge Pump
1x/1.5x/2x
32-Step S²Cwire
Backlight Control
16-Step S²Cwire
Flash Control
Control
Logic
PGND
AGND
OUT
BL1
BL2
BL3
BL4
FL1
FL2
Functional Description
The AAT2848 is an integrated solution for LCD display
applications with a built-in four-channel, 30mA driver for
white LED backlight and two 300mA drivers for flash
LEDs.
The backlight driver uses a dual-mode load switch (1x)
and high-efficiency (1.5x) charge-pump engine. To max-
imize power conversion efficiency, an internal sensing
circuit monitors the voltage required on each constant
current sink output and sets the load switch and charge
pump mode based on the input battery voltage and the
current sink input voltage. As the battery discharges
over time, the charge pump is enabled when any of the
four current sink inputs nears dropout. The charge pump
operates in 1.5x mode for backlight operation. The
charge pump requires only four external components:
two 1μF ceramic capacitors for the charge pump flying
capacitors (C1 and C2), one 4.7μF ceramic input capaci-
tor (C_IN) and one 2.2μF ceramic charge pump output
capacitor (C_OUT).
The four constant current sink inputs (BL1 to BL4) can
drive up to four individual LEDs with a maximum current
of 30mA each. Because the inputs BL1 to BL4 are true
independent constant current sinks, the voltage observed
on any single given input will be determined by the dif-
ference between V_OUT and the actual forward voltage (V_F)
of the LED being driven.
Any unused sink inputs must be connected to the OUT
pin; otherwise, the AAT2848 will operate in 1.5x charge
pump mode regardless of the battery voltage or the V_F of
the diodes connected to the remaining current sinks. The
S²Cwire serial interface at the BENS input enables the
charge pump and sets the backlight current sink level.
Since the current level is programmable, no PWM (pulse
width modulation) or additional control circuitry is needed
to control LED brightness. This feature greatly reduces
the burden on a microcontroller or system IC to manage
LED or display brightness, allowing the user to “set it, and
forget it.” With its high speed serial interface (>1MHz data
rate), the LED current drive can be changed successively
to brighten or dim the LEDs in smooth transitions or in
abrupt steps, giving the user complete programmability
and real-time control of LED brightness.
The flash driver uses a tri-mode load switch (1x) and
high-efficiency (1.5x and 2x) charge-pump engine. To
maximize power conversion efficiency, an internal sens-
ing circuit monitors the voltage required on each con-
stant flash current sink/channel input and sets the load
switch and charge pump mode based on the input bat-
tery voltage and the flash current sink input voltage. As
the battery discharges over time, the charge pump is
enabled when any of the two flash current sink inputs
nears dropout. The charge pump initially starts in 1.5x
mode, and then transitions into 2x mode if the flash cur-
rent sink inputs nears dropout.
2848.2008.05.1.0
www.analogictech.com
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