AAT3156
High Efficiency 1X/1.5X/2X Charge Pump
For White LED Applications
Functional Block Diagram
C1+
C1- C2+
C2-
VIN
1X, 1.5X and 2X
Charge Pump
VOUT
Soft-Start
Control
1MHz
Oscillator
Voltage
Reference
D/A
6 x 16 bit
ROM
D/A
D/A
D/A
D1
D2
D3
D4
D5
D6
EN/SET
AS
2
Cwire
Interface
6 x 16 bit
ROM
D/A
D/A
GND
Functional Description
The AAT3156 is a tri-mode load switch (1X) and
high efficiency (1.5X or 2X) charge pump device
intended for white LED backlight applications. To
maximize power conversion efficiency, an internal
sensing circuit monitors the voltage required on
each constant current sink input and sets the load
switch and charge pump modes based on the input
battery voltage and the current sink input voltage.
As the battery discharges over time, the AAT3156
charge pump is enabled when any of the six cur-
rent sinks near dropout. The charge pump initially
starts in 1.5X mode. If the charge pump output
drops enough for any current sink to become close
to dropout, the charge pump will automatically tran-
sition to 2X mode.
The AAT3156 requires only four external compo-
nents: two 1µF ceramic capacitors for the charge
pump flying capacitors (C1 and C2), one 1µF
ceramic input capacitor (C
IN
), and one 0.33µF to
1µF ceramic charge pump output capacitor (C
OUT
).
The AAT3156 constant current sinks can drive six
individual LEDs with a maximum current of 30mA
per channel. The AS
2
Cwire serial interface enables
the AAT3156 and sets the constant current sink
magnitudes. AS
2
Cwire addressing allows the LED
main channels D1-D4 to be controlled independ-
ently from the LED sub-channels D5-D6.
Constant Current Output Level Settings
The constant current level for the LED channels is
set via the AS
2
Cwire serial interface according to a
logarithmic scale. In this manner, LED brightness
appears linearly when the settings in the scale are
traversed. Because the inputs D1 to D6 are true
independent constant current sinks, the voltage
observed on any single given input will be deter-
mined by the difference between V
OUT
and the
actual forward voltage (V
F
) of the LED being driven.
8
3156.2005.06.1.1
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
