High Efficiency 1X/1.5X/2X Charge Pump
For White LED Applications
Functional Block Diagram
C1+
C1- C2+
C2-
AAT3155
IN
1X, 1.5X, 2X
Charge Pump
OUTCP
Soft-Start
Control
1MHz
Oscillator
Voltage
Reference
D/A
6 x 16 bit
ROM
D/A
D/A
D/A
D1
D2
D3
D4
GND
EN/SET
S
2
Cwire
Interface
Functional Description
The AAT3155 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 AAT3155
charge pump is enabled when any of the four cur-
rent sink inputs nears dropout. The charge pump
initially starts in 1.5X mode. If the charge pump
output droops enough for any current source out-
put to become close to dropout, the charge pump
will automatically transition to 2X mode. The
AAT3155 requires only four external components:
two 1µF ceramic capacitors for the charge pump
flying capacitors (C
1
and C
2
), one 1µF ceramic
input capacitor (C
IN
), and one 0.33µF to 1µF
ceramic charge pump output capacitor (C
OUT
).
The four constant current sink inputs (D1 to D4)
can drive four individual LEDs with a maximum cur-
rent of 20mA each. The unused sink inputs must be
connected to OUTCP, otherwise the part will oper-
ate only in 2X charge pump mode. The S
2
Cwire
serial interface enables the AAT3155 and sets the
current sink magnitudes.
Constant Current Output Level Settings
The constant current sink levels for D1 to D4 are set
via the serial interface according to a logarithmic
scale for the first 12 codes and a separate low-cur-
rent scale for the last four codes. For the first 12
codes, each code is approximately 1.5dB lower than
the previous code. In this manner, LED brightness
appears linear with each increasing code count.
Because the inputs D1 to D4 are true independent
constant current sinks, the voltage observed on any
single given input will be determined by the actual
forward voltage (V
F
) for the LED being driven.
Since the input current sinks of the AAT3155 are
programmable, no PWM (pulse width modulation) or
8
3155.2006.05.1.1
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