AAT3151B
High Efficiency 1X/1.5X Charge Pump
For White LED Applications
Functional Block Diagram
C1+ C1- C2+ C2-
1X and 1.5X
Charge Pump
VIN
VOUT
Soft-Start
Control
1MHz
Oscillator
Voltage
Reference
D/A
D1
D/A
D2
6 x 16 bit
ROM
D/A
D3
AS2Cwire
Interface
EN/SET
D/A
D4
GND
enables the AAT3151B and sets the constant cur-
rent sink magnitudes.
Functional Description
The AAT3151B is a dual mode load switch (1X)
and high efficiency (1.5X) 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 AAT3151B
charge pump is enabled when any of the four cur-
rent sinks near dropout.
Constant Current Output Level Settings
The constant current level for the LED channels is
2
set via the AS Cwire serial interface according to a
logarithmic scale. In this manner, LED brightness
appears to change linearly when the settings in the
scale are traversed. 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 difference between VOUT and the
actual forward voltage (VF) of the LED being driven.
The AAT3151B requires only four external compo-
nents: two 1µF ceramic capacitors for the charge
pump flying capacitors (C1 and C2), one 1µF
ceramic input capacitor (CIN), and one 1µF ceram-
ic charge pump output capacitor (COUT).
Since the constant current levels for the AAT3151B
are programmable, no PWM (pulse width modula-
tion) or additional control circuitry are 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 seri-
al interface (>1MHz data rate), the LED current
The AAT3151B constant current sinks can drive
four individual LEDs with a maximum current of
2
30mA per channel. The AS Cwire serial interface
8
3151B.2007.09.1.0
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