AAT3151
High Efficiency 1X/1.5X/2X Charge Pump
For White LED Applications
Functional Block Diagram
C1+ C1- C2+ C2-
1X, 1.5X and 2X
Charge Pump
VIN
VOUT
Soft-Start
Control
1MHz
Oscillator
Voltage
Reference
D/A
D/A
D/A
D1
D2
D3
6 x 16 bit
ROM
AS2Cwire
Interface
EN/SET
6 x 16 bit
ROM
D/A
D4
GND
per channel. The AS2Cwire serial interface enables
the AAT3151 and sets the constant current sink
magnitudes. AS2Cwire addressing allows the LED
main channels D1-D3 to be controlled independ-
ently from the LED sub-channel D4.
Functional Description
The AAT3151 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 AAT3151
charge pump is enabled when any of the four 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
transition to 2X mode.
Constant Current Output Level Settings
The constant current level for the LED channels is
set via the AS2Cwire 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 AAT3151 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 AAT3151
are programmable, no PWM (pulse width modula-
tion) or additional control circuitry is needed to con-
trol 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
The AAT3151 constant current sinks can drive four
individual LEDs with a maximum current of 30mA
6
3151.2004.10.0.6
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
