AAT3131
High Efficiency 1X/1.5X Fractional
Charge Pump for White LED Applications
Functional Block Diagram
VIN
Soft-Start
Control
1MHz
Oscillator
Voltage
Reference
1X/1.5X
Charge
Pump
C1+
C1-
C2+
C2-
OUT
Current
Reference
Quad
Output
DAC
D1
D2
D3
32x16 bit
ROM
EN/SET
S
2
Cwire
Interface
32x16 bit
ROM
Single
Output
DAC
D4
GND
Functional Description
The AAT3131 is a dual mode load switch (1X) and
high efficiency (1.5X) fractional charge pump
device intended for white LED backlight applica-
tions. The fractional charge pump consists of a low
dropout linear voltage regulator followed by a 1.5X
charge pump with multiple current-source outputs.
To maximize power conversion efficiency, an inter-
nal feedback control sensing circuit monitors the
voltage required on the constant current source
outputs. This control circuit then sets the load
switch and charge pump functions based upon the
input voltage level versus the output voltage level
needed. This function significantly enhances over-
all device efficiency when the input voltage level is
greater than the voltage required at the constant
current source outputs. The 1X load switch/1.5X
charge pump mode decision is based on the volt-
age levels sensed on either the D1 output or the
D4 output, whichever is greater. Switchover
between the 1.5X (charge pump) operating mode
and the 1X (load switch) mode occurs automatical-
ly (as a function of input and output voltages) and
does not require user intervention to maintain max-
imum efficiency.
The AAT3131 requires only four external compo-
nents: 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 output capacitor (C
OUT
). The
LDO/1.5X charge pump output is converted into
three (D1 to D3) constant current outputs to drive
three individual LEDs with a maximum current of
20mA each, and one (D4) constant current output
with a maximum current of 30mA. The current
source output magnitude is controlled by the
EN/SET serial data interface. The interface records
rising edges of the EN/SET pin and decodes them
into 32 addresses corresponding to individual cur-
rent level settings. The 32 addresses are divided
3131.2005.12.1.2
7
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
