AAT3122
High Efficiency 1X/1.5X Fractional Charge
Pump for White LED Applications
Functional Block Diagram
VIN
Soft Start
1MHz
Oscillator
1.5X
Charge
Pump
Voltage
Reference
C1+
C1-
C2+
C2-
OUT
EN/SET
S
2
Cwire
Interface
5
32x8 bit
ROM
8
Current
Mode
DAC
D
GND
Functional Description
The AAT3122 is a dual mode load switch (1X) and
high efficiency (1.5X) fractional charge pump device
intended for white LED backlight applications. The
fractional charge pump consists of a low dropout lin-
ear voltage regulator followed by a 1.5X charge
pump. To maximize power conversion efficiency, an
internal feedback control sensing circuit monitors
the voltage required on the constant current source
output. 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 overall device
efficiency when the input voltage level is greater
than the voltage required at the constant current
source output. Switchover between the 1.5X
(charge pump) operating mode and the 1X (load
switch) mode occurs automatically (as a function of
input and output voltages) and does not require user
intervention to maintain maximum efficiency.
The AAT3122 requires only four external compo-
nents: two 1µF ceramic capacitors for the charge
pump flying capacitors (C
1
and C
2
), one 1µF
ceramic capacitor for C
IN
, and one 0.33µF to 1µF
ceramic capacitor for C
OUT
. The LDO/1.5X charge
pump output is converted into a constant current
output capable of driving up to six individual LEDs
with a maximum of 20mA each. The current source
output magnitude is controlled by the EN/SET seri-
al data S
2
Cwire interface. The interface records ris-
ing edges of the EN/SET pin and decodes them into
32 individual current level settings each 1dB apart.
This is summarized in Figure 1 and Table 1. Code
32 is full scale, and Code 1 is full scale attenuated
by 31dB. The modulo 32 interface wraps states
back to State 1 after the 32nd clock.
3122.2005.11.1.3
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