AAT3122 datasheet(7/12 Pages) AAT

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Functional Block Diagram

Voltage

Reference

Soft Start

1MHz

Oscillator

1.5X

Charge

Pump

S2Cwire

Interface

32x8 bit

ROM

Current

Mode

DAC

VIN

C1+

C1-

C2+

C2-

OUT

GND

EN/SET

AAT3122

High Efficiency 1X/1.5X Fractional Charge

Pump for White LED Applications

3122.2005.11.1.3

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 C2),

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

2Cwire 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.

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