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AAT3123
(Rev.:2003)

High Efficiency 1X/1.5X Fractional Charge Pump for White LED Applications

Advanced Analogic Technologies 

AAT3123/4

High Efficiency 1X/1.5X Fractional Charge

Pump for White LED Applications

Functional Block Diagram

Soft Start

Control

1MHz

Oscillator

Voltage

Reference

VIN

1x/1.5x

Charge

Pump

C1+

C1-

C2+

C2-

OUT

RCeCfuuerrrrreeennnctte

D1

Quad

D2

Output

DAC

D3

D4

EN/SET

S2Cwire

Interface

32 x 8 bit

ROM

Functional Description

The AAT3123/24 is a dual mode Load Switch (1X)

and high efficiency (1.5X) fractional charge pump

device intended for white LED back light 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 internal

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 effi-

ciency when the input voltage level is greater then

the voltage required at the constant current source

outputs. For the AAT3123, the 1X Load Switch / 1.5X

charge pump mode is decided on the voltage sensed

on the output D1. The AAT3124 bases the 1X Load

Switch/1.5X charge pump mode decision is based

on the voltage levels sensed on either the output D1-

D4 output group or the D5-D6 output group, which

ever is greater. 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.

Dual

Output

D5*

DAC

D6*

*AAT 3124 only

GND

The AAT3123/24 requires only four external compo-

nents: two 1µF ceramic capacitors for the charge

pump flying capacitors (C1 and C2), one 1µF ceram-

ic input capacitor (CIN) and one 0.33µF to 1µF ceram-

ic output capacitor (COUT). The LDO/1.5X charge

pump output is converted into four (D1 to D4) or six

(D1-D4 and D5-D6) constant current outputs to drive

four or six individual LEDs with a maximum current of

20mA each. The current source output magnitude is

controlled by the EN/SET serial data S2C interface.

The interface records rising edges of the EN/SET pin,

and decodes them into 32 individual current level set-

tings each 1dB apart (see Current Level Settings

below). 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.

With each EN/SET pulse, the output current increas-

es by 1dB. To decrease the output current by 1dB, 31

EN/SET clock pulses are required. The counter can

be clocked at speeds up to 1MHz, so intermediate

states are not visible. The first rising edge of EN/SET

enables the IC and initially sets the output LED cur-

rent to -31dB, the lowest setting equal to 525µA.

Once the final clock cycle is input for the desired

brightness level, the EN/SET pin is held high to main-

tain the device output current at the programmed

level. The device is disabled 500µs after the EN/SET

pin transitions to a logic low state.

3123.2003.09.1.1

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