AAT2801 Datasheet, PDF(14/20 Page) Advanced Analogic Technologies – Multi-Mode Charge Pump for White LED Backlight and Flash Applications

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AAT2801 Datasheet, PDF (14/20 Pages) Advanced Analogic Technologies – Multi-Mode Charge Pump for White LED Backlight and Flash Applications

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AAT2801

Multi-Mode Charge Pump for

White LED Backlight and Flash Applications

on the voltage at the current source output. Since

the LED to LED VF can vary as much as 1 volt, this

function significantly enhances overall efficiency of

the white LED backlight charge pump section when

the battery input voltage level is greater than the

voltage required at any current source output.

Flash Driver

For flash applications, the voltage doubling

charge pump section is used to support the high

current demand required by the application.

Charge pump regulation is achieved by sensing

the output voltage through an internal resistor

divider network. The free running charge pump

switching frequency is approximately 1MHz. The

charge pump is designed to deliver up to 120mA

of continuous current and 250mA of pulsed cur-

rent for up to 500ms.

The AAT2801 requires only six external compo-

nents: three 1ÂµF ceramic capacitors for the

charge pump flying capacitors (C1, C2 and C3),

one 1ÂµF ceramic input capacitor (CIN), one 1ÂµF

ceramic charge pump output capacitor (CCP), and

one 1ÂµF for the flash output (CF). The four con-

stant current outputs (D1 to D4) drive four individ-

ual LEDs with a maximum current of 30mA each.

The EN/SET S2Cwire serial interface enables the

AAT2801 and sets the current source magni-

tudes. A separate flash enable pin (EN_FLSH) is

provided to independently control the flash charge

pump circuit.

Applications Informationâ

White LED Backlight Section

Constant Current Output Level Settings

The constant current source amplitudes for D1 to D4

are set via the serial interface according to a loga-

rithmic scale, where each code is 1dB greater than

the previous code. In this manner, LED brightness

appears linear with each increasing code count.

Because the outputs D1 to D4 are true independent

constant current sources, the voltage observed on

any single given output will be determined by the

actual forward voltage (VF) for the LED being driven.

Since the output current of the AAT2801 is program-

mable, no PWM (pulse width modulation) or addi-

tional control circuitry is needed to control 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 (1MHz

data rate), the output current of the AAT2801 can be

changed successively to brighten or dim LEDs in

smooth transitions (e.g., to fade-out) or in abrupt

steps, giving the user complete programmability and

real-time control of LED brightness.

The 32 individual current level settings are each

approximately 1dB apart (see Current Level

Settings below). Code 32 is full scale; Code 2 is full

scale attenuated by 31dB; and Code 1 is reserved

as a "no current" setting.

Normalized Current Level Settings

1.00

0.10

0.01

0.00

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Code

2801.2006.04.1.3

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