34844 datasheet(12/37 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34844

FUNCTIONAL DESCRIPTION

INTRODUCTION

FUNCTIONAL DESCRIPTION

INTRODUCTION

LED backlighting has become very popular for small and

medium LCDs, due to some advantages over other

backlighting schemes, such as the widely used cold cathode

fluorescent lamp (CCFL). The advantages of LED

backlighting are low cost, long life, immunity to vibration, low

operational voltage, and precise control over its intensity.

However, there is an important drawback of this method. It

requires more power than most of the other methods, and this

is a major problem if the LCD size is large enough.

To address the power consumption problem, solid state

optoelectronics technologies are evolving to create brighter

LEDs with lower power consumption. These new

technologies together with highly efficient power

management LED drivers are turning LEDs, a more suitable

solution for backlighting almost any size of LCD panel, with

really conservative power consumption.

One of the most common schemes for backlighting with

LED is the one known as “Array backlighting”. This creates a

matrix of LEDs all over the LCD surface, using defraction and

diffused layers to produce an homogenous and even light at

the LCD surface. Each row or column is formed by a number

of LEDs in series, forcing a single current to flow through all

LEDs in each string.

Using a current control driver, per row or column, helps the

system to maintain a constant current flowing through each

line, keeping a steady amount of light even with the presence

of line or load variations. They can also be use as a light

intensity control by increasing or decreasing the amount of

current flowing through each LED string.

To achieve enough voltage to drive a number of LEDs in

series, a boost converter is implemented, to produce a higher

voltage from a smaller one, which is typically used by the

logical blocks to do their function.

The 34844 implements a single channel boost converter

together with 10 input channels, for driving up to 16 LEDs per

string to create a matrix of more than 160 LEDs. Together

with its 90% efficiency and I2C programmable or external

current control, among other features, makes the 34844 a

perfect solution for backlighting small and medium size LCD

panels, on low power portable and high definition devices.

FUNCTIONAL PIN DESCRIPTION

INPUT VOLTAGE SUPPLY (VIN)

IC Power input supply voltage, is used internally to

produce internal voltage regulation (VDC1, VDC3) for logic

functioning, and also as an input voltage for the boost

regulator.

INTERNAL VOLTAGE REGULATOR 1 (VDC1)

This pin is for internal use only, and not to be used for other

purposes. A capacitor of 2.2

μF should be connected between

this pin and ground for decoupling purposes.

INTERNAL VOLTAGE REGULATOR 2 (VDC2)

This pin is for internal use only, and not to be used for other

purposes. A capacitor of 2.2

μF should be connected between

this pin and ground for decoupling purposes.

INTERNAL VOLTAGE REGULATOR 3 (VDC3)

This pin is for internal use only, and not to be used for other

purposes. A capacitor of 2.2

μF should be connected between

this pin and ground for decoupling purposes.

BOOST COMPENSATION PIN (COMP)

Passive terminal used to compensate the boost converter.

Add a capacitor and a resistor in series to GND to stabilize

the system.

IC ENABLE (EN)

The active high enable terminal is internally pulled high

through pull-up resistors. Applying 0V to this terminal would

stop the IC from working.

INPUT/OUTPUT CLOCK SIGNAL (CK)

This terminal can be used as an output clock signal

(master mode), or input clock signal (slave mode), to

synchronize more than one device.

MASTER/SLAVE MODE SELECTION (M/~S)

Setting this pin High puts the device into Master mode,

producing an output synchronization clock at the CK terminal.

Setting this pin low, puts the device in Slave mode, using the

CK pin as an input clock.

EXTERNAL PWM INPUT (PWM)

This terminal is internally pulled down. An external PWM

signal can be applied to modulate the LED channel directly in

absence of an I2C interface.

CLOCK I2C SIGNAL (SCK)

Clock line for I2C communication.

ADDRESS I2C SIGNAL (SDA)

Address line for I2C communication.

34844 Datasheet(PDF) 12 Page - Freescale Semiconductor, Inc

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