Analog Integrated Circuit Device Data



Freescale Semiconductor

13

34844

FUNCTIONAL DESCRIPTION

INTRODUCTION

FUNCTIONAL DESCRIPTION

INTRODUCTION

LED backlighting is 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 of 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

F 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

F 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

F should be connected between this pin and

ground for decoupling purposes. A 1.0 k

resistor between the VDC3 and VDC1 pin is recommended for <-20 °C operation.

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.