LM8207 datasheet(11/22 Pages) NSC

11 / 22 page

Application Section

INTRODUCTION

The performance capabilities of TFT-LCD’s increase rapidly,

with constant improvements such as larger sizes, higher

resolution, and greater brightness. Today’s LCD’s have

screen resolutions of over 1 Mega pixel and higher. The

LM8207 can be used to improve the performance of an LCD.

It is designed for buffering 18 gamma voltage levels and

driving the V

COM level. These voltage levels can be derived

from a highly stable Voltage Reference, which is included in

the LM8207. The LM8207 meets the design requirements

that combine technical improvement with the demand for

cost effective solutions.

The following sections discuss the principle operation of a

TFT-LCD and the principle operation of the LM8207 which

includes sections on each of the following: the Voltage Ref-

erence, the Gamma Buffers, and the V

COM Buffer. After this,

the next sections present a typical LM8207 configuration and

consider the maximum power dissipation. The end of this

application section introduces the evaluation board and pre-

sents layout recommendations.

PRINCIPLE OPERATION OF A TFT-LCD

This section offers a brief overview of the principle operating

of TFT-LCD’s. There is a detailed description of how infor-

mation is presented on the display. An explanation of how

data is written to the screen pixels and how the pixels are

selected is also included.

Figure 1 shows a simplified illustration of an individual LCD

pixel. The top and bottom plates of a pixel consist of Indium-

Tin Oxide (ITO), which is a transparent, electrically conduc-

tive material. ITO lies on the inner surfaces of two glass

substrates that are the front and back glass panels of a TFT

display. Sandwiched between two ITO plates is an insulating

material (liquid crystal). This alters the polarization of light,

depending on how much voltage (V

PIXEL) is applied across

the two plates. Polarizer’s are placed on the outer surfaces

of the two glass substrates. In combination with the liquid

crystal, the polarizer’s create a variable light filter that modu-

lates light transmitted from the back to the front of a display.

A pixel’s bottom plate lies on the backside of a display where

a light source is applied, and the top plate lies on the front,

facing the viewer. For most TFT displays, a pixel transmits

the greatest amount of light when V

PIXEL

≤ ±0.5 V, and it

becomes less transparent as the voltage increases with

either a positive or negative polarity.

For color displays, each pixel is built with three individual sub

pixels. Each sub pixel represents a primary color. These

colors are Red, Green and Blue (RGB). Combining these

three primary colors every user-defined color can be cre-

ated.

Figure 2 shows a simplified diagram of a TFT display, show-

ing how individual pixels are connected to the row, column

and V

COM driver. Each pixel is represented by a capacitor

with a NMOS transistor connected to its top plate. Pixels in a

TFT panel are arranged in rows and columns. Row lines are

connected to the NMOS gates, and column lines to the

NMOS sources. The back plate of every pixel is connected

to a common voltage called V

COM. The voltage applied to the

top plates (also called gamma voltage) controls the pixel

brightness. The column drivers supply this gamma voltage

via the column lines, and ‘write’ this voltage to the pixels one

row at a time. This is accomplished by having the row drivers

selecting an individual row of pixels when the column driver

writes the gamma voltage levels. The row drivers sequen-

tially apply a large positive pulse (typically 25V to 35V) to

each row line. This turns on the NMOS transistors connected

to an individual row, allowing voltage from the column lines

to be written to the pixels.

The V

COM driver (buffer) supplies a common voltage (VCOM)

to all the pixels in a TFT panel. V

COM is a constant DC

voltage that is in the middle of the gamma voltage range. As

a result, when a column driver writes to a row of pixels, the

applied voltages are either positive or negative with respect

to V

COM. In fact, the polarity of a pixel is reversed each time

a row is selected, preventing a pattern from being ‘burned’

into the LCD.

20137930

FIGURE 1. Individual LCD Pixel

20137931

FIGURE 2. TFT Display

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