Application Section

INTRODUCTION

The LM8342 is an integrated combination of a digitally con-

trolled current sink and a non-volatile register (7 bits EE-

PROM). Programming the register can be done using the

I

ometer adjustment, and thereby simplifies the V

tion procedure. With the LM8342, the factory operator can

physically view the screen head-on when performing this

step, easing manufacturing especially for large TFT panel

sizes.

The following sections discuss the principle of operation of a

TFT-LCD and, subsequently give a description of how to use

the LM8342, including the I

trol inputs. After this, two typical LM8342 configurations are

presented. Subsequently an evaluation system is intro-

duced, including a µC-board programming using the I

compatible interface. At the end of this application section

board layout recommendations are given.

PRINCIPLE OF OPERATION OF A TFT-LCD

This section offers a brief overview of the principle of opera-

tion of TFT-LCD’s. It gives a detailed description of how

information is presented on the display. Further an explana-

tion of how data is written to the screen pixels and how the

pixels are selected is 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 is at the inner surfaces of two glass sub-

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

display. Sandwiched between two ITO plates is an insulating

material (liquid crystal). Liquid crystals alter the polarization

of light, depending on how much voltage (V

across the two plates. Polarizers are placed on the outer

surfaces of the two glass substrates. In combination with the

liquid crystal, the polarizers create an electrically variable

light filter that modulates light transmitted from the back to

the front of a display. A pixel’s bottom plate is at the backside

of a display where a light source is applied, and the top plate

is at the front, facing the viewer. For most TFT displays, a

pixel transmits the greatest amount of light when V

PIXEL

≤

creases with either positive or negative polarity.

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

ing how individual pixels are connected to the row, column

and V

with an 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

top plates (i.e. Gamma Voltage) controls the pixel bright-

ness. 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 drivers

write the gamma voltage levels. The row drivers sequentially

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.

20139223

FIGURE 1. Individual LCD Pixel

20139224

FIGURE 2. TFT Display

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