LM8342 datasheet(11/23 Pages) TI1

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DISPLAY

COLUMN DRIVER

ROW

DRIVER

MULTI

SOURCE

DC/DC

CONVERTER

& LDO

TIMING

CONTROL

VCOM

CALIBRATOR

VREF

GAMMA CORRECTION CURVE

LM8207

18 GAMMA BUFFER

VCOM

BUFFER

LM8342

VCOM BUFFER

COLUMN DRIVERS

CSTRAY

VCOM

PIXEL

TFT-LCD PANEL

PIXEL

APPROX

VDD/2

LM8342

www.ti.com

SNOSAM0B – NOVEMBER 2005 – REVISED MARCH 2013

Figure 22. TFT Display

Figure 22 shows a simplified diagram of a TFT display, showing how individual pixels are connected to the row,

column and VCOM driver. Each pixel is represented by a capacitor 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

VCOM. The voltage applied to the top plates (i.e. 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 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.

Figure 23. TFT Panel Block Diagram

Figure 23 shows a block diagram of a TFT panel. The VCOM buffer supplies a common voltage (VCOM) to all the

pixels in a TFT panel. In general, VCOM is a DC voltage that is in the middle of the gamma voltage range. Screen

performance can be optimized by tuning the VCOM voltage in the calibration procedure. Using the LM8342, the

VCOM calibration procedure is simplified by elimination of the potentiometer adjustment task. This task is currently

performed at the factory using a trimmer adjustment tool and visual inspection, when using a stable reference

voltage and a potentiometer as a voltage divider to generate the VCOM voltage.

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