VCOM DRIVER

+

-

COLUMN DRIVERS

CSTRAY

CSTRAY

CSTRAY

VCOM

VCOM

VCOM

APPROX

VDD/2

PIXEL

TFT-LCD PANEL

PIXEL

PIXEL

VCOM

POLARIZER

BOTTOM ITO

PLATE

TOP ITO

PLATE

POLARIZER

TRANSMITTED LIGHT

LIGHT SOURCE

GLASS

SUBSTRATE

LIQUID CRYSTAL

MATERIAL

GLASS

SUBSTRATE

VPIXEL

± POLARITY

LM6584

SNOSA44C – MAY 2003 – REVISED MARCH 2013

www.ti.com

The LM6584 meets these requirements. It has a rail-to-rail input and output, typical gain bandwidth and slew rate

of 15MHz and 15V/

μs, and it can supply up to 320mA of output current. The following sections will describe the

operation of VCOM Drivers, Gamma Buffers, and Panel Repair Buffers, showing how the LM6584 is well suited for

each of these functions.

BRIEF OVERVIEW OF TFT DISPLAY

To better understand these op amp applications, let’s first review a few basic concepts of how a TFT display

operates. Figure 26 is a simplified illustration of an LCD pixel. The top and bottom plates of each pixel consist of

Indium-Tin oxide (ITO), which is a transparent, electrically conductive 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 the two ITO

plates is an insulating material (liquid crystal) that alters the polarization of light to a lesser a greater amount,

depending on how much voltage (VPIXEL) is applied across the two plates. Polarizers are placed on the outer

surfaces of the two glass substrates, which in combination with the liquid crystal create a variable light filter that

modulates 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. On a Twisted

Neumatic (TN) display, which is typical of most TFT displays, a pixel transmits the greatest amount of light when

VPIXEL is less ±0.5V, and it becomes less transparent as this voltage increases with either a positive or negative

polarity. In short, an LCD pixel can be thought of as a capacitor, through which, a controlled amount of light is

transmitted by varying VPIXEL.

Figure 26. Individual LCD Pixel

Figure 27. TFT Display

14

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