LM8207 datasheet(12/22 Pages) NSC

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Application Section (Continued)

Figure 3 shows how the display information is refreshed.

Using the row and column drivers, one pixel is addressed at

the display. The column driver receives the digital color data

from the timing controller. The corresponding gamma volt-

age will be determined, using the gamma correction curve.

In fact, the gamma correction curve is just a voltage refer-

ence with 18 output tabs, which presets the color intensity

settings. This gamma voltage is written to the pixel. The

column driver selects one column at the time; the changing

in the load may affect the ‘tabs’ of the gamma correction

curve. This problem can be solved using ‘gamma buffers’ to

isolate the gamma correction curve from the column driver.

PRINCIPLE OPERATION of the LM8207

The LM8207 combines three basic functions used in TFT

displays:

•

Voltage Reference

To improve picture quality and to reduce brightness varia-

tions, a highly stable reference voltage is available. It has

a low drift over the operation temperature range. This

output voltage (OUT_V

REF) is used as the reference

voltage to define the gamma correction values.

•

Gamma Buffers

The gamma correction curve can be defined easily using

an external chain of precision resistors. To ensure load

independent gamma correction levels, 18 gamma buff-

ers, each having a low output resistance, can be used to

drive the TFT display column drivers.

•

COM Buffer

The V

COM buffer supplies a common voltage, which is

applied to the back plate of all the pixels. Writing color

information to all the pixels will cause high current varia-

tions at the V

COM level so this VCOM buffer is designed for

driving large output currents.

These three functions are discussed in detail in the following

sections.

VOLTAGE REFERENCE

The internal Voltage Reference of the LM8207 can be used

to improve picture stability. This accurate reference is highly

stable over the operation temperature range. The output

voltage (OUT_V

REF) of the Voltage Reference can be set

using two external resistors. In the next two sections, the

possibilities for setting the output voltage of the Voltage

Reference and the operating range of the Voltage Reference

are discussed.

SETTING THE OUTPUT VOLTAGE OF THE VOLTAGE

REFERENCE

The output voltage of the Voltage Reference Amplifier

(OUT_V

REF) can be set using the internal reference in com-

bination with the internal amplifier and two external resistors.

In Figure 4 a typical application circuit for V

REF is given.

20137926

FIGURE 3. Block Diagram of a Typical TFT-LCD

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