0

32

64

96

128 160 192 224 256

INPUT (DIGITAL VALUE)

VGMA3

VGMA12

VGMA1

VGMA18

VCOM

VGMA16

VGMA14

VGMA5

VGMA7

0

2

4

6

8

10

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

LM8207

SNOSAL5A – SEPTEMBER 2005 – REVISED MARCH 2013

www.ti.com

Figure 36. Voltage Reference Headroom vs. Load Current

GAMMA BUFFERS

This section gives an overview for the applications of the gamma buffers and also defines the gamma correction

curve. Specifications for the buffers are derived from their operation range. Also included are the formulas for the

realization of the gamma correction curve using external resistors. An overview is given for the gamma voltage

accuracy, using the LM8207 in combination with external resistors.

As discussed in the section entitled “Principle Operation of a TFT-LCD,” the basic function of the gamma buffers

is to make the gamma correction curve independent of the behavior of the column driver. Writing data to each

subsequent pixel will cause load variations. The gamma buffers have a low impedance output and can handle

these variations without changing the gamma correction curve. A typical gamma correction curve is given in

Figure 37.

Figure 37. Typical Gamma Correction Curve

Each buffer covers a part of the correction curve and, therefore, has its own specifications. All buffers require

that the output should recover quickly from disturbances caused by the switching of the column driver. The

gamma voltage level of each buffer (VGMA1…VGMA18) depends on its position for the levels decrease

sequentially. To best utilize the LM8207, each buffer is optimized for its position in the gamma correction curve.

•

Gamma Buffers 1-2

–

source current to bias the resistive load of the column driver having an open collector structure. To meet

the operating voltage range, these outputs need a resistive load connected to a lower potential sourcing

an output current of at least 1 mA.

•

Gamma Buffers 3-16

–

transitions at the outputs are possible.

16

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