TFT-LCD PANEL

GAMMA

BUFFERS

VDD

+

-

+

-

+

-

+

-

COLUMN

DRIVERS

VGMA1

VGMA2

VGMA3

VGMA5

VGMA4

VGMA6

VGMA7

VGMA8

VGMA9

VGMA10

COLUMN

DRIVERS

VGMA1

VGMA3

VGMA5

VGMA2

VGMA4

CMOS

TRANSMISSION

GATES

RESISTIVE

DAC

HIGH POLARITY GAMMA

LEVEL INPUTS

COLUMN DRIVER

BUFFERS

VGMA6

VGMA8

VGMA10

VGMA7

VGMA9

CMOS TRANSMISSION

GATES

RESISTIVE

DAC

LOW POLARITY GAMMA LEVEL

INPUTS

COLUMN DRIVER

BUFFERS

LM6584

SNOSA44C – MAY 2003 – REVISED MARCH 2013

www.ti.com

Figure 33. Simplified Schematic of Column Driver IC

Figure 34 shows how column drivers in a TFT display are connected to the gamma levels. VGMA1, VGMA5,

VGMA6, and VGMA10 are driven by the Gamma Buffers. These buffers serve as low impedance voltage sources

that generate the display’s gamma levels. The Gamma Buffers’ outputs are set by a simple resistive ladder, as

shown in Figure 34. Note that VGMA2 to VGMA4 and VGMA7 to VGMA9 are usually connected to the column

drivers even though they are not driven by external buffers. Doing so, forces the gamma levels in all the column

drivers to be identical, minimizing grayscale mismatch between column drivers. Referring again to Figure 34, the

resistive load of a column driver DAC (i.e. resistance between GMA1 to GMA5) is typically 10k

Ω to 15kΩ. On a

typical display such as XGA, there can be up to 10 column drivers, so the total resistive load on a Gamma Buffer

output can be as low as 1k

Ω. The voltage between VGMA1 and VGMA5 can range from 3V to 6V, depending on

the type of TFT panel. Therefore, maximum load current supplied by a Gamma Buffer is approximately 6V/1k

Ω =

6mA, which is a relatively light load for most op amps. In many displays, VGMA1 can be less than 500mV below

VDD, and VGMA10 can be less than 500mV above ground. Under these conditions, an op amp used for the

Gamma Buffer must have rail-to-rail inputs and outputs, like the LM6584.

Figure 34. Basic Gamma Buffer Configuration

18

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