Datasheet

TFT-LCD PANEL

GAMMA

BUFFERS

COLUMN

DRIVERS

VGMA1

VGMA2

VGMA3

VGMA5

VGMA4

VGMA6

VGMA7

VGMA8

VGMA9

VGMA10

COLUMN

DRIVERS

LM6588

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SNOSA77D –MAY 2003–REVISED MARCH 2013

Figure 33 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 33. 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 33, the

resistive load of a column driver DAC (i.e. resistance between GMA1 to GMA5) is typically 10kΩ 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 1kΩ. 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/1kΩ =

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

, 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 LM6588.

Figure 33. Basic Gamma Buffer Configuration

Another important specification for Gamma Buffers is small signal bandwidth and slew rate. When column drivers

select which voltage levels are written to a row of pixels, their internal DACs inject current spikes into the Gamma

Lines. This generates voltage transients at the Gamma Buffer outputs, and they should settle-out in less than

1μs to insure a steady output voltage from the column drivers. Typically, these transients have a maximum

amplitude of 2V, so a gamma buffer must have sufficient bandwidth and slew rate to recover from a 2V transient

in 1μs or less.

Product Folder Links: LM6588