levels has a two 6 bit resistive DACs. Typically, the two

DACs will have their 64 resistors grouped into four seg-

ments, as shown in Figure 8. Each of these segments is

connected to external voltage lines, VGMA1 to VGMA10,

which are the Gamma Levels. VGMA1 to VGMA5 set gray-

scale voltage levels that are positive with respect to V

COM

(high polarity gamma levels). VGMA6 to VGMA10 set gray-

scale voltages negative with respect to V

gamma levels).

Figure 9 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 gener-

ate the display’s gamma levels. The Gamma Buffers’ outputs

are set by a simple resistive ladder, as shown in Figure 9.

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, mini-

mizing grayscale mismatch between column drivers. Refer-

ring again to Figure 9, 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 V

ground. Under these conditions, an op amp used for the

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

the LM6584.

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 out-

puts, 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.

Figure 10 illustrates how an op amp responds to a large-

signal transient. When such a transient occurs att=0,the

output does not start changing until T

amp’s propagation delay time (typically 20ns for the

LM6584). The output then changes at the op amp’s slew rate

from t = T

to its final value (V

small-signal frequency response. Although propagation de-

20059233

FIGURE 8. Simplified Schematic of Column Driver IC

20059234

FIGURE 9. Basic Gamma Buffer Configuration

20059235

FIGURE 10. Large Signal Transient Response of an

Operational Amplifier

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