lay and slew limited response time (t=0to T

calculated from data sheet specifications, the small signal

settling time (T

amp’s gain vs. frequency has multiple poles, and as a result,

small-signal settling time can not be calculated as a simple

function of the op amp’s gain bandwidth. Therefore, the only

accurate method for determining op amp settling time is to

measure it directly.

The test circuit in Figure 11 was used to measure LM6584

settling time for a 2V pulse and 1k

Ω load, which represents

the maximum transient amplitude and output load for a

gamma buffer. With this test system, the LM6584 settled to

time fora0to –2V pulse was slightly less, 150ns. These

values are much smaller than the desired response time of

1µs, so the LM6584 has sufficient bandwidth and slew rate

for regulating gamma line transients.

PANEL REPAIR BUFFER

It is not uncommon for a TFT panel to be manufactured with

an open in one or two of its column or row lines. In order to

repair these opens, TFT panels have uncommitted repair

lines that run along their periphery. When an open line is

identified during a panel’s final assembly, a repair line re-

routes its signal past the open. Figure 12 illustrates how a

column is repaired. The column driver’s output is sent to the

other end of an open column via a repair line, and the repair

line is driven by a panel repair buffer. When a column or row

line is repaired, the capacitance on that line increases sub-

stantially. For instance, a column typically has 50pF to

100pF of line capacitance, but a repaired column can have

up to 200pF. Column drivers are not designed to drive this

extra capacitance, so a panel repair buffer provides addi-

tional output current to the repaired column line. Note that

there is typically a 20

Ω to 100Ω resistor in series with the

buffer output. This resistor isolates the output from the

200pF of capacitance on a repaired column line, ensuring

that the buffer remains stable. A pole is created by this

resistor and capacitance, but its frequency will be in the

range of 8MHz to 40MHz, so it will have only a minor effect

on the buffer’s transient response time. Panel repair buffers

transmit a column driver signal, and as mentioned in the

gamma buffer section, this signal is set by the gamma levels.

It was also mentioned that many displays have upper and

lower gamma levels that are within 500mV of the supply

rails. Therefore, op amps used as panel repair buffers should

have rail-to-rail input and stages. Otherwise, they may clip

the column driver signal.

The signal from a panel repair buffer is stored by a pixel

when the pixel’s row is selected. In high-resolution displays,

each row is selected for as little as 11µs. To insure that a

pixel has adequate time to settle-out during this brief period,

a panel repair buffer should settle to within 1% of its final

value approximately 1µs after a row is selected. This is

hardest to achieve when transmitting a column line’s maxi-

mum voltage swing, which is the difference between the

upper and lower gamma levels (i.e. voltage between VGMA1

and VGMA10). For a LM6584, the most demanding applica-

tion occurs in displays that operate from a 13V supply. In

these displays, voltage difference between the top and bot-

tom gamma levels can be as large as 12V, so the LM6584

its final value in approximately 1µs (60mV is approximately

1% of the dynamic range of the high or low polarity gamma

levels). LM6584 settling times for 12V and –12V pulses were

measured in a test circuit similar to the one in Figure 11.V

+

and V

measuring settling time for a 0V to 12V pulse. Likewise, V

+

and V

tling time for a 0V to –12V pulse. In both cases, the LM6584

output was connected to a series RC load of 51

Ω and 200pF.

When tested this way, the LM6584 settled to within 60mV of

12V or –12V in approximately 1.1µs. These observed values

are very close to the desired 1µs specification, demonstrat-

ing that the LM6584 has the bandwidth and slew rate re-

quired for repair buffers in high-resolution TFT displays.

SUMMARY

This application note provided a basic explanation of how

op-amps are used in TFT displays, and it also presented the

specifications required for these op amps. There are three

major op amp applications in a display: V

COM

Driver,

Gamma Buffer, and Panel Repair Buffer, and the LM6584

can be used for all of them. As a V

can supply large values of output current to regulate V

COM

load transients. It has rail-to-rail input common-mode range

and output swing required for gamma buffers and panel

repair buffers. It also has the necessary gain bandwidth and

slew-rate for regulating gamma levels and driving column

repair lines. All these features make the LM6584 very well

suited for use in TFT displays.

20059236

FIGURE 11. Gamma Buffer Settling Time Test Circuit

20059237

FIGURE 12. Panel Repair Buffer

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