+

-

PANEL

REPAIR

BUFFER

20 TO 100:

COLUMN DRIVER

OPEN

COLUMN

LINE

TFT-LCD

PANEL

PANEL

REPAIR

LINE

LM6588

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SNOSA77D – MAY 2003 – REVISED MARCH 2013

capacitance on that line increases substantially. 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 additional 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 11

μs. 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 1

μs after a row is

selected. This is hardest to achieve when transmitting a column line’s maximum voltage swing, which is the

difference between the upper and lower gamma levels (i.e. voltage between VGMA1 and VGMA10). For a

LM6588, the most demanding application occurs in displays that operate from a 16V supply. In these displays,

voltage difference between the top and bottom gamma levels can be as large as 15V, so the LM6588 needs to

transmit a ±15V pulse and settle to within 60mV of its final value in approximately 1

μs (60mV is approximately

1% of the dynamic range of the high or low polarity gamma levels). LM6588 settling times for 15V and –15V

pulses were measured in a test circuit similar to the one in Figure 35. V+ and V− were set to 15.5V and –0.5V,

respectively, when measuring settling time for a 0V to 15V pulse. Likewise, V+ and V− were set to 0.5V and

–15.5V when measuring settling time for a 0V to –15V pulse. In both cases, the LM6588 output was connected

to a series RC load of 51

Ω and 200pF. When tested this way, the LM6588 settled to within 60mV of 15V or –15V

in approximately 1.1

μs. These observed values are very close to the desired 1μs specification, demonstrating

that the LM6588 has the bandwidth and slew rate required for repair buffers in high-resolution TFT displays.

Figure 36. Panel Repair Buffer

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:

VCOM Driver, Gamma Buffer, and Panel Repair Buffer, and the LM6588 can be used for all of them. As a VCOM

Driver, the LM6588 can supply large values of output current to regulate VCOM 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 LM6588 very well suited for use in TFT displays.

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