LM6584MA NSC [National Semiconductor], LM6584MA Datasheet - Page 14

• Manufacturer Part Number: LM6584MA
• Description: TFT-LCD Quad, 13V RRIO High Output Current
• Manufacturer: NSC [National Semiconductor]

www.national.com
TFT Display Application
lay and slew limited response time (t = 0 to 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 1kΩ load, which represents
the maximum transient amplitude and output load for a
gamma buffer. With this test system, the LM6584 settled to
within
time for a 0 to –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 20Ω 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
FIGURE 11. Gamma Buffer Settling Time Test Circuit
±
30mV of 2V pulse in approximately 170ns. Settling
SR
to T
SET
) cannot. This is because an op
(Continued)
SR
) can be
20059236
14
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
needs to transmit a
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 51Ω 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
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
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.
−
−
were set to 0.5V and –12.5V when measuring set-
were set to 12.5V and –0.5V, respectively, when
FIGURE 12. Panel Repair Buffer
±
12V pulse and settle to within 60mV of
COM
Driver, the LM6584
COM
20059237
Driver,
COM
+
+