LM6584MA NSC [National Semiconductor], LM6584MA Datasheet - Page 13

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

TFT Display Application
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
(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
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 V
ground. Under these conditions, an op amp used for the
Gamma Buffer must have rail-to-rail inputs and outputs, like
the LM6584.
FIGURE 8. Simplified Schematic of Column Driver IC
DD
, and VGMA10 can be less than 500mV above
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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 at t = 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-
FIGURE 10. Large Signal Transient Response of an
FIGURE 9. Basic Gamma Buffer Configuration
PD
to T
SR
F
Operational Amplifier
. From t = T
) at a speed determined by the op amp’s
SR
to T
SE
PD
T, the output settles
, which is the op
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