BUF07703_16 Datasheet, PDF (15/32 Pages) Texas Instruments – Multi-Channel LCD Gamma Correction Buffer

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INPUT VOLTAGE RANGE GAMMA BUFFERS
Figure 36 shows a typical gamma correction curve
with 10 gamma correction reference points (GMA1
through GMA10). As can be seen from this curve, the
voltage requirements for each buffer varies greatly.
The swing capability of the input stages of the various
buffers in the BUFxx703 is carefully matched to the
application. Using the example of the BUF07703 with
six gamma correction channels, buffers 1 to 3 have
input stages that include V_DD, but will only swing
within 1V to GND. Buffers 1 through 5 have only a
single NMOS input stage. Buffers 4 through 6 have
only a single PMOS input stage. The input range of
the PMOS input stage includes GND.
V_DD1
GMA1
GMA2
GMA3
GMA4
GMA5
GMA6
GMA7
GMA8
GMA9
GMA10
V_SS1
0
10
20
30
40
Input Data HEX0
Figure 36. Gamma Correction Curve
OUTPUT VOLTAGE SWING GAMMA
BUFFERS
The output stages have been designed to match the
characteristic of the input stage. Once again using
the example of the BUF07703 means that the output
stage of buffer 1 swings very close to V_DD, typically
V_CC − 100mV at 5mA; its ability to swing to GND is
limited. Buffers 2 and 3 have smaller output stages
with slightly larger output resistances, as they will not
have to swing as close to the positive rail as buffer 1.
Buffers 4 through 6 swing closer to GND than V_DD.
Buffer 6 is designed to swing very close to GND,
typically GND + 100mV at a 5mA load current. See
the Typical Characteristics for more details. This
approach significantly reduces the silicon area and
cost of the whole solution. However, due to this
architecture, the correct buffer needs to be connected
to the correct gamma correction voltage.
BUF07703
BUF06703
BUF05703
SBOS269C – MARCH 2003 – REVISED APRIL 2010
Connect buffer 1 to the gamma voltage closest to
V_DD, and buffers 2 and 3 to the sequential voltages.
Buffer 6 should be connected to the gamma
correction voltage closest to GND (or the negative
rail), buffers 4 and 5 to the sequential higher
voltages.
COMMON BUFFER (V_COM)
The common buffer output of the BUF07703 and
BUF05703 has a greater output drive capability than
the gamma correction buffers, to meet the heavier
current demands of driving the common node of the
LCD panel. It was also designed to drive heavier
capacitive loads and still remain stable, as shown in
Figure 37.
45
40 ^{VDD = 10V}
R_L = 2kW
35 V_COM
30
25
20
15
10
5
0
10
100
Load Capacitance, C_L (pF)
1000
Figure 37. Phase Shift vs Load Capacitance
CAPACITIVE LOAD DRIVE
The BUFxx703 has been designed to be able to
sink/source DC currents in excess of 10mA. Its output
stage has been designed to deliver output current
transients with little disturbance of the output voltage.
However, there are times when very fast current
pulses are required. Therefore, in LCD source-driver
buffer applications, it is quite normal for capacitors to
be placed at the outputs of the reference buffers.
These are to improve the transient load regulation.
These will typically vary from 100pF and more. The
BUFxx703 gamma buffers were designed to drive
capacitances in excess of 100pF and retain effective
phase margins above 50°, as shown in Figure 38.
Copyright © 2003–2010, Texas Instruments Incorporated
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