buf06703 Datasheet, PDF (16/25 Pages) Burr-Brown (TI) – MULTI-CHANNEL LEC GAMMA CORRECTION BUFFER

BUF07703
BUF06703
BUF05703
SBOS269A – MARCH 2003 – REVISED JUNE 2003
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.
VDD1
GMA1
GMA2
GMA3
GMA4
GMA5
GMA6
GMA7
GMA8
GMA9
GMA10
VSS10
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. 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.
16
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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 = 10 V
RL = 2 kΩ
35 _VCOM
30
25
20
15
10
5
0
10
100
CL – Load Capacitance – 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.
140
BUF07703: Channels 1 to 6
120 _{BUF06703: Channels 1 to 6}
100 ^{BUF05703: Channels 1 to 4}
80
60
VDD = 10 V
RL = 2 kΩ
40
20
0
10
100
CL – Load Capacitance – pF
1000
Figure 38. Phase Shift Between Output and Input
vs Load Capacitance for the gamma buffers