BUF20800 Datasheet, PDF (14/27 Pages) Burr-Brown (TI) – 18-Channel GAMMA VOLTAGE GENERATOR with Two Programmable VCOM Channels

BUF20800
SBOS329D − JUNE 2005 − REVISED OCTOBER 2006
PROGRAMMABLE V_COM
The V_COM channels of the BUF20800 can swing to 2V from
the positive supply rail while sourcing 100mA and to 1V
above the negative rail while sinking 100mA (see Figure 4,
typical characteristic Output Voltage vs Output Current).
To store the gamma and the V_COM values, an external
EEPROM is required. During power-up of the LCD panel,
the timing controller can then read the EEPROM and load
the values into the BUF20800 to generate the desired
V_COM voltages, as illustrated in Figure 11 and Figure 12.
The V_COM channels can be programmed independently
from the gamma channels.
SDA
SCL
BUF20800
V_{COM OUT1}
V_{COM OUT2}
V_COM
OUT1
OUT2
OUT17
Gamma
References
OUT18
Control Interface
Figure 12. BUF20800 Used for Programmable
V_COM
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REFH AND REFL INPUT RANGE
Best performance and output swing range of the
BUF20800 are achieved by applying REFH and REFL
voltages that are slightly below the power-supply
voltages. Most specifications have been tested at
REFH = V_S − 200mV and REFL = GND + 200mV. The
REFH internal buffer is designed to swing very closely to
V_S and the REFL internal buffer to GND. However, there
is a finite limit on how close they can swing before
saturating. To avoid saturation of the internal REFH and
REFL buffers, the REFH voltage should not be greater
than V_S −100mV and REFL voltage should not be lower
than GND + 100mV. Figure 13 shows the swing capability
of the REFH and REFL buffers.
The other consideration when trying to maximize the
output swing capability of the gamma buffers is the
limitation in the swing range of output buffers (OUT1−18,
V_COM1, and V_COM2), which depends on the load current. A
typical load in the LCD application is 5−10mA. For
example, if OUT1 is sourcing 10mA, the swing is typically
limited to about V_S − 200mV. The same applies to OUT18,
which typically limits at GND + 200mV when sinking
10mA. An increase in output swing can only be achieved
for much lighter loads. For example, a 3mA load typically
allows the swing to be increased to approximately V_S −
100mV and GND + 100mV.
Connecting REFH directly to V_S and REFL directly to GND
does not damage the BUF20800. As discussed above
however, the output stages of the REFH and REFL buffers
will saturate. This condition is not desirable and can result
in a small error in the measured output voltages of
OUT1−18, V_COM1, and V_COM2. As described above, this
method of connecting REFH and REL does not help to
maximize the output swing capability.
18
17
16
REFH OUT (sourcing), Code = 3FFh
15
V_REFL = 1V, V_REFH = 17.8V
R_LOAD Connected to GND
3
REFL OUT (sinking), Code = 000h
2
V_REFL = 0.2V, V_REFH = 17V
R_LOAD Connected to 18V
1
0
0 10 20 30 40 50 60 70 80 90 100
Output Current (mA)
Figure 13. Reference Buffer Output Voltage vs
Output Current
14