BUF20820 Datasheet, PDF (17/32 Pages) Burr-Brown (TI) – 18-Channel GAMMA VOLTAGE GENERATOR with Two Programmable VCOM Channels

BUF20820
www.ti.com
PROGRAMMABLE V_COM
The V_COM channels of the BUF20820 can swing to 2.5V
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). The gamma and the V_COM values can be
permanently stored in the internal OTP memory. The V_COM
channels can be programmed independently from the
gamma channels. Figure 14 shows the BUF20820 being
used for V_COM voltages.
BUF20820
V_{COM OUT1}
V_{COM OUT2}
V_COM
OUT1
OUT2
SBOS330C − DECEMBER 2005 − REVISED OCTOBER 2006
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 15 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 BUF20820. 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_{COM OUT1}, and V_{COM OUT2}. As described above,
this method of connecting REFH and REFL does not help
to maximize the output swing capability.
SDA
SCL
OUT17
OUT18
Gamma
References
Control Interface
Figure 14. BUF20820 Used for Programmable
V_COM
REFH AND REFL INPUT RANGE
Best performance and output swing range of the
BUF20820 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
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 15. Reference Buffer Output Voltage vs
Output Current
CONFIGURATION FOR 20 GAMMA CHANNELS
The V_COM outputs can be used as additional gamma
references in order to achieve two additional gamma
channels (20 total). The V_COM outputs will behave the
same as the OUT1−9 outputs when sourcing or sinking
smaller currents (see the Typical Characteristics,
Figure 4). The V_COM outputs are better able to swing to the
positive rail than to the negative rail. Therefore, it is better
to use the V_COM outputs for higher reference voltages; see
Figure 16.
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