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| 型號： | BUF20820EVM |
| 廠商： | Texas Instruments |
| 文件頁數(shù)： | 9/29頁 |
| 文件大小： | 0K |
| 描述： | EVAL MOUDLE FOR BUF20820 |
| 標(biāo)準(zhǔn)包裝： | 1 |
| 系列： | * |
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BUF20820
SBOS330E DECEMBER 2005 REVISED OCTOBER 2008
www.ti.com
17
PROGRAMMABLE VCOM
The VCOM 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 VCOM values can be
permanently stored in the internal OTP memory. The VCOM
channels can be programmed independently from the
gamma channels. Figure 14 shows the BUF20820 being
used for VCOM voltages.
BUF20820
SDA
SCL
R
e
gi
s
ter
Control Interface
Gamma
References
OUT1
OUT2
OUT17
OUT18
V
COM
V
COM OUT1
V
COM OUT2
Figure 14. BUF20820 Used for Programmable
VCOM
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 = VS 200mV and REFL = GND + 200mV. The
REFH internal buffer is designed to swing very closely to
VS 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 VS 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 (OUT118,
VCOM OUT1, and VCOM OUT2), which depends on the load
current. A typical load in the LCD application is 510mA.
For example, if OUT1 is sourcing 10mA, the swing is
typically limited to about VS 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
VS 100mV and GND + 100mV.
Connecting REFH directly to VS 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
OUT118, VCOM OUT1, and VCOM OUT2. As described above,
this method of connecting REFH and REFL does not help
to maximize the output swing capability.
18
17
16
15
3
2
1
0
O
u
tp
ut
V
o
lt
ag
e
(V
)
0
10
20
100
40
50
60
70
80
90
30
Output Current (mA)
REFL OUT (sinking), Code = 000h
V
REFL =0.2V, VREFH = 17V
R
LOAD Connected to 18V
REFH OUT (sourcing), Code = 3FFh
V
REFL =1V, VREFH =17.8V
R
LOAD Connected to GND
Figure 15. Reference Buffer Output Voltage vs
Output Current
CONFIGURATION FOR 20 GAMMA CHANNELS
The VCOM outputs can be used as additional gamma
references in order to achieve two additional gamma
channels (20 total). The VCOM outputs will behave the
same as the OUT19 outputs when sourcing or sinking
smaller
currents
(see
the
Typical
Characteristics,
Figure 4). The VCOM outputs are better able to swing to the
positive rail than to the negative rail. Therefore, it is better
to use the VCOM outputs for higher reference voltages; see
Figure 16.
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