BUF08821 Datasheet, PDF (17/27 Pages) Texas Instruments – Programmable Gamma-Voltage Generator and VCOM Calibrator with Integrated Two-Bank Memory

www.ti.com ................................................................................................................................................. SBOS438C – AUGUST 2008 – REVISED AUGUST 2009
END-USER SELECTED GAMMA CONTROL
Because the BUF08821 has two banks of nonvolatile
memory, it is well-suited for providing two levels of
gamma control by using the BKSEL pin, as shown in
Figure 17. When the state of the BKSEL pin changes,
the BUF08821 updates all 9 programmable buffer
outputs simultaneously in less than 10μs.
To update all nine programmable output voltages
simultaneously via hardware; use this procedure:
Toggle the BKSEL pin to switch between Gamma
Curve 0 (stored in Bank0) and Gamma Curve 1
(stored in Bank1).
All DAC/V_COM registers update in less than 5μs, and
all output voltages settle within the next 5μs.
5V
BKSEL
BUF08821
Switch
OUT1
Change in
Output Voltages
OUT8
I²C
pin toggles between the new gamma curves after the
data is updated. The gamma curve data in the
nonvolatile memory may be restored for each curve
individually by issuing a general acquire command for
the desired bank. To restore both curves, a general
acquire command must be issued for each bank.
OUTPUT PROTECTION
The BUF08821 output stages can safely source and
sink the current levels indicated in Figure 1 and
Figure 2. However, there are other modes where
precautions must be taken to prevent to the output
stages from being damaged by excessive current
flow. The outputs (OUT1 through OUT8 and the
V_COM) include ESD protection diodes, as shown in
Figure 18. Normally, these diodes do not conduct and
are passive during typical device operation. Unusual
operating conditions can occur where the diodes may
conduct, potentially subjecting them to high, even
damaging current levels. These conditions are most
likely to occur when a voltage applied to an output
exceeds (V_S) + 0.5V, or drops below GND – 0.5V.
One common scenario where this condition can occur
is when the output pin is connected to a sufficiently
large capacitor, and the BUF08821 power-supply
source (V_S) is suddenly removed. Removing the
power-supply source allows the capacitor to
discharge through the current-steering diodes. The
energy released during the high current flow period
causes the power dissipation limits of the diode to be
exceeded. Protection against the high current flow
may be provided by placing current-limiting resistors
in series with the output, as shown in Figure 10.
Select a resistor value that restricts the current level
to the maximum rating for the particular pin.
V_S
Figure 17. Gamma Control
DYNAMIC GAMMA CONTROL
Dynamic gamma control is a technique used to
improve the picture quality in LCD television
applications. This technique typically requires
switching gamma curves between frames. The
BUF08821 is well-suited to implementing dynamic
gamma control. The BUF08821 has two banks of
nonvolatile memory (one for each gamma curve), and
two sets of storage registers (one for each gamma
curve). The BKSEL pin enables the device to switch
between gamma curves; the BUF08821 outputs fully
settled within 10μs.
The BUF08821 reads gamma curve data from each
nonvolatile memory bank into the respective storage
registers upon power-on. The BKSEL pin toggles
between the two gamma curves. It is also possible to
update the gamma curves by writing new data to the
storage registers via the I²C interface. The BKSEL
BUF08821
ESD Current
Steering Diodes
OUTX
or
V_COMX
Figure 18. Output Pins ESD Protection
Current-Steering Diodes
Copyright © 2008–2009, Texas Instruments Incorporated
Product Folder Link(s): BUF08821
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