BUF22821 Datasheet, PDF (18/25 Pages) Texas Instruments – Programmable Gamma-Voltage Generator and VCOM Calibrator with Integrated Two-Bank Memory

BUF22821
SBOS399 – JUNE 2007
STATIC GAMMA CHANNELS
The BUF22821 offers two static gamma buffers.
These two analog signal paths can be used to
provide additional gamma channels. The
STATOUTH pin is a buffered version of the
STATINH pin. The STATOUTL pin is a buffered
version of the STATINL pin. For typical output swing,
see the Typical Characteristics.
END-USER SELECTED GAMMA CONTROL
Because the BUF22821 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 19. When the state of the BKSEL pin
changes, the BUF22821 updates all 24
programmable buffer outputs simultaneously after
750µs (±80µs).
To update all 24 programmable output voltages
simultaneously via hardware:
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 and output voltages are
updated simultaneously after approximately 750µs.
5V
BKSEL
BUF22821
Switch
OUT1
Change in
Output Voltages
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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. Using the
BKSEL pin to switch between two gamma curves will
not likely lead to good results because of the 750µs
it takes to transfer the data from the nonvolatile
memory to the DAC register. However, dynamic
gamma control can still be accomplished by storing
two gamma curves in an external EEPROM and
writing directly to the DAC register (nonvolatile).
The double register input structure saves
programming time by allowing updated DAC values
to be pre-stored into the first register bank. Storage
of this data can occur while a picture is still being
displayed. Because the data are only stored into the
first register bank, the DAC/V_COM output values
remain unchanged—the display is unaffected. At the
beginning or the end of a picture frame, the
DAC/V_COM outputs (and therefore, the gamma
voltages) can be quickly updated by writing a '1' in bit
15 of any DAC/V_COM register. For details on the
operation of the double register input structure, see
the Updating the DAC Outputs section.
To update all 24 programmable output voltages
simultaneously via software:
STEP 1: Write to registers 1–24 with bit 15 always
'0'.
STEP 2: Write any DAC/V_COM register a second time
with identical data. Make sure that bit 15 is set to '1'.
All DAC/V_COM channels are updated simultaneously
after receiving the last bit of data.
OUT22
I²C
Figure 19. Gamma Control
18
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