BUF12840_12 Datasheet, PDF (17/26 Pages) Texas Instruments – Programmable Gamma-Voltage Generator with Integrated Two-Bank Memory and External EEPROM

BUF12840
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DYNAMIC GAMMA CONTROL
Dynamic gamma control is a technique used to
improve the picture quality in LCD TV applications.
The brightness in each picture frame is analyzed and
the gamma curves are adjusted on a frame-by-frame
basis. The gamma curves are typically updated
during the short vertical blanking period in the video
signal. Figure 19 shows a block diagram using the
BUF12840 for dynamic gamma control.
The BUF12840 is ideally suited for rapidly changing
the gamma curves as a result of its unique topology:
• Double register input structure to the DAC
• Fast serial interface
• Simultaneous updating of all DACs by software.
See the Read/Write Operations section to write to
all registers and the Output Latch sections.
END-USER SELECTED GAMMA CONTROL
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 output values remain
unchanged—the display is unaffected. During the
vertical sync period, the DAC outputs (and therefore,
the gamma voltages) can be quickly updated either
SBOS519A – OCTOBER 2010 – REVISED JULY 2011
by using an additional control line connected to the
LD pin, or through software—writing a '1' in bit 15 of
any DAC register. For details on the operation of the
double register input structure, see the Output Latch
section.
Example: Update all 12 registers simultaneously via
software.
Step 1: Check if the LD pin is placed in a high
state.
Step 2: Write DAC registers 1-12 with bit 15
always '0'.
Step 3: Write any DAC register a second time
with identical data. Make sure that bit 15 is '1'. All
DAC channels are updated simultaneously after
receiving the last bit of data.
Example: Update all 12 registers simultaneously via
hardware.
Step 1: Toggle the BKSEL pin to the desired
gamma curve, either Bank0 or Bank1.
Step 2: Toggle the LD pin low. When this occurs,
all 12 internal DAC registers are updated after
1µs. The output then slews to the new voltage
level. The time to change between two gamma
voltage settings is then dependent on the slew
rate of the DAC plus the gamma buffer and the
change in voltage required. This value can be
obtained by referring to the Large-Signal Step
Response curve (Figure 9).
Digital
Picture
Data
Histogram
Gamma
Adjustment
Algorithm
Black White
Timing Controller/Microcontroller
BUF12840
Gamma References
A through L
Source Driver Source Driver
Figure 19. Dynamic Gamma Control
Copyright © 2010–2011, Texas Instruments Incorporated
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