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STATIC GAMMA CHANNELS

DYNAMIC GAMMA CONTROL

END-USER SELECTED GAMMA CONTROL

To update all 24 programmable output voltages

BUF22821

I C

Changein

OutputVoltages

OUT1

OUT22

BKSEL

Switch

BUF22821

SBOS399 – JUNE 2007

The BUF22821 offers two static gamma buffers.

Dynamic gamma control is a technique used to

These two analog signal paths can be used to

improve

the

picture

quality

LCD

television

provide

additional

gamma

channels.

The

applications.

This

technique

typically

requires

STATOUTH

buffered

version

the

switching gamma curves between frames. Using the

STATINH pin. The STATOUTL pin is a buffered

BKSEL pin to switch between two gamma curves will

version of the STATINL pin. For typical output swing,

not likely lead to good results because of the 750

µs

see the Typical Characteristics.

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

Because the BUF22821 has two banks of nonvolatile

writing directly to the DAC register (nonvolatile).

memory, it is well-suited for providing two levels of

The

double

input

structure

saves

gamma control by using the BKSEL pin, as shown in

programming time by allowing updated DAC values

Figure 19. When the state of the BKSEL pin

to be pre-stored into the first register bank. Storage

changes,

the

BUF22821

updates

all

of this data can occur while a picture is still being

programmable buffer outputs simultaneously after

displayed. Because the data are only stored into the

750

µs (±80µs).

first register bank, the DAC/VCOM output values

To update all 24 programmable output voltages

remain unchanged—the display is unaffected. At the

simultaneously via hardware:

beginning or the end of a picture frame, the

DAC/VCOM outputs (and therefore, the gamma

Toggle the BKSEL pin to switch between Gamma

voltages) can be quickly updated by writing a '1' in bit

Curve 0 (stored in Bank0) and Gamma Curve 1

15 of any DAC/VCOM register. For details on the

(stored in Bank1).

operation of the double register input structure, see

All DAC/VCOM registers and output voltages are

the Updating the DAC Outputs section.

updated simultaneously after approximately 750

µs.

simultaneously via software:

STEP 1: Write to registers 1–24 with bit 15 always

'0'.

STEP 2: Write any DAC/VCOM register a second time

with identical data. Make sure that bit 15 is set to '1'.

All DAC/VCOM channels are updated simultaneously

after receiving the last bit of data.

Figure 19. Gamma Control

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