BUF16821 datasheet(19/29 Pages) TI

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END-USER SELECTED GAMMA CONTROL

DYNAMIC GAMMA CONTROL

BUF16821

I C

Changein

OutputVoltages

OUT1

OUT16

BKSEL

Switch

BUF16821

www.ti.com ...................................................................................................................................................................................................... SBOS428 – JULY 2008

Because the BUF16821 has two banks of nonvolatile

Dynamic gamma control is a technique used to

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

improve

the

picture

quality

LCD

television

gamma control by using the BKSEL pin, as shown in

applications.

This

technique

typically

requires

Figure 17. When the state of the BKSEL pin changes,

switching gamma curves between frames. Using the

the BUF16821 updates all 18 programmable buffer

BKSEL pin to switch between two gamma curves

outputs simultaneously after 750

µs (±80µs).

does not often provide good results because of the

750

µs required to transfer the data from the

To update all 18 programmable output voltages

nonvolatile memory to the DAC register. However,

simultaneously via hardware, toggle the BKSEL pin to

dynamic gamma control can still be accomplished by

switch between Gamma Curve 0 (stored in Bank0)

storing two gamma curves in an external EEPROM

and Gamma Curve 1 (stored in Bank1).

and writing directly to the DAC register (volatile).

All DAC/VCOM registers and output voltages are

The

double

input

structure

saves

updated simultaneously after approximately 750

µs.

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/VCOM output values

remain unchanged—the display is unaffected. At the

beginning or the end of a picture frame, the

DAC/VCOM outputs (and therefore, the gamma

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

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

operation of the double register input structure, see

the Updating the DAC Outputs section.

To update all 18 programmable output voltages

simultaneously via software, perform the following

actions:

STEP 1: Write to registers 1–18 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 17. Gamma Control

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