7.4.2 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 11 shows a block diagram using the BUF20800-Q1 for dynamic gamma control and V_COM output.

The BUF20800-Q1 is ideally suited for rapidly changing the gamma curves because 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 to write to all registers and the Output Latch sections.

The double register input structure saves programming time by allowing updated DAC values to be pre-loaded 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 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 the details on the operation of the double register input structure, see the Output Latch section.

Example: Update all 18 gamma registers simultaneously via software.

Step 1: Check if LD pin is placed in HIGH state.

Step 2: Write DAC Registers 1−18 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 will be updated simultaneously after receiving the last bit of data. (Note: this step may be eliminated by setting bit 15 of DAC 18 to ‘1’ in the previous step.)

Figure 11. Dynamic Gamma Control