LM8207 datasheet(17/22 Pages) NSC

17 / 22 page

Application Section (Continued)

COM BUFFER

The V

COM buffer supplies a common voltage to the back

plate of all the pixels in a TFT panel. When column drivers

write to the pixels, current pulses will occur onto the V

COM

line. These pulses are the result of charging the capacitance

between V

COM and the column lines. This capacitance is a

combination of stray capacitance and pixel capacitance. This

stray capacitance varies between panel sizes but typically

ranges from 16 pF to 33 pF per column. Pixel capacitance is

in the order of 0.5 pF and contributes very little to these

pulses because only one pixel at a time is connected to a

column. Charging this capacitance can result in short posi-

tive or negative current pulses of 100 mA or more, depend-

ing on the panel size. The V

COM buffer is designed to handle

these pulses. A V

COM buffer is basically a voltage regulator

that can sink or source current in large capacitive loads. The

COM buffer should recover very fast from these distur-

bances. The operating voltage of the V

COM buffer is in the

middle of the gamma voltage range.

The typical application in Figure 11 shows the V

COM buffer

supplying a common voltage to the back plate of the display.

This level can be adjusted by changing the value of the

resistors. Increasing the value of R

1 or decreasing the value

of R

2 will decrease the VCOM level. Increasing the value of

2 or decreasing the value of R1 will increase the VCOM

level.

Another, more flexible, solution is to use National Semicon-

ductor’s programmable V

COM calibrator, the LM8342. The

COM level can be adjusted using an I

2C interface. See the

LM8342’s datasheet for more detailed information about this

part.

LM8207 CONFIGURATION

A complete configured typical application of the LM8207 is

given in Figure 12. All three basic functions of the LM8207

are discussed in the previous sections. Details for setting the

Voltage Reference are given in the “Voltage Reference”

section. Calculations for defining a gamma correction curve

are given in the section entitled “Gamma Buffers.” Defining

and adjusting the V

COM level is discussed in the “VCOM

Buffer” section. The LM8207 is an 18 channel gamma buffer

plus a V

COM buffer. In certain applications some of the

gamma buffers or the V

COM buffer may not be used. In such

cases it is recommended that the unused buffer input pins be

tied to the input voltage range value.

20137928

FIGURE 11. VCOM Buffer

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