LM8207 datasheet(14/22 Pages) NSC

14 / 22 page

Application Section (Continued)

GAMMA BUFFERS

This section gives an overview for the applications of the

gamma buffers and also defines the gamma correction

curve. Specifications for the buffers are derived from their

operation range. Also included are the formulas for the real-

ization of the gamma correction curve using external resis-

tors. An overview is given for the gamma voltage accuracy,

using the LM8207 in combination with external resistors.

As discussed in the section entitled “Principle Operation of a

TFT-LCD,” the basic function of the gamma buffers is to

make the gamma correction curve independent of the be-

havior of the column driver. Writing data to each subsequent

pixel will cause load variations. The gamma buffers have a

low impedance output and can handle these variations with-

out changing the gamma correction curve. A typical gamma

correction curve is given in Figure 8.

Each buffer covers a part of the correction curve and, there-

fore, has its own specifications. All buffers require that the

output should recover quickly from disturbances caused by

the switching of the column driver. The gamma voltage level

of each buffer (VGMA1…VGMA18) depends on its position

for the levels decrease sequentially. To best utilize the

LM8207, each buffer is optimized for its position in the

gamma correction curve.

•

Gamma Buffers 1-2

Operating voltage range: V

DD to VSS +2V. Due to the

operating voltage, only negative transitions at the output

are possible. Positive transitions will exceed the supply

voltage V

DD. These buffers are able to source current to

bias the resistive load of the column driver having an

open collector structure. To meet the operating voltage

range, these outputs need a resistive load connected to a

lower potential sourcing an output current of at least

1 mA.

•

Gamma Buffers 3-16

Operating voltage range: V

DD – 1VtoVSS + 1V. Due to

the operating range, both positive and negative transi-

tions at the outputs are possible.

•

Gamma Buffers 17-18

Operating voltage range: V

DD -2toVSS. Due to the

operating voltage, only positive transitions at the output

are possible. Negative transitions will exceed the nega-

tive supply voltage V

SS. These buffers are able to sink

current from the resistive load of the column driver having

an open collector structure. To meet the operating volt-

age range, these outputs need a resistive load connected

to a higher potential sinking an output current of at least

1mA

Example:

A typical application using the LM8207 is given in Figure 9.

The

corresponding

gamma

correction

curve

(VGMA1...VGMA18) is defined in Table 1. The Voltage Ref-

erence supplies the 14.4V to the resistor network. The cal-

culations for the resistor values and for setting the Voltage

Reference are shown in the section “Voltage Reference.”

20137922

FIGURE 7. Voltage Reference Headroom vs. Load

Current

20137923

FIGURE 8. Typical Gamma Correction Curve

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