LM8207MT datasheet(16/22 Pages) NSC

16 / 22 page

Application Section (Continued)

TABLE 1. Resistor Values for Defining the Gamma

Correction Curve

Gamma Curve Definition

VGMA

Node

VGMA Voltage

Calculated Resistance (

Ω)

11.59

210

11.38

2200

9.18

670

8.51

670

7.84

430

7.41

280

7.13

980

6.15

6.07

170

5.90

5.82

980

4.84

280

4.56

430

4.13

670

3.46

730

2.73

2190

0.54

210

0.33

330

Changing the gamma correction curve, in combination with

the load of the column drivers can impact the behavior of the

gamma buffers. Gamma buffers 1 and 2 are designed for

operating voltages near V

DD, and will source the current into

the column drivers. Gamma buffers 17 and 18 are designed

for operating voltages near V

SS and will sink this current.

Buffers 3 to 16 are designed to operate in the mid-voltage

range and can sink or source current. Under special circum-

stances, by increasing the voltage gap between gamma

buffer 1 and gamma buffer 2, in combination with a low

impedance load of the column driver between these outputs,

the output of buffer 2 has to sink more current than possible,

and can saturate. This will result in a setting error of the

inputs of the column driver.

For buffer 17 and 18 an identical situation can occur, by

increasing the operating voltage range of buffer 17 with

respect to buffer 18.

A simple and cost effective solution is to lower the resistance

between buffer 2 and 3 or buffer 16 and 17, using an

additional by-pass resistor R

S. This method is presented in

Figure 10. This will not affect the desired voltage levels, and

buffer 3 which has a larger linear output current spec will sink

the current instead of buffer 2. The resistor value R

S can be

calculated by the voltage drop divided by the current. The

resistor value should be low enough to sink this current,

otherwise buffer 2 and/or buffer 17 will still saturate.

GAMMA VOLTAGE ACCURACY

Adding buffers to the tabs of the gamma correction resistor

chain will make the values more independent of the load

variations. Unfortunately, there are some other effects that

will influence the gamma values. The following effects deter-

mine the accuracy of each gamma voltage.

Major effects are:

•

Variation of the internal voltage reference. This can be

found in the Electrical Characteristics Table. This is the

maximum variation between parts.

•

Variation of the feedback resistors used for setting the

output voltage of the voltage reference (OUT_V

REF). Us-

ing high accuracy resistors will result in a small variation

of the output voltage between different boards

•

The accuracy of the resistors obtained from the gamma

correction voltage curve. The gamma correction curve

will be affected by the accuracy of the resistors. This will

vary over different boards. Temperature variations will not

affect this curve.

•

Output offset voltage (V

OS) of the buffers. Variations of

OS (output offset voltage) of the buffers, will affect the

gamma correction curve. The contribution of V

higher for the buffers driving the lower gamma voltages.

Minor effects are:

•

Input current (I

BIAS) of the gamma buffers. Variations of

the input current (I

BIAS) of the gamma buffers caused by

temperature changes, will affect the gamma correction

voltages.

20137946

FIGURE 10. Using additional by-pass resistor to

increase current sinking capability

LM8207MT Datasheet(PDF) 16 Page - National Semiconductor (TI)

LM8207MT Datasheet(HTML) 16 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Datasheet Download

Link URL