LM8207MTSLASHNOPB datasheet(19/32 Pages) TI1

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OUT_1

OUT_2

OUT_3

LM8207

COLUMN DRIVER

IOUT_1

IOUT_2

IOUT_3

OUT_4

IOUT_4

BY-PASS RESISTOR

LM8207

www.ti.com

SNOSAL5A – SEPTEMBER 2005 – REVISED MARCH 2013

Figure 39. Using additional by-pass resistor to increase current sinking capability

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 determine the accuracy of each gamma voltage.

Major effects are:

•

Variation of the internal voltage reference. This can be found in the 16V 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_VREF).

Using 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 (VOS) of the buffers. Variations of VOS (output offset voltage) of the buffers, will affect the

gamma correction curve. The contribution of VOS is higher for the buffers driving the lower gamma voltages.

Minor effects are:

•

Input current (IBIAS) of the gamma buffers. Variations of the input current (IBIAS) of the gamma buffers caused

by temperature changes, will affect the gamma correction voltages.

VCOM BUFFER

The VCOM 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 VCOM line. These pulses are the result of charging

the capacitance between VCOM 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 positive or negative

current pulses of 100 mA or more, depending on the panel size. The VCOM buffer is designed to handle these

pulses. A VCOM buffer is basically a voltage regulator that can sink or source current in large capacitive loads.

The VCOM buffer should recover very fast from these disturbances. The operating voltage of the VCOM buffer is in

the middle of the gamma voltage range.

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