1.2 Block Diagram
1.2.4 Data Driving Method
Before introducing the VCOM driver, we need to discuss the data driving method of the
TFT-LCD driver first. The liquid crystal acts like the light gate, controlled by the voltage across it, to adjust the angle of the liquid crystals, resulting in brightness control. Unlike the spontaneous light emitting display panel, the TFT-LCD panel need extra backlight source such as Cold Cathode Fluorescent Light (CCFL) or Light Emitting Diode (LED) to display a picture.
The electric field applied to the liquid crystals is oriented. If we change the electric field with an opposite direction, that is so-called polarity change [2]. This polarity change would not and should not result in the transmitted luminance of the liquid crystals, but the angles of them. Because the DC blocking effect of the orientation layer and the dc residue effect of TFT-LCD panel, we must apply polarity change by line or by frame.
There are two methods of polarity change, which will be explained in the following contexts. Fig. 10 shows the inversion types adopted in the TFT-LCD driver to do the polarity change for preventing the liquid crystals from dc residue effect and increase life time.
Fig. 10. Inversion Types of TFT-LCD Driver.
Frame inversion updates the pixel voltage polarity in the form of frame. We can see all of the sub-pixels in frame 1 are positive, and in frame 2 are negative.
Line inversion updates the pixel voltage polarity in the form of line (gate driver
direction). We can see in frame 1, the sub-pixels of GODD are positive and those of GEVEN are negative. But in frame 2, the sub-pixels of GODD are negative and those of GEVEN are positive.
Column inversion updates the pixel voltage polarity in the form of column. We can see in frame 1, the sub-pixels of SODD are positive and those of SEVEN are negative. But in frame 2, the sub-pixels of SODD are negative and those of SEVEN are positive.
Dot inversion updates the pixel voltage polarity in the form of pixel. We can see in frame 1, all of the polarity of the sub-pixel is contrary to the neighbor ones. And in frame 2, all of the polarity of the sub-pixel change.
AC VCOM Data Driving Method
VCOM voltage is a reference common electro plate voltage inside the panel. AC VCOM means the VCOM voltage level changes polarity by line or by frame to avoid the dc residue effect of the liquid crystals. In Fig. 11, VCOM voltage changes polarity by line or by frame between the voltage level VCOMH and VCOML. The VCOMH and VCOML voltage depend on the LC characteristics for the different panel makers.
In the positive polarity period, the voltages across the liquid crystals are gray scale voltages (V0+ ~ V63+) minus VCOML, these voltages are the positive sign voltages, and then, in the negative polarity period, the voltages across the liquid crystals are gray scale voltages (V0+ ~ V63+) minus VCOMH, these voltages are the negative sign voltages.
If we well adjust VCOMH and VCOML voltage level to let the positive sign voltage just equal to the absolute value of the negative sign voltage, by doing so can prevent the liquid crystals from dc voltage residue and increase their life time.
AC VCOM Driving Method
DC VCOM Data Driving Method
Fig. 12 shows the DC VCOM driving method. DC VCOM means the VCOM voltage level always keeps a dc value at anytime. By this way, the job of polarity change is controlled only by the source drivers.
In the positive polarity period, the voltages across the liquid crystals are gray scales voltage (V0+
~ V63+
) minus VCOM, these voltages are the positive sign voltages, and then, in the negative polarity period, the voltages across the liquid crystals are gray scale voltages (V0+
~ V63+
) minus VCOM, these voltages are the negative sign voltages.
If we well adjust VCOM voltage level to let the positive sign voltage just equal to the absolute value of the negative sign voltage, by doing so can prevent the liquid crystals from dc voltage residue and increase their life time. But different from the AC VCOM, the gray scale voltages range (V0+
~ V63+
) of the DC VCOM method must large than the AC VCOM to
maintain the same voltage levels across the liquid crystals.
DC VCOM Driving Method V0
+
V1 +
V62 +
V63 +
V63
-V62
-V1
-V0
-Gray Scale Voltage
VCOM waveform
Positive Polarity Negative Polarity 1 line/frame
Fig. 12. DC VCOM Driving Method.
Comparison Between AC VCOM and DC VCOM
TABLE I. shows the advantages and disadvantages comparisons of AC VCOM and DC VCOM. Let’s gain further insight into the differences between the AC VCOM and the DC VCOM data driving method.
For example, in AC VCOM case, assume Gray scale voltage V0+ =4V
Gray scale voltage V0− =0.5V VCOML voltage VCOML=−0.5V VCOMH voltage VCOMH =5V
(1)
In the positive polarity, the voltage across the liquid crystal is 4.5V, and in the negative polarity, the voltage across the liquid crystal is -4.5V. Because the Gray scale voltage range is 4V to 0.5V, the source driver only need the 5V device for this design.
In DC VCOM case, assume Gray scale voltage V0+ =4V
Gray scale voltage V0− =−5V VCOM voltage VCOM =−0.5V
(2)
In the positive polarity, the voltage across the liquid crystal is 4.5V, and in the negative polarity, the voltage across the liquid crystal is -4.5V. Because the Gray scale voltage range is 4V to -5V, the 5V device can not meet this design in the source driver. In this case, the source driver needs the 12V device and this will increase the wafer cost.
In TABLE I, the image quality of DC VCOM is better than AC VCOM, but the cost of DC VCOM is more than AC VCOM because the higher voltage process is needed. This work uses AC VCOM for design because the cost of AC VCOM is lower than DC VCOM.
TABLE I. COMPARISONS OF DATA DRIVING METHODS
Characteristics AC VCOM DC VCOM
Inversion Type Frame, Line inversion
Frame, Line, Column,
Dot inversion
Image quality Poor Better
Power dissipation Higher Lower
Process Lower voltage process Higher voltage process
Cost Lower Higher