This application claims the priority benefit of Taiwan application serial no. 96116272, filed on May 8, 2007. The entirety the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification.
1. Field of the Invention
The present invention generally relates to an Optically Compensated Bend mode Liquid Crystal Display (OCB LCD), and more particularly to a pixel and driving method thereof for an OCB LCD.
2. Description of Related Art
To correspond with our modern living style, the volume and weight of video or image devices are decreasing all the time. Although the conventional Cathode Ray Tube (CRT) still has its advantages, the electron gun structure renders the display bulky, and the radiation produced by the electrons bombarding the fluorescent screen is also harmful to the human eyes. Therefore, through the rapid development of optical-electrical technology and semiconductor processing technology, flat panel displays such as Liquid Crystal Display (LCD), Organic Light-Emitting Display (OLED) or Plasma Display Panel (PDP) have become the mainstream display product.
In the field of LCD, the OCB LCD has a fast response and a wide viewing angle. With these advantages, OCB LCD has great potential for developing into large LCD. However, the leakage current problem of Thin Film Transistor (TFT) is the common bottleneck encountered by all kinds of LCD, particularly for OCB LCD.
When the liquid crystal is in a bend state, the only thing that needs to be considered is that the white state voltage VPCW (the voltage across liquid crystals during displaying a white image) must be higher than the critical voltage VCR. Otherwise, the liquid crystal may drop back from bend state with higher free energy to the splay state with lower free energy. However, due to the effect of leakage current from TFT or liquid crystal capacitor, the white state voltage VPCW in partial areas of the display panel may be lower than the critical voltage VCR. Therefore, the liquid crystals in the areas may revert back to the splay state so that an abnormal image is displayed. To resolve this problem, the white state voltage VPCW is normally increased so as to maintain the liquid crystals in the bend state with higher free energy. Although this method can prevent the appearance of abnormal images, the light transmittance of the liquid crystals with a white image is sacrificed.
Accordingly, the present invention provides a pixel that can be applied to an Optically Compensated Bend mode Liquid Crystal Display. Together with the driving method of the present invention, a low white state voltage in the bend state is obtained so as to increase the light transmittance during displaying a white image.
The present invention also provides a Liquid Crystal Display that utilizes an over-driving voltage to maintain the white state voltage of the OCB LCD at a lower voltage potential without getting into the splay state. As a result, the light transmittance with a white image is increased so that a brighter white image is obtained.
The present invention also provides a pixel driving method. After charging a liquid crystal capacitor, an over-driving voltage is coupled to the liquid crystal capacitor so as to prevent the liquid crystal from getting into the splay state. Therefore, the white state voltage can be lower so that the light transmittance with a white image is increased and a brighter white image is obtained.
According to an embodiment of the present invention, a pixel is provided. The pixel includes a transistor, a liquid crystal capacitor, a storage capacitor and a coupling capacitor. A first end of the transistor is connected to a data line, and a gate of the transistor is connected to a scan line. The liquid crystal capacitor and the storage capacitor are coupled between a second end of the transistor and a common voltage. The coupling capacitor is connected between the second end of the transistor and a select line.
In a preferred embodiment of the present invention, after a driving voltage is outputted to the liquid crystal capacitor and the storage capacitor by the data line, the select line inputs an over-driving voltage to the coupling capacitor. Due to the capacitor coupling between the liquid crystal capacitor and the coupling capacitor, the over-driving voltage forms an impulse signal on the driving voltage. The foregoing impulse signal increases the pixel voltage due to coupling effect on capacitor and then lowers back to the original pixel voltage so that the root-mean-square value of the liquid crystal voltage is higher than the critical voltage. Therefore, even if the lowest pixel voltage of the liquid crystal is not high enough, the liquid crystal can still be maintained in the bend state to display images normally.
The present invention also provides a liquid crystal display that includes a first scan line, a plurality of data lines and a first select line. The first scan line corresponds with a plurality of first pixels. The data lines are used for driving the first pixels. The first select line outputs a first over-driving voltage to the first pixels according to the driving polarities of the data lines. After the data lines output driving voltages to the first pixels, the first select line outputs a first over-driving voltage to the first pixels.
In a preferred embodiment of the present invention, the liquid crystal display further includes a second scan line and a second select line. The second scan line corresponds with a plurality of second pixels. The data lines also drive the second pixels. The second select line outputs a second over-driving voltage to the second pixels according to the driving polarities of the data lines. After the data lines output driving voltages to the second pixels, the second select line outputs a second over-driving voltage to the second pixels.
The present invention also provides another liquid crystal display that includes a first data line and a first select line. The first data line is used for driving a plurality of first pixels. The first select line outputs a first over-driving voltage to the first pixels according to the driving polarities of the first data lines. After charging one of the first pixels through the first data line, the first select line outputs the first over-driving voltage to the first pixels.
According to a preferred embodiment of the present invention, the liquid crystal display further includes a second data line and a second select line. The second data line is used for driving a plurality of second pixels. The second select line outputs a second over-driving voltage to the second pixels according to the driving polarities of the second data lines. After charging one of the second pixels through the second data line, the second select line outputs the second over-driving voltage to the second pixels.
From another point of view, the present invention also provides another liquid crystal display that includes a scan line, a first select line and a second select line. The scan line corresponds with a first pixel and a second pixel. The first pixel corresponds with a first data line and the second pixel corresponds with a second data line. The first select line outputs a first over-driving voltage to the first pixel according to the driving polarity of the first data line. The second select line outputs a second over-driving voltage to the second pixel according to the driving polarity of the second data line. After charging the first pixel through the first data line, the first select line outputs a first over-driving voltage to the first pixel. After charging the second pixel through the second data line, the second select line outputs a second over-driving voltage to the second pixel. The first pixel is adjacent to the second pixel. Furthermore, the driving polarities of the first data line and the second data line are opposite to each other.
According to a preferred embodiment of the present invention, the foregoing liquid crystal displays can be row inversion mode, column inversion mode and dot inversion mode. Because the over-driving voltage increases the ability of the driving voltage to drive the liquid crystals, the liquid crystals will not change state due to an insufficiently high pixel voltage (lower than the critical voltage) during the liquid crystals displaying a white image.
The present invention also provides a pixel driving method that includes the following steps. First, a voltage is provided to a pixel. Thereafter, a coupling capacitor is used to couple an over-driving voltage to the pixel.
According to a preferred embodiment of the present invention, because the over-driving voltage forms an impulse signal on the white state voltage of the pixels in the foregoing pixel driving method, the overall root-mean-square (RMS) value of the white state voltage is greater than a critical value during the reaction time of the liquid crystals. Therefore, even though the lowest white state voltage is lower than the critical voltage, the liquid crystals can still be maintained in the bend state for displaying images normally. As a result, the operating voltage range of the pixel is increased.
From another point of view, the present invention also provides a pixel structure that includes a substrate, an insulation layer, a passivation layer, a pixel electrode, a common voltage connecting line and a select line. The insulation layer is formed on the substrate, the passivation layer is formed on the insulation layer, and the pixel electrode is formed on the passivation layer. The common voltage connecting line is located between the substrate and the insulation layer. The select line is also located between the substrate and the insulation layer. The common voltage connecting line and the pixel electrode form a storage capacitor while the select line and the pixel electrode form a coupling capacitor.
From another point of view, the present invention also provides a pixel structure that includes a substrate, an insulation layer, a passivation layer, a pixel electrode, a common voltage connecting line and a select line. The insulation layer is formed on the substrate, the passivation layer is formed on the insulation layer, and the pixel electrode is formed on the passivation layer. The common voltage connecting line is located between the substrate and the insulation layer. The select line is located between the insulation layer and the passivation layer. The common voltage connecting line and the pixel electrode form a storage capacitor while the select line and the pixel electrode form a coupling capacitor.
In a preferred embodiment of the present invention, the storage capacitor and the liquid crystal capacitor of the pixel structure can obtain an impulse signal from the coupling capacitor so that the root-mean-square value of the pixel voltage is higher than the critical voltage. Therefore, the white state voltage can be closer to the critical voltage so as to increase the light transmittance of liquid crystals during displaying a white image.
In the present invention, the coupling capacitor is connected in parallel with the liquid crystal capacitor so that the coupling capacitor can provide an over-driving voltage to the liquid crystal capacitor. The preferred embodiment of the present invention at least includes:
1. Even though the lowest pixel voltage of the liquid crystal is not high enough, the liquid crystal can still be maintained in the bend state so as to the image can be still displayed normally.
2. During displaying a white image, the liquid crystals will not change state due to insufficient driving ability of the driving voltage.
3. The operating voltage range of the pixel is increased.
4. The white state voltage can be closer to the critical voltage so as to increase the light transmittance of the liquid crystals during displaying a white image.
In order to male the aforementioned and other objects, features and advantages of the present invention comprehensible, preferred embodiments accompanied with figures are described in detail below.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.
The light transmittance of a liquid crystal is determined by the electric field applied to the liquid crystal. When the electric field applied to the liquid crystal is changed before the liquid crystal starts to react, the deviations of the liquid crystal are affected by the change in the electric field due to the viscosity coefficient and the elasticity coefficient of the liquid crystal material. In other words, the arrangement of the liquid crystal is determined by the mean of the torque applied to the liquid crystal by the electric field per unit time, and the torque is proportional to the square of the magnitude of the electric field. Because the light transmittance is related to the arrangement of the liquid crystal, the light transmittance is related to the Root-Mean-Square (RMS) value of the driving voltage. The equation is as follow:
wherein, V(t) is a voltage function of time, and T is the variation period of V(t).
Using the theory mentioned above, a group or multiple groups of impulse signals can be applied to the pixel voltage to resolve the problem encountered in the prior technique. As shown in
The gate voltage VG indicates the gate driving voltage on a scan line used for turning on a corresponding scan line. The data line supply voltage VDL indicates the voltage outputted by the data line. The over-driving voltage VSEL outputted by the select line can have the waveform of an impulse signal (for example, PS, PS1 and PS2). The common voltage VCOM indicates the voltage level of the common voltage.
Through the impulse signal PS, though when the white state voltage with lower potential VPCWF, which has a lower potential, is finally equal to or lower than the critical voltage VCR, the liquid crystal still will not change into the hard controlled splay state. Therefore, in actual applications, the foregoing method allows the white state voltage VPCW to operate closer to the critical voltage VCR. As show in
In addition, the storage capacitor CST1 is connected in parallel with the liquid crystal capacitor CLC so as to increase the energy storage capacity of the liquid crystal capacitor CLC. After charging the liquid crystal capacitor CLC, negative effects such as a leakage current in the transistor TFT may lead to a lowering of the pixel voltage. The parallel connection between the storage capacitor CST1 and the liquid crystal capacitor CLC can generate a larger capacitance, and store more electric charges so that the rate of lowering of the pixel voltage can be attenuated.
After a voltage is stored in the liquid crystal capacitor CLC and the storage capacitor CST1 by the data line DL through the transistor TFT, the select line SEL couples an over-driving voltage (for example, VSEL) to the liquid crystal capacitor CLC through the coupling capacitor CST2. Within a short period of time, the pixel voltage is increased, and the over-driving voltage generates an impulse signal (for example, PS in
The driving method and pixel structure in the foregoing embodiment can be applied to an OCB LCD with frame inversion mode, row inversion mode, column inversion mode and dot inversion mode.
The over-driving voltages output by the select lines SEL1˜SELN will change according to the driving polarities of the data lines DL1˜DLM. When the data lines DL1˜DLM drive with positive polarity, the over-driving voltage is positive. On the other hand, when the data lines DL1˜DLM drive with negative polarity, the over-driving voltage is negative. Therefore, the embodiment in
The LCD in
In another embodiment of the present invention, the display can dispose two select lines in each row of pixel capacitors (to indicate the equivalent capacitance of the pixels) so as to adapt to the driving method with dot inversion mode.
For a description of the pixel structures in
The present invention also provides six kinds of layouts that can be used to form the pixel structure shown in
Next,
First, the layouts in
From a processing point of view, refer to
In
The pixel structures shown in
From another point of view, the foregoing embodiment can be regarded as a driving method.
In the aforementioned pixel structure, the liquid crystal capacitor obtains an impulse signal from the coupling capacitor so that the RMS pixel voltage is higher than the critical voltage. As a result, the white state voltage is closer to the critical voltage, and the light transmittance of the liquid crystal displaying a white image is increased.
In summary, because the coupling capacitor provides an over-driving voltage to produce an impulse signal in the voltage provided by the data line in the embodiment of the present invention, the following advantages are produced:
1. Even though the white state voltage of the liquid crystal is lower than the critical voltage, the liquid crystal can still be maintained in the bend state and display images normally.
2. When the OCB LCD displays a white image, the liquid crystals with OCB mode will not change to a splay state due to insufficient driving ability of the driving voltage.
3. The voltage operating range of the pixel is expanded.
4. The white state voltage can be closer to the critical voltage so that the light transmittance of the liquid crystals is increased during displaying a white image.
5. The pixel structure in the foregoing embodiments can be applied to OCB LCD with different types of pixel polarity inversions.
6. Anyone skilled in the art can easily implement the foregoing embodiments according to the process layout disclosed in those embodiments without incurring extra cost.
It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.
Number | Date | Country | Kind |
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96116272 | May 2007 | TW | national |