Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2003-03-12
2004-03-09
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S141000
Reexamination Certificate
active
06704067
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display apparatus, and in particular, to the apparatus capable of curbing image quality degradation due to crosstalk and implementing high image quality and reduction of driving voltage and power consumption.
The liquid crystal display apparatus is widely used as a display portion of mobile equipment represented by a notebook PC and a portable telephone taking advantage of its characteristics of low profile, light weight and super low power consumption, and is beginning to be widespread as a monitor for a desktop PC and a liquid crystal television because it shows performance superior to the display apparatuses such as a CRT (Cathode Ray Tube) in-terms of low profile, light weight, high-resolution display and so on.
As for a display principle of the liquid crystal display, an IPS mode (In-plane Switching Mode) characterized by a wide viewing angle, MVA (Multi Domain Vertical Alignment), OCB (Optically Compensated Birefringence) and so on are used in addition to mainstream TN (Twisted Nematic).
Among these display modes, the in-plane switching mode has an excellent viewing angle characteristic and is capable of gathering on one of the substrates almost all the components of a liquid crystal panel such as active elements, electrodes and a color filter in addition, and so it is expected as a mode capable of inexpensively implementing a high-resolution liquid crystal display apparatus having no problem of electrode pattern alignment between the upper and lower substrates.
As for a challenge in an early development stage of the in-plane switching mode, there is improvement in the image quality degradation due to the crosstalk from a row line for applying a signal voltage based on image data. It occurs, when the row line and a pixel electrode are closely positioned, because variation of the signal voltage is overlapd at a certain ratio on a pixel potential as a crosstalk voltage due to a coupling capacitance between the line and the electrode. In order to prevent it, a configuration for using a second pixel electrode also as a screening electrode was devised, and is disclosed in JP-A-6-202127 specification.
Another challenge of the in-plane switching mode is that, as a liquid crystal is driven by a horizontal electric field, the driving voltage is apt to rise when an inter-electrode distance is widened to increase an opening ratio. If the liquid crystal driving voltage becomes high, it is necessary to enhance dielectric strength of a driving element placed around the liquid crystal panel for the sake of applying a voltage to the liquid crystal, and it is also necessary, in the case where the driving element is comprised of active elements such as thin film transistors formed by polysilicon or amorphous silicon of a low temperature process on a glass substrate, to enhance the dielectric strength of the thin film transistors. In addition, the dielectric strength required for the active elements in the pixel portion also becomes high. Due to enhancement of the dielectric strength of the active elements, there is a danger of damaging some of the characteristics of the liquid crystal display apparatus such as limitation of higher resolution due to increased area and increase in non-display area of the driving element, increase in the process of enhancing the dielectric strength of the thin film transistors, and increase in glass periphery area.
The authors hereof devised a differential driving mode, as the configuration capable of applying a sufficient voltage to the liquid crystal and also reducing the voltage of the dielectric strength of the driving element even with a decreased number of lines, wherein a plurality of TFTs are placed in one pixel and the liquid crystal is driven by a differential of the voltages written by these TFTs to display an image, which mode was disclosed in JP-A-6-148596 and JP-A-6-202073 specifications.
In order to more clarify an object of the present invention, a basic configuration common among all the active matrix type liquid crystal display modes and a driving mode thereof will be described first, and then the object of an active matrix type liquid crystal display apparatus will be described by taking the in-plane switching mode as an example.
FIG. 22
shows an equivalent circuit diagram of the active matrix type liquid crystal display apparatus. At a start of a selected period, the potential for rendering an active element
203
on is given to a row line
201
by a gate driver
106
, the potential based on image data is given to a row line
202
by a gate driver
107
, and the potential based on the image data is given to a pixel electrode
210
via the active element
203
. A liquid crystal
208
and a holding capacitance
205
connected in parallel are charged by a potential difference between the potential of the pixel electrode
210
and that of a common line
209
to a second pixel electrode
204
and the holding capacitance
205
of the liquid crystal. In the cases of having on the opposite substrate side a second pixel electrode which is plane and common among all the pixels as in the TN mode, the MVA mode or the OCB mode, the second pixel electrode
204
is normally formed on the opposite substrate side and the common line
209
of the holding capacitance
205
is formed on the same substrate as the active element as shown in this drawing. On the other hand, in the configuration such as the in-plane switching mode capable of forming the pixel electrode
210
and the second pixel electrode
204
on the same substrate, it is possible to connect the common line
209
to the second pixel electrode
204
. At the end of the selected period, the potential for rendering the active element
203
off is given to the row line
201
, so that the writing is completed. Charging of the liquid crystal
208
and the holding capacitance
205
is finished in a very short time compared with an optical response of the liquid crystal. At this time, transmittance shown by the liquid crystal
208
corresponds to an absolute value of the voltage which is written and is not dependent on polarity of the voltage.
The crosstalk in the active matrix type liquid crystal display apparatus in the in-plane switching mode will be described by referring to FIG.
21
.
FIG. 21
shows an equivalent circuit of the active matrix type in-plane switching mode, and
FIG. 23
shows a plane layout diagram thereof. The overall configuration as the liquid crystal display apparatus is omitted since it is the same as FIG.
22
. This drawing is the equivalent circuit diagram and at the same time, it shows a placement which is almost plane, where the second pixel electrode
204
is normally connected to the common line
209
of a constant potential so that the second pixel electrode
204
acts as an electrical shield against voltage variation of Vd1 and Vd2 of the row line
202
so as to stabilize the potential of the pixel electrode
210
. Accordingly, it is possible, by taking sufficient width of the second pixel electrode
204
, to be hardly influenced by potential variation of the row line
202
so as to display a high-quality image without crosstalk. However, if the width of the second pixel electrode
204
is rendered smaller for the purpose of increasing the opening ratio or in the case of small pixel area due to high resolution, the voltage variation of the row line
202
to which the voltage based on the image data is applied is transferred via capacitance coupling of a parasitic capacitance
631
, so that crosstalk voltage is overlapd on the pixel electrode
210
. In this case, there are the cases where, in the in-plane switching mode that is a normally black mode, contrast reduction due to rise in black display luminance, the crosstalk in a row direction in half-tone display, or a flicker or an afterimage due to superimposition of asymmetrical voltage by the polarity may be observed.
Of such image quality degradation, a curbing method by a driving mode has been adopted as to the flicker and the crosstalk in
Aono Yoshinori
Hiyama Ikuo
Konno Akitoyo
Tsumura Makoto
Yamamoto Tsunenori
Chung David
Parker Kenneth
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