JP3603900B2 - Liquid crystal display - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to a liquid crystal display device.
[0002]
[Prior art]
Conventionally, in a liquid crystal display device, an analog image display signal applied to a pixel electrode every one horizontal period is inverted to a positive side and a negative side with respect to a potential near a common substrate potential facing the pixel electrode via a liquid crystal layer. There was a way. FIG. 3 shows the driving waveform. The image display drive waveform in FIG. 3 includes a positive side video signal portion waveform A period and a horizontal retrace interval a period, and a negative side video signal portion B period and a horizontal retrace interval b period. At the boundary C between the positive side and the negative side, the potential changes linearly. However, in this driving method, regarding the potential of the pixel electrode, the potential of the data line is affected by the parasitic capacitance existing between the pixel electrode and the adjacent data line every horizontal scanning period. For example, in a state where the pixel potential holds the positive potential, when the data line selects the next horizontal scanning period, the potential of the data line becomes a negative potential, causing capacitive coupling due to the parasitic capacitance, and the pixel electrode potential becomes It will swing to the negative potential. This increases the threshold voltage of the liquid crystal and lowers the effective potential. On the image display, there is a possibility that it may cause a decrease in contrast and an increase in crosstalk, thereby causing a serious defect in display quality.
[0003]
[Problems to be solved by the invention]
As described in the related art, in the 1H inversion drive in which the analog video signal is inverted positively or negatively with respect to the potential near the common substrate electrode every horizontal scanning period, insufficient contrast or crosstalk which is a display problem occurs. It is in.
[0004]
[Means for Solving the Problems]
In order to solve such a problem, the present invention provides a plurality of pixel electrodes arranged in a matrix, a data line for supplying an image signal to the pixel electrode via a transistor, and a pixel electrode and a liquid crystal through a liquid crystal. A liquid crystal display device comprising: a common electrode facing the liquid crystal display; wherein the image signal supplied to the data line is a positive image signal that makes the potential of the pixel electrode positive with respect to the potential of the common electrode. A negative image signal for setting the potential of the pixel electrode to a negative potential is alternately switched every horizontal scanning period, and the image signal uses a step-like waveform indicating a step change of a plurality of levels. Switching between the waveform of the positive image signal and the waveform of the negative image signal is performed, and the waveform generating circuit for generating the waveform of the image signal includes a waveform obtained by differentially amplifying the video signal and a polarity inversion amplification. A polarity inversion generating circuit for generating a stepped waveform that generates a stepped waveform indicating the stepwise change of the plurality of levels by sampling the voltage of the sine waveform, and generating a waveform from the polarity inversion generating circuit every horizontal scanning period. And a switch for generating a waveform of the image signal by switching to a stepwise waveform indicating a stepwise change of the plurality of levels during a period in which the waveform is switched between a positive polarity and a negative polarity while alternately switching to a different polarity. It is characterized by the following.
In particular, in the present invention, an electro-optical material is sealed between a pair of substrates, a plurality of pixel electrodes arranged in a matrix on one of the substrates, a switching transistor connected to the pixel electrode, and the switching transistor. An image display device comprising: an image display unit having a plurality of data lines for supplying an image signal to a transistor; and image sampling means for sampling an analog image signal and supplying a sampled image signal to the data line. The analog image display signal is inverted in a positive polarity and a negative polarity with respect to the common substrate potential every horizontal scanning period, and at the time of the inversion, the potential waveform of the analog image display signal has a stepped waveform or a ramp waveform. There is a feature.
[0005]
【Example】
FIGS. 1A and 1B show an example of a drive waveform of an analog video signal according to the present invention, and FIGS. 2A and 2B show examples of a drive circuit for generating the waveform. FIG. 4 shows an equivalent circuit diagram of an active matrix substrate for driving a liquid crystal of the liquid crystal display device used in the present invention. An operation example in which data is written and held in a pixel having an N-channel pixel transistor will be described below.
[0006]
In FIG. 4, the gate of the analog switch 32 is sequentially opened by the shift register 31 to write an analog video signal to the data line 37. When the pixel gate electrode 36 has a high potential, a predetermined data signal is written to the pixel electrode during one horizontal scanning period in which the pixel transistor 33 opens. Thereafter, the pixel transistor is closed when the gate potential becomes low. Then, the potential written to the pixel is held by the pixel holding capacitor 35. This holding state is continued until a pixel writing operation in the next field is performed, and this operation is repeated. At this time, a timing chart of the pixel potential waveform 41 of any one pixel, the pixel transistor gate potential waveform 43, the common electrode potential 45, and the video potential waveform 44 of the data line connected to the source of the arbitrary one pixel pixel transistor Is shown in FIG. FIGS. 5A, 5B, and 5C respectively show a pixel electrode waveform, a gate line waveform, and a data line video signal waveform. The amplitude of each waveform represents voltage, and the horizontal axis represents time.
[0007]
The data line video waveform 44 alternately repeats a period of positive potential and a period of negative potential with respect to the common electrode potential 45. The half cycle time is equal to the time during which the pixel gate electrode is open. That is, after the writing operation to the pixel is completed, the gate line of the next stage is opened, and during the writing operation of the next stage pixel, a signal having a polarity opposite to that of the data line video signal written to the own pixel exists in the data line. This polarity inversion is alternately repeated for each horizontal scanning period. ΔVSG is considered to be a potential drop caused by the parasitic capacitance 38 between the pixel and the data line in FIG. 4, thereby reducing the effective voltage applied to the liquid crystal. The parasitic capacitance of the pixel and the data line is a parameter determined by the design value of the active matrix substrate. In a fine-pitch device requiring high resolution, the ratio of this parasitic capacitance to the pixel holding capacitance increases, and the value of ΔVSG further increases. growing.
[0008]
In the conventional example, the pixel electrodes and the data lines are formed in the same layer as shown in FIG. The outline of the process will be described below with reference to FIG. The source, drain, and channel of the thin film transistor 51 are formed on a glass substrate 58 with polycrystalline Si 56, and after the gate insulating film 57 is formed, the gate electrode 55 and the scanning lines are formed with polycrystalline Si doped with a metal or an impurity. Thereafter, ion implantation is performed by self-alignment based on the gate electrode to form source and drain portions. Thereafter, an insulating film is formed and a contact hole is formed. Thereafter, an ITO layer 53 serving as a pixel electrode is formed, and a metal such as AL or Cr serving as the data line 52 is formed on the same layer. The potential drop to ΔVSG in FIG. 4 is caused by the capacitance between the ITO layer 53 and the metal layer 52. The space between the ITO layer 53 serving as the pixel electrode and the metal layer 52 serving as the data line is required to be 3 μm or more in order to prevent the disturbance of the alignment due to the electric field in the horizontal direction of the liquid crystal. This parallel capacitance was suppressed to about 2% of the total pixel capacitance in a device having a pixel pitch of about 40 μm square.
[0009]
However, in recent years, it has become necessary to adopt a structure in which the ITO layer 53 as a pixel electrode overlaps the data line as shown in FIG. In addition, in a device that requires a smaller liquid crystal panel module as a whole and a fine pitch is required, the pixel electrode and the data line are formed in separate layers as shown in FIG. Take an overlapping structure. The insulating film 54 is formed of SiO2 or the like, but if the film thickness is increased, the insulating film 54 is also a second-layer insulating film, and cracks and the like occur, so that it cannot be made too thick. For this reason, the former is a parallel capacitance, while the latter is an overlapping capacitance with a limited maximum film thickness, and the capacitance between the data line 52 and the pixel electrode 53 is larger. In addition, because of the fine pitch, there is a limit to the overall pixel holding capacity. Therefore, the ratio of the parasitic capacitance to the pixel storage capacitance increases, and the value of ΔVSG increases. In a device having a pixel pitch of about 30 μm, the parasitic capacitance between the pixel electrode and the data line reaches about 15%, and the value of ΔVSG cannot be ignored.
[0010]
In a device having such limitations in device specifications and process design, as shown in FIGS. 1 (a) and 1 (b), a high-frequency component is reduced by changing the potential waveform to a staircase wave or a ramp wave when the video signal is inverted. The method of the present invention for suppressing the amplitude of a signal through a parasitic capacitance between a line and a pixel electrode is effective. Therefore, in a device with a fine pitch in which the ratio of the parasitic capacitance to the pixel holding capacitance is structurally large, it can be an effective means for limiting the value of ΔVSG to a small value. By reducing ΔVSG, the absolute value of the difference between the pixel potential waveform and the common electrode potential in FIG. 5 increases, and the effective voltage applied to the liquid crystal throughout the holding period increases. Also, when there is a potential difference depending on the location in the screen of the video signal, the difference in the interference effect between the data line and the pixel via the capacitance in the screen becomes small. Therefore, it is possible to prevent a decrease in contrast and a crosstalk defect on a display.
[0011]
Next, FIG. 2A shows an example of a circuit for generating the staircase video waveform shown in FIG. A polarity inversion amplification waveform is generated from the video signal by a polarity inversion generating circuit including an inverting amplifier 11 and a differential amplifier 12. Here, VWP is a white field voltage of a positive field when the liquid crystal display is a normally white display, and VWM is a white level voltage of a negative field. Further, a one-horizontal-scanning-period changeover switch (1HSW) 13 generates a waveform of a different polarity every one horizontal scanning period. Next, a staircase waveform is generated by a sample and hold circuit composed of an analog switch 16 and a capacitor 17 which are switched by a sampling pulse 15 with the sine waveform voltage 14 as an input. The staircase wave is synthesized with the inverted video waveform by the changeover switch 18 that switches at the timing within the retrace interval of one horizontal scanning period. Thus, the staircase waveform of FIG. 1A can be created.
[0012]
FIG. 2B shows the ramp generation circuit of FIG. 1B. The video signal is polarity-reversed and amplified by the polarity reversal amplifier circuit in the same manner as the staircase wave generation circuit of FIG. Further, an appropriate rectangular potential waveform 19 (Vsig) is integrated to form a ramp waveform, and is synthesized with the inverted video waveform by a changeover switch that switches at a timing within a retrace interval of one horizontal scanning period. Thus, the ramp wave shown in FIG. 2B can be produced. By adding such a circuit to the liquid crystal driving circuit, it is possible to display a liquid crystal display having a fine pitch, which is restricted in design, without causing crosstalk or a decrease in contrast.
[0013]
In the present invention, when the video signal is inverted as shown in FIG. 1, by changing the potential waveform to a staircase wave or a ramp wave, the high-frequency component can be reduced, and the amplitude of the signal passing through the parasitic capacitance between the data line and the pixel electrode can be reduced. It is possible. Therefore, in a device with a fine pitch in which the ratio of the parasitic capacitance to the pixel holding capacitance is structurally large, it can be an effective means for limiting the value of ΔVSG to a small value. By reducing ΔVSG, the absolute value of the difference between the pixel potential waveform and the common electrode potential in FIG. 5 increases, and the effective voltage applied to the liquid crystal throughout the holding period increases. Also, when there is a potential difference depending on the location in the screen of the video signal, the difference in the interference effect between the data line and the pixel via the capacitance in the screen becomes small. Therefore, it is possible to prevent a decrease in contrast and a crosstalk defect on a display.
[Brief description of the drawings]
FIG. 1 is a waveform diagram of a video signal for driving a liquid crystal display device of the present invention.
FIG. 2 is a diagram of an example of a waveform generation circuit of FIG. 1;
FIG. 3 is a diagram showing a conventional video signal waveform for driving a liquid crystal display device.
FIG. 4 is an equivalent circuit diagram of a liquid crystal display device.
FIG. 5 is a diagram showing a pixel potential waveform, a gate potential waveform, a data line potential waveform, and a common electrode potential of a certain pixel.
FIG. 6 is a cross-sectional view of a pixel portion of a liquid crystal display device.
[Explanation of symbols]
11 Inverting amplifier circuit 12 Differential amplifier circuit 13 1H switch 14 Sine potential waveform 15 Sampling pulse 16 ... Analog switch 17 ... Sample holder capacity 18 ... Inversion switch 19 ... Ramp wave generation reference voltage 20 ... Integrator 31 Shift register 32 Analog switch 33 Pixel transistor 34 Video line 35 Pixel holding capacitor 36 ... Pixel gate electrode, scanning line 37, data line 38, parasitic capacitance 41, pixel potential waveform 42, data line, pixel Voltage drop due to parasitic capacitance between electrodes △ VSG
43 gate line potential waveform 44 data line potential waveform 45 common electrode potential 51 thin film transistor 52 data line 53 ..... Pixel electrode 54 ..... Interlayer insulating film

Claims (1)

A liquid crystal display device comprising a plurality of pixel electrodes arranged in a matrix, a data line for supplying an image signal to the pixel electrode via a transistor, and a common electrode opposed to the pixel electrode via a liquid crystal. So,
The image signal supplied to the data line includes a positive image signal for setting the potential of the pixel electrode with respect to the potential of the common electrode to a positive potential and a negative image signal for setting the potential of the pixel electrode to a negative potential. The image signal is alternately switched every horizontal scanning period, and the image signal uses a staircase waveform indicating a step change of a plurality of levels, and the waveform of the positive image signal and the negative image signal are used. Switch between the waveforms of
A waveform generating circuit for generating a waveform of the video signal, a polarity inversion generating circuit for generating a waveform obtained by differentially amplifying the video signal and a waveform obtained by inverting and amplifying the polarity of the video signal; A stepwise waveform indicating a change is generated, and the waveform from the polarity inversion generating circuit is alternately switched to a different polarity every horizontal scanning period, and the plurality of waveforms are switched during a period in which the waveform switches between a positive polarity and a negative polarity. A switch for generating a waveform of the image signal by switching to a step-like waveform indicating a step change in level.

Images