Patent Application
20050104835
The present invention relates to a method for use in driving a liquid crystal display device.
A liquid crystal display device includes a liquid crystal display panel (also called a liquid crystal display element or a liquid crystal cell). The liquid crystal display panel includes a pair of substrates which face each other in an opposed manner, and liquid crystal composition is sandwiched between the two substrates. Pixels are formed on the substrate in a matrix array. These pixels constitute a display part of the liquid crystal display device.
Each pixel includes a pair of electrodes constituted of a pixel electrode and a counter electrode. By use of an electric field which is generated in response to a voltage applied between these electrodes, the optical transmissivity of the liquid crystal is controlled.
As examples of a liquid crystal display device, a vertical electric field type and an in-plane switching type are known. In the vertical electric field type, pixel electrodes are formed on one substrate and counter electrodes are formed on another substrate. In the in-plane switching type, the pixel electrodes and the counter electrodes are formed on the same substrate.
In these liquid crystal display devices, an AC driving method which periodically inverts the polarities of a voltage applied to the liquid crystal layer is performed. This AC driving method is adopted to prevent a deterioration of the liquid crystal which tends to occur when a DC voltage is applied to the liquid crystal. As one AC driving method, there is a known method in which a DC voltage is applied to the counter electrodes, and signal voltages of positive polarity and negative polarity, using a counter electrode voltage as a reference voltage, are alternately applied to the pixel electrodes.
Assuming a period in which all pixels of a liquid crystal display part are driven as one frame, there is a known driving method which changes over the polarities of voltages applied to pixel electrodes for every frame (hereinafter called a frame-inversion driving method). An example of the frame-inversion driving method is disclosed in Pub. No.: US 2002/0008800.
However, it has been found that, even when a liquid crystal display device is driven using the frame-inversion driving method, there arises a drawback, such as sticking (after image) or the like.
As a cause of these drawbacks, it is estimated that ionic impurities (hereinafter called ions) are present in a trace amount in the inside of the liquid crystal in a state in which the ions are unevenly distributed. This sticking is a phenomenon in which, for example, a fixed image is displayed for a fixed period, and, thereafter, even when the whole surface is changed over to another image, the previous fixed image remains. It has been known that such sticking is relevant to a phenomenon in which the light modulation quantity of the liquid crystal becomes different between the positive-polarity signal frame and the negative-polarity signal frame. That is, this phenomenon is a phenomenon in which unevenly distributed ions remain on a sticking image region, so that, even when the signals are eliminated, the remaining undistributed ions induce a light modulation of the liquid crystal.
Accordingly, it is an object of the present invention to provide a method which can be used for driving a liquid crystal display device to reduce the uneven distribution of ions in a liquid crystal layer.
A summary of representative aspects of the invention disclosed in the present application is as follows.
The present invention is directed to a method of driving a liquid crystal display device including pixel electrodes and counter electrodes, the method comprising the steps of:
The present invention is also directed to a method of driving a liquid crystal display device having a display region on which a plurality of pixels are formed, pixel electrodes which are provided to the pixels, counter electrodes which face the pixel electrodes in an opposed manner, and an image memory which stores display data, the method comprising the steps of:
The present invention is also directed to a method of driving a liquid crystal display device having a display region on which a plurality of pixels are formed, pixel electrodes which are provided to the pixels, counter electrodes which face the pixel electrodes in an opposed manner, and an image memory which stores display data, the method comprising the steps of:
Here, the present invention is not limited to the above-mentioned constitution, and various modifications can be made without departing from the technical concept of the present invention.
Embodiments of a liquid crystal display device according to the present invention will be explained hereinafter in conjunction with the drawings.
Although a liquid crystal display device which is used in a projector is mainly considered by way of example in the explanation presented hereinafter, the present invention is applicable to other liquid crystal display devices.
This is because other liquid crystal display devices are the same as the liquid crystal display device used in a projector with respect to the fact that each pixel of the liquid crystal display device includes a pair of electrodes, light modulation is performed in response to an electric field applied between the electrodes, and AC driving is used for obviating deterioration of the liquid crystal; and, at the same time, other liquid crystal display devices also have the same task to be solved as the liquid crystal display device that is considered in conjunction with the embodiments of the present invention to be described herein.
<<Equivalent Circuit>>
On the substrate, there are gate signal lines GL, which extend in the direction x and are arranged in parallel in the direction y, and drain signal lines DL, which extend in the direction y and are arranged in parallel in the direction x. Regions which are surrounded by these respective signal lines constitute pixel regions. Here, a liquid crystal display part of the liquid crystal display device is constituted of an array of these respective pixel regions.
Each pixel region is provided with a switching element SW, which is driven in response to a scanning signal received from the gate signal line GL, and a pixel electrode PX to which a video signal from the drain signal line DL on one side of the pixel region is supplied by way of the switching element SW.
An electric field is generated between the pixel electrode PX and a counter electrode (not shown in the drawing), and, hence, the orientation direction of the liquid crystal composition of the pixel region is changed, thus generating a light modulation. The counter electrodes can be formed on either one of the two substrates which form the liquid crystal display panel.
Further, on the pixel region, a capacitive element Cadd which stores charges during an OFF period of the switching element SW, is formed. In
<<Constitution of a Pixel>>
The reflective-type liquid crystal display device is used in a projector or the like. In the projector, light from a light source is radiated to the reflective-type liquid crystal display device, and reflection light is radiated from the reflective-type liquid crystal display device. The reflection light is enlarged by way of an optical system and is projected on a screen.
In the reflective-type liquid crystal display device, of the respective substrates SUB1, SUB2 which are arranged to face each other with liquid crystal LQ therebetween, one substrate SUB2 is formed as a transparent substrate and the other substrate SUB1 is constituted of a semiconductor substrate. On a liquid-crystal-LQ-side surface of the semiconductor substrate SUB1, switching elements SW are formed. The switching elements SW are formed of a diffusion layer DF, an insulation layer INS, lines ML and the like which are formed on a surface of the semiconductor substrate SUB1. Further, above the lines ML or the like, capacitive elements are formed of conductive layers and the like which are overlapped relative to each other by way of insulation films, wherein one electrode is indicated by symbol CD in the drawing.
On the surface of the semiconductor substrate SUB1, pixel electrodes PX, which are formed of metal or the like (for example, aluminum) and have a favorable reflectance efficiency, are formed. Further, an orientation film AS1, which is directly brought into contact with the liquid crystal, is formed such that the orientation film AS1 also covers the pixel electrodes PX, wherein the initial orientation direction of molecules of the liquid crystal is determined by the orientation film AS1.
On the other hand, on a liquid-crystal-LQ-side surface of the transparent substrate SUB2, which is arranged to face the semiconductor substrate SUB1 by way of the liquid crystal LQ, the counter electrodes CT, which are formed of a light transmitting material (for example, ITO: Indium Tin Oxide), are formed. An orientation film AS2, which is brought into contact with the liquid crystal LQ, is formed such that the orientation film AS2 also covers the counter electrodes CT. The initial orientation direction of the molecules of the liquid crystal LQ also can be determined by the orientation film AS2.
Spacers SP are arranged between the semiconductor substrate SUB1 and the transparent substrate SUB2 in a scattered manner, for example, thus making the layer thickness “d” of the liquid crystal LQ uniform using the spacers SP.
Also, in the liquid crystal display device having such pixels, light from a light source which is radiated from the outside of the transparent substrate SUB2 reaches the pixel electrode PX through the transparent substrate SUB2 and the liquid crystal LQ. As described above, the pixel electrodes PX are formed of a metal having a favorable reflection efficiency or reflectance, and, hence, the light is reflected on the pixel electrodes PX and is radiated to the outside of the transparent substrate SUB2 through the liquid crystal LQ and the transparent substrate SUB2.
Here, the liquid crystal display device which represents the subject of the present invention is not limited to the above-mentioned constitution. That is, the semiconductor substrate may be formed of a transparent substrate and a reflection plate may be formed between the substrates or outside the substrates. Further, the present invention is also applicable to a transmissive-type liquid crystal display device, as opposed to the reflective-type liquid crystal display device.
Here, the transmissive-type liquid crystal display device is a liquid crystal display device which uses a transparent substrate in place of the above-mentioned semiconductor substrate. In such a device, light is incident on one transparent substrate and is irradiated after passing through the liquid crystal and the other transparent substrate. A cold cathode ray tube or a light emitting diode is used as the light source, and the light sources are arranged on a back surface of the liquid crystal display device on a viewer's side. Further, the pixel electrodes are formed on a liquid-crystal-side surface of one transparent substrate, and counter electrodes are formed on a liquid-crystal-side surface of the other transparent substrate. All of the respective electrodes are formed of a light transmitting material over substantially the whole area of both pixel electrodes. Alternatively, the pixel electrodes and the counter electrodes are formed in a strip shape on a liquid-crystal-side surface of one transparent substrate, wherein the pixel electrodes and the counter electrodes are alternately arranged in a state in which they are spaced apart from each other.
<<Driving Method>>
In the liquid crystal display device having such a constitution, a display start signal is inputted to the liquid crystal display device from the outside. Upon receiving the display start signal, the liquid crystal display device sequentially supplies the scanning signal (ON signal) to the respective gate signal lines GL shown in
With respect to the respective pixels for one line, which is constituted of a group of pixels arranged in parallel in the direction x, video signals are supplied to the respective pixel electrodes PX through the switching elements SW, which are simultaneously turned on with the pixels, and this operation is transferred to each respective pixel of the next lines.
Such operations are repeated until the scanning signal reaches the last gate signal line GL. When the scanning signal is supplied to the last gate signal line GL, the writing of the video signals for one screen is completed. The period from a point of time at which the display start signal is inputted from the outside to a point of time at which another display start signal is inputted again will be referred to hereinafter as one frame period (hereinafter also called as “frame”) of the liquid crystal display device. Here, in general, the vertical synchronizing signal is used as the display start signal.
On the other hand, the signal which becomes the reference with respect to the video signal (counter voltage Vcom) is supplied to the counter electrodes CT, and an electric field which corresponds to the voltage between the counter electrode CT and the pixel electrode PX is generated with a value which corresponds to the video signal.
Here, as the video signals, positive-side signals and negative-side signals which exhibit a positive-negative symmetry with respect to the signals applied to the counter electrodes are prepared. Then, the liquid crystal display device adopts, in general, a driving method (an AC driving method) in which, for example, the positive-side signals are used at the time of displaying the image of the first frame and the negative-side signals are used at the time of displaying the image of the next frame. When the electric field in one direction is continuously applied to the liquid crystal, the liquid crystal is deteriorated, and, hence, the direction of the electric field applied to the liquid crystal is changed for every frame.
The duty ratio is the time ratio of the positive polarity and the negative polarity when a repeating cycle of writing the positive polarity and the negative polarity of the voltage signal applied to the liquid crystal is set as one cycle.
However, even in such a case, it has been found that a sticking phenomenon arises with respect to the liquid crystal.
The inventors of the present invention have investigated a cause of the phenomenon and have estimated the following as the cause of the phenomenon. That is, for example, the pixel electrodes PX and the counter electrodes CT differ in shape, material and the like, and, hence, the flow of a substance which is charged with ions or the like (hereafter called ions) from the pixel electrode PX, to the counter electrode CT differs from the flow of ions from the counter electrode CT to the pixel electrode PX whereby the ion concentration in the inside of the liquid crystal differs in a direction perpendicular to the electrode substrate.
To explain one example of sticking, it is a phenomenon in which, even when a fixed image in which white, black and other gray scales are present in a mixed form is displayed for a fixed period and, thereafter, the fixed image is changed over to an intermediate gray scale over the whole screen, the previous fixed image remains. This phenomenon is relevant to a phenomenon in which the light modulation quantity of the liquid crystal differs between the positive signal frame and the negative signal frame. That is, the ions which are unevenly distributed at the time of displaying the fixed image remain on the sticking display region, and, hence, even in a state in which the signal is not applied, the remaining unevenly distributed ions induce a light modulation of the liquid crystal.
As shown in
When a steady-state fixed potential difference arises between the semiconductor substrate SUB1 and the transparent substrate SUB2 due to a certain cause at the time of driving the liquid crystal display device, the ions are unevenly distributed in the vicinity of the substrates. During the period in which the liquid crystal display device is driven, being induced by the stationary potential difference, the quantity of unevenly distributed ions is increased. Since the ions are charged, due to the fact that they are unevenly distributed, a potential difference is generated between the pixel electrode PX and the counter electrode CT.
The counter voltage (common voltage) Vcom is arranged at approximately the intermediate level between the signals of positive and negative polarities, such that a difference is not generated on the displayed gray scale between the video signals of positive polarity and those of negative polarity, that is, the light modulation quantity of the liquid crystal becomes equal between the time of applying the voltage of positive polarity and the time of applying the voltage of negative polarity (hereinafter called the accommodated counter voltage Vcom).
When a potential difference is generated between the pixel electrode PX and the counter electrode CT due to the unevenly distributed ions and this potential difference is increased, as indicated by a line DRF in
Next, in conjunction with
In the case shown in
In the case shown in
In the normally black mode, when the video signal +Vsig of positive polarity is applied to the pixel electrodes PX, the images are displayed more brightly than the normal display, and when the video signal (−)Vsig of negative polarity is applied to the pixel electrodes PX, the images are displayed so as to be darker than the normal display, and, hence, the counter voltage Vcom is adjusted to the positive side relative to the center voltage of the positive polarity signal voltage and the negative polarity signal voltage.
In
In the case shown in
In the normally black mode, when the video signal +Vsig of positive polarity is applied to the pixel electrodes PX, the images are displayed so as to be darker than the normal display, and when the video signal (−)Vsig of negative polarity is applied to the pixel electrodes PX, the images are displayed so as to be brighter than the normal display, and, hence, the counter voltage Vcom is adjusted to the negative side relative to the center voltage of the positive polarity signal voltage and the negative polarity signal voltage.
With respect to
In
In
When the video signal +Vsig of positive polarity is applied to the pixel electrode PX, a display that is darker than the normal display is produced, and when the video signal (−)Vsig of negative polarity is applied to the pixel electrode PX, a display that is brighter than the normal display is produced, and, hence, the counter voltage Vcom is adjusted to the proper voltage. In the above-mentioned case, the counter voltage Vcom is adjusted to the negative side relative to the initial voltage.
However, when the counter voltage Vcom is adjusted, the electric field generated in the inside of the liquid crystal is changed, and, hence, an uneven distribution of the ionic impurities takes on a distribution that is different from the distribution before adjustment (the uneven distribution quantity being increased). Accordingly, there arises a difference in the contrast in the display between the time of the positive polarity signal and the time of the negative polarity signal again.
As a cause of the uneven distribution of the ionic impurities, the materials of the upper and lower substrate electrodes, an interface treatment process of the orientation film and the like a factor. For example, the ionic impurities which are induced by signal voltages of positive polarity and negative polarity applied from the outside and which reach the interfaces of the electrodes are attracted to the orientation film or the like or the ionic impurities are easily removed from the orientation film due to the changeover of the polarities of the signal voltage. The uneven distribution of the ionic impurities is caused between the upper and lower substrates due to the easiness of the above-mentioned attraction or removal of the ionic impurities.
When the ionic impurities are unevenly distributed in the inside of the liquid crystal and the uneven distribution is adjusted by adjusting the light modulation quantity of the liquid crystal based on the difference between the signal voltage of positive polarity and the signal voltage of negative polarity or the Vcom voltage, it is preferable that the uneven distribution of the ionic impurities is not generated again. However, the adjustment of electric field on the liquid crystal simultaneously acts on the ionic impurities in the inside of the liquid crystal and increases the quantity of unevenly distributed ionic impurities, and, hence, it is difficult to completely suppress the sticking which is generated due to the uneven distribution of the ionic impurities.
For purposes of suppressing the uneven distribution of ions, the driving method employed by this embodiment changes the ratio between the period in which the video signal of positive polarity is applied to the pixel electrodes PX and the period in which the video signal of negative polarity is applied to the pixel electrodes PX.
The driving method will be explained in conjunction with
Due to such setting of the periods of positive polarity and negative polarity, in one certain frame and the frame which follows next, the polarity of the video signal which is supplied to the respective pixels at the same position can be driven such that, as shown in the drawing, when the polarity of ions which are unevenly distributed in the pixel electrode PX is negative, the time that the positive polarity is applied is short and the time that the negative polarity is applied is long, as indicated by the symbol SIG1. On the other hand, when the polarity of ions which are unevenly distributed in the pixel electrode PX is positive, the time that the negative polarity is applied is short and the time that the positive polarity is applied is long, as indicated by the symbol SIG2.
Due to such a driving method, the uneven distribution of the ions in the inside of the liquid crystal layer is prevented by changing each time ratio of the positive polarity of the video signal.
In
Also in
It is needless to say that the flow of the ions from the pixel electrode PX to the counter electrode CT may take the following mode depending on the degree of difference of the flow of ions from the counter electrode CT to the pixel electrode PX. That is, as shown in
That is, with respect to the liquid crystal of each pixel, in the respective polarities of the signal (voltage) applied in a sequentially changed-over manner, it is sufficient that the signal application time for one polarity is different from the signal application time for the other polarity. In this case, the polarity of the signal (voltage) which is applied to the liquid crystal is determined based on the value of the voltage applied to the counter electrode CT and the value of the voltage applied to the pixel electrode PX; wherein, when the liquid crystal is driven such that the polarity of the reference voltage signal applied to the counter electrode CT is changed, the liquid crystal display device is driven in a state in which the polarity of the video signal which is applied to the pixel electrode PX is changed to have the above-mentioned relationship.
In view of the above, it should be apparent that the driving method is not limited to the methods shown in
Line inversion driving is a method in which, in sequentially driving groups of pixels (lines), each of which is constituted of respective pixels which are arranged in parallel in the x axial direction from the upper side to the lower side, for example, the respective pixels of one pixel group are driven with the positive polarity (negative polarity), and, thereafter, the respective pixels of the next one pixel group are driven with the negative polarity (positive polarity), and such driving is sequentially repeated such that a reverse polarity relationship is established each time the frame is changed over.
In this case, the driving may be performed such that, for example, first of all, the data is written by selecting only lines of positive polarity, and, thereafter, only the data of negative polarity is written. That is, the operation to select every one other gate signal line GL is performed twice in one frame.
Further, row inversion driving is a method in which, in the same manner as the line inversion driving method, in sequentially driving groups of pixels (lines), each of which is constituted of respective pixels which are arranged in parallel in the x axial direction from the upper side to the lower side, for example, in driving the respective pixels of one pixel group, the respective pixels are driven in the order of positive polarity, negative polarity, positive polarity, negative polarity, . . . from the left side to the right side, for example. Also, in the next pixel group, the respective pixels are driven in order of positive polarity negative polarity, positive polarity, negative polarity, . . . from the left side to the right side, and, thereafter, such driving is repeated to establish a reverse polarity relationship at the time of changing over the frame.
Further, dot inversion driving is a method in which, in the same manner as the line inversion driving method, in sequentially driving groups of pixels (lines), each of which is constituted of respective pixels which are arranged in parallel in the x axial direction from the above-side to the lower side, for example, in driving the respective pixels of one pixel group, the respective pixels are driven in order of positive polarity, negative polarity, positive polarity, negative polarity, . . . from the left side to the right side, for example. Also, in the next pixel group, the respective pixels are driven in the order of negative polarity, positive polarity, negative polarity, positive polarity . . . from the left side to the right side, and, thereafter, such driving is repeated to establish a reverse polarity relationship at the time of changing over the frame.
In the above-mentioned row inversion driving and dot inversion driving, first of all, the data of positive polarity or negative polarity of a certain line is written, and, thereafter, the gate signal lines GL are selected again from the head line and only the data of negative polarity or positive polarity is written. Here, into the pixels in which the data of positive polarity or negative polarity is already written, it is possible to write so-called black data. In this manner, it is possible to realize a black insertion, which is preferable in a moving image display.
Then, based on the result of an arithmetic operation performed by the control circuit uCOM, the output timing of a clock signal (for example, corresponding to the polarity changeover signal CK2 in
By adopting the constitution shown in
Here, with respect to the above-mentioned information obtained from the liquid crystal display panel PNL, it is preferable to obtain the information from the pixel for the inspection which is formed on the region of the liquid crystal display panel PNL, for example, which is formed separately on a position slightly remote from the liquid crystal display part thereof. This is because of the fact that, when the pixel for inspection is provided in the inside of the liquid crystal display part, the pixel for inspection becomes an obstacle when a viewer watches images. Although the number of pixels for inspection may be one pixel, for example, it is preferable that a plurality of pixels are arranged close to each other to have a sufficient light quantity.
The pixel for inspection is driven under conditions that are equal to the conditions for driving respective pixels of the liquid crystal display part. The duty ratio at which the voltage is applied by changing over the positive polarity and the negative polarity, signals applied to the counter electrodes CT and the signals applied to the pixel electrodes PX, are also equal to those used for driving the liquid crystal display part.
Further, the sensor DTC is arranged to face the pixel for inspection in an opposed manner so as to detect the quantity of light emitted from the pixel. An output of the sensor DTC is transmitted to the control circuit uCOM, and the difference between the quantity of light when the signal of positive polarity is applied to the pixel and the quantity of light when the signal of negative polarity is applied to the pixel is calculated by the control circuit uCOM.
When the difference becomes 0, this implies that the quantity of light at the time of applying the signal of positive polarity to the pixel for inspection and the quantity of light at the time of applying the signal of negative polarity to the pixel for inspection is equal. That is, this implies that, under the current situation, in so far as the pixel is concerned, the value of the duty ratio at the time of applying the voltage by changing over the positive polarity and the negative polarity is proper or appropriate. This also implies that the value of the duty ratio is appropriate or proper also with respect to the respective pixels of the liquid crystal display part.
When the difference assumes a value other than 0, this implies that the value of the duty ratio is not proper, and, hence, the correction of the duty ratio becomes necessary. For example, when the quantity of light at the time of applying the signal of positive polarity to the pixel (pixel for detection) is larger than the quantity of light at the time of applying the signal of negative polarity to the pixel (pixel for detection), the value of the duty ratio is made to approximate the proper value by decreasing the time for applying the signal of positive polarity or by increasing the time for applying the signal of negative polarity. In the same manner, when the quantity of light at the time of applying the signal of positive polarity to the pixel (pixel for detection) is smaller than the quantity of light at the time of applying the signal of negative polarity to the pixel (pixel for detection), the value of the duty ratio is made to approximate the proper value by increasing the time for applying the signal of positive polarity or by decreasing the time for applying the signal of negative polarity. Such a control is also performed by the control circuit uCOM based on the above-mentioned arithmetic operation values. Here, the sensor may be constituted of a sensor which detects only blue light which receives the largest influence from the ionic material or a sensor which can detect all of three primary colors in color.
In driving such a liquid crystal display device, the deterioration of the liquid crystal of each pixel can be properly reduced by adding a point of view, that is, the difference in the flow of ions in respective positive-polarity and negative-polarity applied states. Further, it is also possible to obviate a drawback that, due to the generation of the electrical imbalance attributed to the difference in respective times of application of positive and negative polarities, the reference voltage (Vcom) applied to the counter electrode CT drifts, and, hence, the brightness at the time of writing the positive electrode and negative electrode can be changed.
In this change of brightness, when a display of black (0 gray scale) is to be displayed, for example, the positive polarity (0 gray scale) and the negative polarity (10 gray scale) are alternately changed over, and, hence, a display having a gray scale of (0+10)/2=5 becomes a display in a state in which black is shifted to the white-side gray scale (looking whitish) by five gray scales. According to the driving method of this embodiment, the generation of such a phenomenon also can be prevented.
Here, in the embodiment explained in conjunction with
Further, the duty ratio may be changed along with a lapse of time using a timer TM, as shown in
In this case, in the liquid crystal display device in which the above-mentioned control circuit uCOM and the like are incorporated, it is necessary to recognize the change of some elements, which show the degree of the progress of the deterioration of the liquid crystal along with the lapse of time, and, hence, the characteristics are stored in a memory (not shown in the drawing), and the proper duty ratio is set based on the information stored in the memory. Here, as the element which indicates the degree of the deterioration of the liquid crystal or the like, an accumulated light quantity of a light source or the like, which is radiated to the liquid crystal display device, can be given as an example.
Although the present invention has been explained in conjunction with the above-mentioned embodiments, the inventors of the present invention have found that the influence of the ionic impurities can be sufficiently reduced by setting the duty ratio from 55 percent to 70 percent.
The above-mentioned respective embodiments can be respectively used in a single form or in combination. This is because the respective embodiments can produce the respective advantageous effects in a single form or synergistically.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-389146 | Nov 2003 | JP | national |
