Method for Improving the Gradational Display of an Active Type Liquid Crystal Display Unit

This invention relates to a method of driving an active matrix type liquid crystal display unit.

Research and development of active matrix type liquid crystal display units in various fields have been conducted for the purpose of applying the technology to thin or flat television sets and the like.

An equivalent circuit diagram of the foregoing active matrix type liquid crystal display unit will now be described with respect to FIG. 3. In this drawing, SW.sub.i,SW.sub.i+1 designate switching elements made of transistors or the like, which are selected (turned on) and non-selected (turned off) in accordance with signals sent to scanning signal lines XL.sub.i,XL.sub.i+1. YL.sub.j,YL.sub.j+1 designate picture signal lines for supplying picture signals to pixel electrodes PX.sub.i,PX.sub.i+1 connected with the selected switching elements SW.sub.i,SW.sub.i+1. LC.sub.i,LC.sub.i+1 designate liquid crystal layers corresponding to individual pixels which are sandwiched by the pixel electrodes PX.sub.i,PX.sub.i+1 and a common electrode COMON. ST.sub.i,ST.sub.i+1 designate stick capacitors connected with the pixel electrodes PX.sub.i,PX.sub.i+1, which are provided for holding individual voltages supplied from the picture signal lines YL.sub.j,YL.sub.j+1. STACK designates a stick capacitor electrode for the stick capacitors ST.sub.i,ST.sub.i+1.

FIG. 9 is a time chart showing a method of driving the active matrix type liquid crystal display unit shown in FIG. 3. In this drawing, X.sub.i,X.sub.i+1 designate scanning signals applied to the scanning signal lines XL.sub.i,XL.sub.i+1, with the value of logic level "1" indicating "selection" and with the value of logic level "0" indicating "non-selection." Specifically, a selection signal of logic level "1" is supplied during a horizontal interval T.sub.H per vertical interval T.sub.V. Y.sub.j designates a picture signal applied to the picture signal line YL.sub.j, whose polarity is inverted about a reference voltage V.sub.C per vertical interval T.sub.V. This alternating-current drive mode is adopted for the purpose of preventing direct current from being applied to the liquid crystal. COM designates a common voltage applied to the common electrode COMON, which is always maintained at the reference voltage V.sub.C. PXL designates a pixel voltage applied to the pixel electrode PX.sub.i. In this connection, the stick capacitor ST.sub.i holds the value of the picture signal Y.sub.j supplied to the pixel electrode PX.sub.i when the switching element SW.sub.i is selected, even when the switching element SW.sub.i is brought into the "non-selection" mode. PXL-COM designates the signal of the pixel voltage PXL minus the common voltage COM, i.e., the voltage applied to the liquid crystal layer LC.sub.i, which has the same waveform as that of the pixel voltage PXL because the common voltage has a constant value V.sub.C. It should be noted that the voltage applied to the stick capacitor electrode STACK has the constant value V.sub.C.

FIG. 10 shows a light transmittance characteristic of the liquid crystal obtained in accordance with the foregoing driving method. In this drawing, the abscissa represents the effective voltage V.sub.LC applied to the liquid crystal layer and the picture signal voltage V.sub.SIG, whereas the ordinate represents the liquid crystal light transmittance T. According to the foregoing driving method, as described above, the voltage (PXL-COM) applied to the liquid crystal layer has a constant value V.sub.SIG because the common voltage COM is constant. Thus, its effective voltage is also "V.sub.SIG ". Therefore, the effective voltage V.sub.LC applied to the liquid crystal layer is identical with the picture signal voltage V.sub.SIG.

However, with such an active matrix type liquid crystal display unit, a gradational display is made by segmenting the span from 100% (white) to 0% (black) of the liquid crystal light transmittance T. Practically, the gradational display is attained by dividing the picture signal voltage V.sub.SIG so as to correspond to discrete values of light transmittance. Therefore, to obtain a fine gradational display, the voltage width of the picture signal voltage V.sub.SIG corresponding to where the light transmittance T varies from 100% to 0% must be large. Consequently, the ratio of the change in liquid crystal light transmittance .DELTA.T to the change in picture signal voltage .DELTA.V.sub.SIG must be made as small as possible. According to the foregoing driving method, however, the picture signal voltage V.sub.SIG is completely identical with the effective voltage V.sub.LC applied to the liquid crystal layer. Therefore, if the effective voltage applied to the liquid crystal layer is .DELTA.V.sub.LC, the following expression is obtained:

Since the range of the effective voltage V.sub.LC applied to the liquid crystal layer corresponding to where the light transmittance varies from 100% to 0% is generally as small as a few volts, it is difficult to make the foregoing ratio of .DELTA.T/.DELTA.V.sub.SIG small. Thus, the foregoing driving method could hardly realize a sufficient gradational display.

The present invention has been devised in view of the foregoing problems of the prior art. It is therefore an object of the present invention to provide a method of driving an active matrix type liquid crystal display unit which is capable of attaining a sufficient gradational display.

To achieve the foregoing object, the present invention provides a method of driving an active matrix type liquid crystal display unit having stick capacitors, in which an alternating voltage is used as a common voltage and/or a stick capacitor voltage such that, within a given range of liquid crystal light transmittance, the ratio of the change in liquid crystal light transmittance .DELTA.T to the change in picture signal voltage .DELTA.V.sub.SIG becomes smaller than the ratio of the change in liquid crystal light transmittance .DELTA.T to the change in effective voltage applied to the liquid crystal layer .DELTA.V.sub.LC.

It is preferable that the period of the alternating voltage be an integral multiple or integral fraction of a horizontal interval and further that the period of the alternating voltage be no longer than the period of a vertical interval.

FIG. 1 is a time chart showing waveforms of signals according to a first embodiment of the present invention;

FIG. 2 is a transmittance characteristic graph of a liquid crystal display that is obtained with the signals according to the first embodiment of the present invention;

FIG. 3 is an electric circuit diagram showing a portion of an active matrix type liquid crystal display unit with which the present invention can be used;

FIG. 4 is an electric circuit diagram showing a portion of another active matrix type liquid crystal display unit with which the present invention can be used;

FIG. 5 is a time chart showing waveforms of signals according to a second embodiment of the present invention;

FIG. 6 is a time chart showing waveforms of signals according to a third embodiment of the present invention;

FIG. 7 is a time chart showing waveforms of signals according to a fourth embodiment of the present invention;

FIG. 8 is a time chart showing waveforms of signals according to a fifth embodiment of the present invention;

FIG. 9 is a time chart showing waveforms of signals of a conventional system; and

FIG. 10 is a transmittance characteristic graph of a liquid crystal display according to the prior art.

Embodiments of the present invention will now be described with reference to the drawings.

For convenience, FIG. 3 showing the equivalent circuit of the active matrix type liquid crystal display unit will again be referred to hereinafter.

In FIG. 1, X.sub.i,X.sub.i+1 designate scanning signals applied to the scanning signal lines XL.sub.i,XL.sub.i+1, with the value of logic level "1" indicating "selection" and with the value of logic level "0" indicating "non-selection." Specifically, a selection signal of logic level "1" is supplied during a horizontal interval T.sub.H per vertical interval T.sub.V. Y.sub.j designates a picture signal applied to the picture signal line YL.sub.j, whose polarity is inverted about a reference signal V.sub.C per vertical interval T.sub.V. COM designates a common voltage applied to the common electrode COMON, which is an alternating voltage having an amplitude V.sub.COM that alternates about the reference voltage V.sub.C at a period identical with that of the horizontal interval T.sub.H. PXL designates a pixel voltage applied to the pixel electrode PX.sub.i. PXL-COM designates the signal of a stick capacitor voltage PXL minus the common voltage COM, i.e., the voltage applied to the liquid crystal layer LC.sub.i. It should be noted that the voltage of the stick capacitor electrode STACK has a constant value V.sub.C.

As will be appreciated, when the scanning signal X.sub.i is in the state of non-selection, i.e., when the switching element SW.sub.i is OFF, the liquid crystal layer LC.sub.i and the stick capacitor ST.sub.i are connected in series between the common electrode COMON and the stick capacitor electrode STACK with the pixel electrode PX.sub.i serving as a connecting point. If the capacitance of the liquid crystal layer is C.sub.LC and the capacitance of the stick capacitor is C.sub.ST, and assuming that the voltage of the common electrode COMON and the voltage of the stick capacitor electrode STACK change by values .DELTA.V.sub.COM and .DELTA.V.sub.STK, respectively, to cause a change .DELTA.V.sub.PX in the voltage of the pixel electrode PX.sub.i, then the following relationship holds:

This first embodiment of the present invention represents the case where the capacitance C.sub.LC of the liquid crystal layer is negligibly small compared with the capacitance C.sub.ST of the stick capacitor, i.e., C.sub.LC <<C.sub.ST, to cause no change in the voltage of the stick capacitor electrode STACK, i.e., .DELTA.V.sub.STK =0. In this case, therefore, the pixel voltage PXL of the pixel electrode PX.sub.i does not change even if the common voltage COM of the common electrode COMON changes, i.e., .DELTA.V.sub.PX =0 in expression (1). Accordingly, the voltage PXL-COM applied to the liquid crystal layer LC.sub.i is that shown in the drawing. Further, the effective voltage in this case is given by: ##EQU1## this being different from the picture signal voltage V.sub.SIG.

FIG. 2 shows the light transmittance characteristic of the liquid crystal layer obtained using the foregoing driving method. In this drawing, the abscissa represents the effective voltage V.sub.LC applied to the liquid crystal layer and the picture signal voltage V.sub.SIG, whereas the ordinate represents the liquid crystal light transmittance T. This characteristic was obtained by taking the amplitude V.sub.COM of the common voltage COM to be 2 volts, modifying expression (2), and calculating the voltage V.sub.SIG from the value of V.sub.LC. As will be appreciated from this drawing, although the change of effective voltage V.sub.LC applied to the liquid crystal layer where the light transmittance of the liquid crystal layer varies from 100% to 0% is about 2 volts, the change of picture signal voltage V.sub.SIG becomes as large as about 4 volts. At the same time, the ratio of the change in liquid crystal light transmittance .DELTA.T to the change in picture signal voltage .DELTA.V.sub.SIG becomes smaller than the ratio of the change in liquid crystal light transmittance .DELTA.T to the change in effective voltage applied to the liquid crystal layer .DELTA.V.sub.LC, substantially over the whole range of light transmittance.

As will be appreciated from the above, the foregoing driving method can make the value of .DELTA.T/.DELTA.V.sub.SIG small to attain a sufficient gradational display.

FIG. 5 is a time chart showing a second embodiment according to the present invention. In this embodiment, the alternating period of the common voltage COM is set to one half the horizontal interval T.sub.H. The transmittance characteristic of the liquid crystal layer is identical with that of the first embodiment shown in FIG. 2, and the same effects as those of the first embodiment are obtained.

FIG. 6 is a time chart showing a third embodiment according to the present invention. In this embodiment, the alternating period of the common voltage COM is set to two times the horizontal interval T.sub.H.

In this embodiment, the picture signal Y.sub.j is inverted about the reference voltage V.sub.C per horizontal interval T.sub.H. Where the alternating period of the common voltage COM is set to n times the horizontal interval T.sub.H (n=2, 3, 4 . . . ) as in this embodiment the foregoing point is significant. Of course, the transmittance characteristic of the liquid crystal layer is identical with that of the first embodiment shown in FIG. 2, and the same effects as those of the first embodiment are obtained.

FIG. 7 is a time chart showing a fourth embodiment according to the present invention. In this embodiment of the present invention, it is not necessary to make the common voltage COM have a square waveform as in the first, second and third embodiments, but the common voltage may be modified as shown in this drawing. Using the common voltage COM shown in this embodiment, the characteristic of the picture signal voltage V.sub.SIG shown in FIG. 2 can be changed to any desired shape to obtain the picture signal V.sub.SIG best adapted for the characteristic of the effective voltage V.sub.LC applied to the liquid crystal layer.

FIG. 8 is a time chart showing a fifth embodiment according to the present invention. This embodiment differs from the foregoing first, second, third and fourth embodiments, that is, the relationship between the capacitance C.sub.LC of the liquid crystal layer and the capacitance C.sub.ST of the stick capacitor is different from the case of C.sub.LC <<C.sub.ST of the first embodiment. Specifically, concurrently with the addition of an alternating voltage to the common voltage COM, an alternating voltage is added also to the stick capacitor voltage STK. In this case, it is preferable to keep the pixel voltage PXL unchanged, i.e., .DELTA.V.sub.PX =0 in expression (1).

This embodiment shows the case where the capacitance C.sub.LC of the liquid crystal layer is equal to the capacitance C.sub.ST of the stick capacitor (C.sub.LC =C.sub.ST). In this case, the condition of .DELTA.V.sub.PX =0 in expression (1) is satisfied if the amplitude V.sub.COM of the common voltage COM is equal to the voltage amplitude V.sub.STK of the stick capacitor electrode (V.sub.COM =V.sub.STK) and their alternating phases are opposite.

By obtaining the voltage PXL-COM applied to the liquid crystal layer on the basis of the foregoing condition, it will be recognized that the same operation as that of the first embodiment results as shown in the drawing. Therefore, the transmittance characteristic of the liquid crystal layer is identical with that of the first embodiment shown in FIG. 2, and the same effects as those of the first embodiment are obtained.

Even when the condition C.sub.LC =C.sub.ST is not met, it is preferable to make the common voltage COM and the stick capacitor voltage STK opposite in phase and to modify these voltages so as to meet the following relationship:

It should be noted that the present invention is not necessarily limited to the foregoing embodiments, but may be modified to a system in which an alternating voltage is added to the common voltage COM and/or the stick capacitor voltage STK such that the ratio of the change in liquid crystal light transmittance .DELTA.T to the change in picture signal voltage .DELTA.V.sub.SIG is smaller than the ratio of the change in liquid crystal light transmittance .DELTA.T to the change in effective voltage applied to the liquid crystal layer .DELTA.V.sub.LC, over a given range of light transmittance T.

In this case, it is preferable that the period of the alternating voltage be no longer than the period of the vertical interval T.sub.V. The reason is that if the alternating period is longer than the period of the vertical interval, flicker or the like appears in the display.

The circuit usable in the present invention may be configured as shown in FIG. 4, as well as that shown in FIG. 3.

In the circuit of FIG. 4, the stick capacitors ST.sub.i+1, ST.sub.i+2 are provided between the pixel electrodes PX.sub.i+1, PX.sub.i+2 (not shown) and the scanning signal lines X.sub.i, X.sub.i+1. Therefore, by considering the scanning signal lines X.sub.i, X.sub.i+1 as the stick capacitor electrodes, the foregoing embodiments can be applied without modification.

Further, according to the present invention, since an alternating voltage identical in frequency with the alternating voltage for the common voltage COM/stick capacitor voltage STK is applied to the liquid crystal layer, the domain of the liquid crystal observed in the prior art is reduced, whereby the quality of display can be enhanced.

Since the ratio of the change in light transmittance of the liquid crystal layer .DELTA.T to the change in picture signal voltage .DELTA.V.sub.SIG can be made small, a sufficient gradational display can be attained to enhance the quality of display.

Further, since the domain of the liquid crystal is reduced, the quality of display is enhanced.