Driving Circuit for Producing Varying Signals for a Liquid Crystal Display Apparatus

1. Field of the invention

This invention relates to a driving circuit for a liquid crystal display apparatus, and more particularly to a driving circuit for a liquid crystal display apparatus in which thin film transistors are used as switching elements.

2. Description of the prior art

FIG. 6 shows a driving circuit for driving an active matrix type LCD apparatus 1 in which thin film transistors (TFTS) are arranged as switching elements in a matrix form. The driving circuit shown in FIG. 6 comprises a source driver 2, a data driver 3, a controller 4, and a polarity-inverting circuit 5. When a DC voltage is applied to the liquid crystal in the LCD apparatus 1, electrochemical reaction occurs in the liquid crystal, thereby deteriorating the liquid crystal. In order to prevent such deterioration from occurring, the driving circuit is provided with the polarity-inverting circuit 5 so that the LCD apparatus 1 is AC-driven.

The polarity-inverting circuit 5 generally comprises an amplifier, an inverter which inverts the polarity of the output of the amplifier, and a switching circuit which alternatingly selects either of the outputs of the amplifier and inverter to output the selected output. The polarity-inverting circuit 5 converts input video signals into polarity-inverted signals (AC signals). FIG. 7 shows gray scale video signals. For example, the polarity-inverting circuit 5 converts the video signals of FIG. 7 into polarity-inverted signals shown in FIG. 8.

When the LCD apparatus I displays the same time for a long period of time, the pattern is "memorized" in the liquid crystal, with the result in that some extent of time is required to completely distinguish this memorized pattern. Even when another pattern is to be displayed, therefore, this memorized pattern also appears as a residual image on the apparatus 1 (i.e., the residual image phenomenon occurs). This residual image phenomenon greatly impairs the image quality.

The driving circuit of this invention, which overcomes the above-discussed and numerous other disadvantages and deficiencies of the prior art, comprises a polarity-inverting circuit for converting input video signals into polarity-alternating signals, and said polarity-inverting circuit has input-output voltage characteristics which are at least partially non-linaer.

In preferred embodiments, the polarity-inverting circuit has input-output characteristics which are linear in a positive region, and non-linear in a negative region.

Alternatively, the polarity-inverting circuit may have input-output characteristics which are non-linear in a positive region, and linear in a negative region.

Thus, the invention described herein makes possible the objectives of:

(1) providing a driving circuit which can drive an LCD apparatus with an improved image quality; and

(2) providing a driving circuit which can drive an LCD apparatus without causing the residual image phenomenon.

This invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings as follows:

FIG. 1A illustrates the first and fourth quadrants of a voltage plot representing positive and negative regions, respectively showing the input-output characteristics of a polarity-inverting circuit used in a driving circuit according to the invention.

FIG. 1B is a block diagram illustrating the principal portion of the polarity-inverting circuit.

FIG. 2 is a circuit diagram of an amplifying unit used in the polarity-inverting circuit of FIG. 1B.

FIG. 3 illustrates the fourth quadrant of a voltage plot representing a negative region showing the input-output characteristics of FIG. 1A in more detail.

FIG. 4 illustrates the first and fourth quadrants of a voltage plot representing positive and negative regions, respectively showing the input-output characteristics of a polarity-inverting circuit used in another driving circuit according to the invention.

FIG. 5 is a graph showing the relationship between applied voltages and DC levels in the embodiments.

FIG. 6 is a block diagram showing an LCD apparatus and a driving circuit.

FIG. 7 shows a waveform of video signals input to a polarity-inverting circuit.

FIG. 8 shows a waveform of signals output from a conventional polarity-inverting circuit.

FIG. 9 illustrates the first and fourth quadrants of a voltage plot representing positive and negative regions, respectively showing the input-output characteristic of polarity-inverting circuit used in a conventional driving circuit.

FIG. 10 is an equivalent circuit diagram of a pixel portion of an LCD apparatus.

FIG. 11 is a cross section of a TFT.

FIG. 12 shows a waveform of a gate signal.

FIG. 13 is a graph illustrating the relationship between the pixel capacitance and the applied voltage.

Before describing embodiments of the invention, the generation mechanism of the residual image phenomenon will be described. FIG. 10 shows an equivalent circuit of a picture element (pixel) of the LCD apparatus 1 (FIG. 6). Each pixel is provided with a TFT 13. FIG. 11 shows the sectional structure of the TFT 13. The source electrode 13s and drain electrode 13d of the TFT 13 are connected to a source line 11 and a pixel electrode 14, respectively. A gate line 12 which perpendicularly intersects the source line 11 functions also as the gate electrode of the TFT 13. The numerals 18 and 19 in FIG. 11 indicate a gate insulating film, and a semiconductor film, respectively. In the pixel having the above-mentioned structure, a parasitic capacitance C.sub.gd is formed between the gate line 12 and the drain electrode 13d, and a pixel capacitance C.sub.LC is formed between the pixel electrode 14 and a counter electrode 17 which is opposite to the pixel electrode 14.

The signal for driving the TFT 13 will be described with reference to FIG. 12 which illustrates the waveform of the gate signal applied to the gate line 12. In FIG. 12, V.sub.ON indicates the ON-voltage at which the TFT 13 is ON, and V.sub.OFF the OFF-voltage at which the TFT 13 is OFF. The level of the gate signal (i.e., the gate voltage) is changed from V.sub.OFF to V.sub.ON at time T.sub.1, so that the TFT 13 turns ON and the potential of the drain electrode 13d and pixel electrode 14 begins to increase towards the voltage level applied to the source line 11. In this way, the "writing" of the pixel is performed. At time T.sub.2, then, the level of the gate signal is reduced from V.sub.ON to V.sub.OFF, thereby turning OFF the TFT 13.

The potential of the counter electrode 17 remains unchanged. As a result of the change of the level of the gate signal from V.sub.ON to V.sub.OFF at time T.sub.2, therefore, the potential of the drain electrode 13d and pixel electrode 14 (hereinafter, referred to as "the drain potential") is shifted by

This drain potential which has been shifted by .DELTA.V is maintained until the next writing (i.e., between times T.sub.2 and T.sub.3). In other words, the drain potential is offset by .DELTA.V with respect to the signal applied to the source line 11.

In the expression (1) which indicates the offset voltage .DELTA.V, C.sub.LC changes in accordance with the applied voltage (r.m.s.), while V.sub.ON, V.sub.OFF and C.sub.gd are constant. FIG. 13 shows a relationship between C.sub.LC and the applied voltage (r.m.s.) in an LCD apparatus using the TN type liquid crystal (which is widely employed in TFT LCD apparatus). In an LCD apparatus using the TN type liquid crystal, the transmittance of the liquid crystal is changed by varying the level of the applied voltage so that images are displayed on the LCD apparatus. In other words, the value of .DELTA.V de pends on the contents to be displayed. In the case that V.sub.ON -V.sub.OFF =20 V, C.sub.gd =0.1 pF, C.sub.LC =0.6 pF, and C//.sub.LC =1.4 pF, the offset voltage .DELTA.V can be calculated as follows: ##EQU1##

As seen from above, the offset voltage .DELTA.V which is caused by the parasitic capacitance C.sub.gd of the TFT 13 is changed in a large degree (in the above example, as much as about 1.5 V) in accordance with the contents of images to be displayed. When the same pattern is displayed for a long period of time, therefore, offset voltages .DELTA.V of different levels are applied to each pixel according to the respective contents of the pattern to be displayed therein. This means that DC voltages of different levels are applied to respective pixels for a long period of time. This prolonged application of DC voltages causes electro chemical changes in the components of each pixel (the liquid crystal, the orientation film, the protection film, etc.). These changes are memorized in the respective pixel of the LCD apparatus 1. Even when signals for the next pattern are applied to the pixels (or when offset voltages .DELTA.V of other levels are applied to the pixels), it requires a considerable period of time to extinguish the memorized changes from the pixels. These memorized or remaining changes appear as residual images.

In this way, the residual image phenomenon is caused by the fact that the levels of offset voltages .DELTA.V change in accordance with the contents of patterns to be displayed. Hence, if the changes of offset voltages .DELTA.V can be corrected or compensated, the problem of the residual image phenomenon will be solved.

In this specification, when the input-output voltage characteristics substantially satisfy the relationship that an output voltage increases equally in proportion to an increase of an input voltage, the voltage characteristics (in a positive or negative polarity region, i.e., a V+ or V- region) have a substantially fixed ratio of, for example, one-to-one, which is a "substantially linear" output. If the output voltage characteristics include a portion where the output voltage does not increase in proportion to an increase of input voltage, the voltage characteristics vary (at a transition) to a ratio of input voltage to output voltage different from a one-to-one or substantially linear ratio. Thus an output voltage (which includes such a transition) has two different ratios which taken together and when viewed as a whole is a substantially "non-linear" output.

FIG. 1A shows the input-output characteristics of a polarity-inverting circuit used in a driving circuit according to the invention. In FIG. 1A, the solid line LA indicates the input-output characteristics of the embodiment, and the broken line LB that of the prior art. The driving circuit according to the invention may be generally constructed in the same manner as that of the prior art shown in FIG. 6. In this embodiment, however, the polarity-inverting circuit 5 is constructed so that the input-output characteristics in a positive region are linear in a manner similar to that of the prior art, and that the input-output characteristics in a negative region are nonlinear unlike that of the prior art (in which the input-output characteristics in both the positive and negative regions are linear). In this embodiment, the non-linear characteristics of the output of the polarity-inverting circuit in a negative region provide an input/output relationship that, even when inputs of the same level are respectively supplied to the embodiment and to a circuit of the prior art, the output level of the embodiment is smaller than that of the prior art circuit, thereby correcting or compensating changes of the offset voltages .DELTA.V. As a result of this correction or compensation of the changes of the offset voltages .DELTA.V, the drain potential (DC level) is substantially constant irrespective of the contents of patterns to be

As shown in FIG. 5, the DC level of signals output from the polarity-inverting circuit 5 changes in accordance with the contents of patterns to be displayed (which correspond to the AC amplitude). To the drain electrode 13d and pixel electrode 14, are applied signals the level of which is the sum of the level of the output signal and the offset voltage .DELTA.V (which depends on the contents of a pattern to be displayed). Therefore, the drain potential is substantially constant irrespective of the contents of pa-,terns to be displayed. Even when the same pattern is displayed for a long period of time, consequently, the contents of the pattern are not memorized in the respective pixels, with the result that the residual image phenomenon does not occur in the LCD apparatus 1.

FIG. 1B shows the principal portion of the polarity-inverting circuit 5. In this embodiment, instead of the amplifier and inverter in a conventional circuit, the polarity-inverting circuit 5 comprises two amplifying units 5A and 5B. The amplifying unit 5A is a non-inverting amplifying unit having linear input-output characteristics, and the amplifying unit 5B is an inverting amplifying unit having non-linear input-output characteristics. The outputs V.sub.+ and V.sub.- of the amplifying units 5A and 5B are alternatingly selected by a switching circuit (not shown) for each field to be output, in the same manner as in a conventional circuit. The amplifying unit 5B will be described in more detail with reference to FIG. 2. The amplifying unit 5B comprises an operational amplifier 51. Video signals V.sub.in are supplied to the inverting input terminal of the amplifier 51 through a resistor R.sub.1. Between the inverting input terminal (V.sub.B) and the output V.sub.- of the amplifier 51, is connected a resistor R.sub.2. A series circuit of a resistor R.sub.3, a diode D.sub.1 and a resistor R.sub.5 is connected in parallel with the resistor R.sub.1. A power source V.sub.R is coupled to the junction point of the diode D.sub.1 and the resistor R.sub.5 via a resistor R.sub.6. In parallel with the resistor R.sub.2, a series circuit of resistors R.sub.7 and R.sub.4 and a diode D.sub.2 is connected. At the junction point of the resistors R.sub.7 and R.sub.4, a power source V.sub.CC is coupled through a resistor R.sub.8. The power source V.sub.R is also connected to the non-inverting input terminal of the amplifier 51 via a resistor R.sub.9 which is connected in series with a resistor R.sub.10 to ground.

FIG. 3 illustrates in more detail the input-output characteristics of the amplifying unit 5B. When the video input signal V.sub.in is small (region A in FIG. 3), both the diodes D.sub.1 and D.sub.2 are OFF. In this case, the relationship between the input V.sub.in and output V.sub.- of the amplifying unit 5B follows:

wherein V.sub.c is the potential of the non-inverting input terminal of the operational amplifier 51, and changes as the line La shown in FIG. 3. The gain .vertline.A.vertline. is R.sub.2 /R.sub.1.

When the input V.sub.in increases to reach the voltage V.sub.1, only the diode D.sub.1 is ON so that the series circuit of the resistors R.sub.3 and R.sub.5 is connected in parallel with the resistor R.sub.1. This can be achieved by adequately setting the values of the resistors. In this case, the gain .vertline.A.vertline. is

The voltage V.sub.1, which is a changing point, is

wherein V.sub.F means the voltage drop of the diodes (about 0.7 V in the case where the diodes are silicon diodes). The relationship between the input V.sub.in and output V.sub.- changes as the line Lb shown in FIG. 3.

When the input V.sub.in further increases to reach the voltage V.sub.2, the diode D.sub.2 turns ON while the diode D.sub.1 remains ON. This ON operation causes the series circuit of the resistors R.sub.4 and R.sub.7 to be connected in parallel with the resistor R.sub.2. Therefore, the gain .vertline.A.vertline. drops and can be expressed by the following

The relationship between the input V.sub.in and output V.sub.- changes as the line Lc shown in FIG. 3. The voltage V.sub.2, which is another changing point, is

FIG. 4 shows the input-output characteristics of a polarity-inverting circuit used in another driving circuit according to the invention, in which the amplifying unit 5A has non-linear input-output characteristics and the amplifying unit 5B has linear input-output characteristics. In this embodiment, as shown by the solid line LC, the input-output characteristics in the negative region are linear in a manner similar to that of the prior art, and that the input-output characteristics in the positive region are non-linear unlike that (the broken line LB) of the prior art (in which the input-output characteristics in both positive and negative regions are linear). According to this embodiment, the drain potential can be maintained substantially constant in the similar manner as the above-described embodiment.

Alternatively, both the amplifying units 5A and 5B may have non-linear input-output characteristics, so that the input-output characteristics of the polarity-inverting circuit are non-linear in both positive and negative regions.

In the embodiments, the polarity-inverting circuits have non-linear input-output characteristics by which the drain potential is maintained constant. The kind of non-linear input-output characteristics are not restricted to the above, provided that the variation of the offset voltage can be suppressed.

As seen from above, the driving circuit according to the invention can drive an LCD apparatus without causing the residual image phenomenon. Therefore, the driving circuit according to the invention is very useful in driving an LCD apparatus used in office automation equipment in which the same pattern may be displayed for a long period of time.

It is understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be construed as encompassing all the features of patentable novelty that reside in the present invention, including all features that would be treated as equivalents thereof by those skilled in the art to which this invention pertains.