Abstract:
The present invention is directed to a flat display device and the driving method thereof having a plurality of display picture elements which are defined by liquid crystal cell portions formed between the scanning and the signal electrodes arranged in the form of a matrix wherein at least scanning electrodes of said scanning and signal electrodes are driven from both terminals of the electrodes by individual driving circuits. A circuit is also included that creates a short circuit across the electrode thus protecting the flat display panel from degradation due to excessive current flowing across the panel before the voltage waveform is stabilized.

Description:
This is a continuation of application Ser. No. 08/026,234, filed Mar. 2, 1993, now U.S. Pat. No. 6,091,392, which is a continuation of application Ser. No. 07/783,728, filed Oct. 28, 1991, now abandoned which is a continuation of application Ser. No. 07/391,593, filed on Jul. 10, 1989 and now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a flat liquid crystal display device, and more particularly to a flat display device having a plurality of display elements which are defined by liquid crystal cell portions formed between the scanning and the signal electrodes arranged in the form of a matrix and the method of driving the flat display device. In particular, the present invention relates to a driving method which is effective for the improvement of the display quality in the flat display device. 
     One method for driving a flat display panel is shown in Japanese Patent Laid-Open No. 38935/78 and Japanese Patent Laid-Open No. 52686/83. This method discloses a driving circuit connected to one end of transparent electrodes in order to drive a display panel. 
     However, in the above-mentioned prior art, since large capacity dot matrix type liquid crystal panels are required in many applications, the transparent electrodes increase in length and width. The electrode resistance, R, and capacitance, C, across the terminal of the driving circuit to the end of the electrode increases. This increase results in degradation of the display. For example, when the liquid crystal panel has 640×400 dots and is driven at a duty ratio of 1/200, R=10˜60 KΩ, and C=800˜2000 pF. 
     When the driving waveform changes, there is a delay period of up to several tens of μsec. in which the display elements stabilize. This delay time is significant relative to one scanning period (60˜80 μs). Due to the delay time, the effective driving voltage applied to the respective display elements varies during stabilization from the predetermined value obtained by the voltage standard method. As a result, unevenness of color contrast occur, as shown in FIG.  2 . The quality of the display is reduced so much that it may be difficult to distinguish the non-selected and selected regions. 
     FIG. 2 shows one embodiment of the prior art wherein unevenness of color contrast is generated between the non-selected regions  11  and  12  when every other horizontal lines is ON. As shown in the upper side of the portion  11 , when there are a large number of signal electrodes  14  and every other horizontal line is in the ON state, the display is too light. In the upper side of the portion  12 , when there are fewer signal electrodes  15  and every other horizontal line is in the ON state, the display is too dark. The color contrast is likely to be more even when the resistance of the scanning electrodes is high. 
     Increasing the thickness of the electrodes in order to reduce the resistance is ineffectual because of the inferior alignment and increased cost of the panel due to the reduction of the throughput in the manufacturing. Therefore, there is a limit to the thickness of the transparent electrodes that can be achieved. 
     In order to eliminate the above problems, the object of the present invention is to drive the electrodes of the liquid crystal panel from both terminals thereof, thereby providing a flat display device possessing a high quality display and little unevenness of contrast. 
     SUMMARY OF THE INVENTION 
     The flat display device and method of driving the same according to the present invention has a construction in which the scanning and signal electrodes are arranged in the form of a matrix. Display elements are formed at the crossing points therebetween. The scanning electrodes are driven from both terminals of the electrodes by driving circuits. 
     According to the above method of the present invention, the resistance of the transparent electrodes becomes approximately one quarter the resistance of the transparent electrodes when the electrodes are driven from only one side. The output resistance of the driving circuit become equivalently one half and the effect of the voltage variation of the display elements is also less because the electrode is being driven from both terminals. Therefore, elements are not likely to be affected by the varying voltage, so that the quality of display is improved over that of the prior art. 
     Accordingly, it is an object of this invention to provide an improved flat display device which substantially reduces contrast problems. 
     It is another object of the invention to provide an improved flat display device which applies a voltage to both scanning driving circuits to prevent current from flowing across the liquid crystal. 
     Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification. 
     The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the apparatus embodying features of construction, combinations of elements and arrangement of parts which are adapted to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying drawings, in which: 
     FIG. 1 is a perspective view of the flat display device in accordance with the present invention. 
     FIG. 2 is perspective view illustrating the problems with the prior art. 
     FIG. 3 is a schematic showing an electronic model of the flat display of FIG.  1 . 
     FIG. 4 is a circuit diagram of the driving circuit utilizing short preventing circuitry in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a driving method according to one embodiment of the present invention. The display elements are formed on the crossing portions between the scanning and signal electrodes, arranged in the form of a matrix. The flat display device is driven by two segment and scanning side driving circuits of the transparent electrode (i.e. ITO (Indium Tin Oxide)). The amplitudes of the voltages simultaneously applied to the both ends of the scanning and signal side driving circuits are equal. A shift clock input, XSCL, is input to a shift register  2 . In addition, the display data, XD, is switched in parallel into shift register  2 . Shift clock input, XSCL, is synchronized with a latch signal, LP, by a latch  3 , and display data, XD, is converted through a level shifter  4  and driver  5  to a liquid crystal driving waveform having an amplitude equal to one of four voltage levels employed in the voltage standard method to either activate or deactivate each display element. 
     FIG. 1 also illustrates the functioning of the scanning driving circuits. A shift register  6  receives a start pulse, YD, and latch signal, LP. The output of shift register  6  is converted to a liquid crystal driving waveform by a level shifter  7  and a driver  8 . 
     When power is applied to the flat display device (i.e. power-on), the circuits become unstable, and the output voltages of the two driving circuits which are connected to each other through a transparent electrode are not equal. Since the liquid crystal driving voltage is 20-40 V, a current of several hundred μA—several mA is applied across each electrode during each output period. This amount of current causes degradation of the driving circuit and liquid crystal panel. Therefore a driving circuit is provided to control the output for a predetermined time until the waveform of the driving circuit is stabilized by a prohibit signal, INH, at the time of power-on in order to create a short circuit across the liquid crystal panel. 
     FIG. 3 is an electronic model of the driving circuit according to a dot matrix flat display device of the present invention. In FIG. 3, 5×3 matrix panel is driven from both terminals of the scanning electrodes. Display element A signifies the display element farthest from either terminal of a scanning electrode driven by a dual driving mechanism in accordance with the present invention. Display element B represents the display element farthest from the terminals of a scanning electrode driven by a single driving circuit in accordance with the prior art. 
     Resistance values between the driving circuit and the portions A and B are as follows: 
     Â: (3r c +R c )×½Ω 
     {circle around (B)}: 5r c +R c Ω 
     Herein, r c  and r s  are the resistance between display elements, and R c  and R s  are the output resistances of the driving circuits. R c  is greater than or equal to 1 KΩ. When the number of display elements is increased, r c  increases to several hundred or more KΩ and the value of R c  becomes negligible. Therefore, when the number of display elements is increased, the resistance ratio Â:{circle around (B)} approaches 1:4. Since the capacitor, C, coupled to the electrodes is not changed, the display quality obtained according to the driving method of the present invention is the same as that of the prior driving method while the resistance of the transparent electrodes is reduced to one quarter. Since the display quality of the present invention is the same as that of the prior art, a liquid crystal cell having twice as many display elements can be realized while preserving the high resolution of the flat display. 
     FIG. 4 shows one embodiment of the scanning driving circuit. The circuitry which is surrounded by a dotted line  42  shows the driving circuits for one scanning electrode. Two circuits are utilized for each electrode. Each circuit is coupled to the next and previous electrode circuits. The portion surrounded by the dotted line  43  shows circuitry which utilizes high A.C. voltages as opposed to the other portions of the circuit which utilize digital logic. In FIG. 4, a shift register which is operated by a shift clock, SCK, is comprised of a D type flip flop  21 . A latch signal, LP, shown in FIG. 1 is input to the shift clock, SCK. Therefore, the output D n+1  is input to the D type flip flop input, D n , of the next electrode driving circuit in accordance with shift clock, SCK. 
     In addition, a start pulse, YD, is input to flip flop  21 . The output of flip flop  21 , Q n  is connected to the inputs I and {overscore (I)} of the level shifter  23   a  through NOR gate  24  which can force the driver output, OUT, to an equivalent electric level depending on the state of prohibit signal, INH. The outputs O and {overscore (O)} of the level shifter are connected to the transfer gates  26  and  27  of a co-compensative transistor. In addition, the outputs, O and {overscore (O)}, are connected to the input of the NOR gate  32  and NAND gate  31  respectively. NOR gate  32  and NAND gate  31  are utilized to change the non-selected potential to A.C. The signal which is obtained by combining the frame signal, FR, with prohibit signal, INH, through the NOR gate  36  is input to the other sides of NAND gate  31  and NOR gate  32  through the level shifter  23   b  and an invertor  38 . The outputs of NAND gate  31  and NOR gate  32  are connected to a transfer gate  28  of a P channel transistor and a transfer gate  29  of an N channel transistor, respectively to control the output of the non-selected levels V 1  and V 4 . 
     The selected voltages, V 0  and V 5  are multiplexed by the transfer gate  40  of the P channel transistor and the transfer gate  41  of the N channel transistor. The output O of the level shifter  23   b  acts as a gate input to each transistor. In addition level shifter  23   b  outputs, O and {overscore (O)} are also connected to the gates of co-compensative transistors  26  and  27 . 
     When only the output, Q n , of shift register  21  is high, transfer gates  26  and  27  are conductive and the “select” voltage level is transmitted to the output, OUT. When prohibit signal, INH, is high, transfer gates  26  and  27  are conductive, thus the driver output OUT remains high (i.e. V 5  or V). Therefore, when the flat display device and thus the driving circuit is ON, and prohibit signal, INH, is high, the outputs of drivers  8 , OUT, are at the same voltage level, so that current cannot flow across the electrode and damage the liquid crystal. As a matter of course, the level of the equivalent voltage potential need not be only V 5  but can also be V 0 , V 1 , V 4  or high impedance. Further, it is possible to implement this dual driving circuit for the signal electrodes also. 
     On the other hand, if the driving method of the present invention has TAB construction wherein a semiconductor IC is used for driving and bonded to flexible tape or COG (Chip On Glass) construction, the driving circuits are connected to both terminals of the liquid crystal cell and are stored easily. In particular, COG construction is superior in that there are wires between the driving circuit and electrodes, and the connecting resistance is kept to a minimum. 
     Furthermore, even if the driving method of the present invention is only applied to the scanning electrodes, such a construction is effective in preventing the phenomenon shown in FIG.  2 . Since the amplitude of the driving voltage of the scanning electrodes is about 5 to 10 times larger than that of the signal electrodes, the delay time during charge and discharge is likely to effect the display quality. This method is most suitable for color liquid crystal cells which have narrow electrode pitches. 
     In addition, the resistance of the signal electrodes can also be reduced by utilizing thicker transparent electrode films or by coupling a different metal having low resistance to the transparent electrodes, so that the resistance of the scanning electrodes is reduced equivalently. The display quality can be improved in this manner without great cost. 
     A simple matrix type LCD in the flat display device is explained above. The deterioration of the display quality which may be generated is dependent on the resistance of the display electrodes. This can be improved by the method of the present invention. Therefore, the method of the present invention is widely applicable for active type liquid crystal displays having TFT (thin film transistor) or MIM (metal-insulator-metal), or for flat display till PDP (plasma display panel) wherein much current flows or ELD (electro-luminescence display). 
     As mentioned above, in accordance with the present invention, since the transparent electrode resistances are reduced to one quarter, this method of driving has the following advantages. 
     First, in the case of a passive type matrix liquid crystal cell, the voltage applied to a liquid crystal cell approaches a predetermined value which is obtained by the voltage standardizing method. Even if the display device has low or moderate capacitance and is driven by a two-frame A.C. driving method, it is possible to obtain a high contrast display. 
     Secondly, a large scale panel display having fine pitch can be obtained without deterioration of the display quality. 
     In addition, since it is not necessary to excessively reduce the output resistance of the driving circuit, it is possible to use an IC having more pins and at a lower cost than those utilized in the prior art. 
     Finally, since the present invention utilizes thin electrodes, it is possible to obtain flat display panels which are low in cost. 
     It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in carrying out the above method and in the constructions set forth without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
     It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.