Flat type display device and driving method and assembling method therefor

Abstract: A display device has a plurality of scanning conductor lines (307) and a plurality of signal conductor lines (308) intersecting the scanning conductor lines (307), a first switching element (304) formed at each intersection between the scanning conductor lines (307) and the signal conductor lines (308), a scanning conductor line drive element (305) for driving the scanning conductor lines, and a signal conductor line drive element (306) for driving the signal conductor lines. A second switching element (309) is provided between each scanning conductor line (307) and the scanning conductor line drive element (305), and the second switching element (309) is put into an open state in synchronization with a fall in level of a video signal given to the signal line (308) to electrically disconnect the scanning conductor line (307) from the scanning conductor line drive element (305).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flat type display device employing liquid crystals, plasma, a thin film EL (Electro Luminescence), or the like driven through scanning of its display screen, such as a matrix type liquid crystal display device, and relates to a driving method and an assembling method for the display device. 2. Description of the Prior Art Fig. 1 shows a conventional thin film transistor (TFT) matrix type liquid crystal display device. The liquid crystal display device is constructed by arranging in a matrix form unit cells each composed of a pixel P and a thin film transistor T one terminal of which is connected to the pixel P. The gates of the thin film transistors T, T, ... are connected to a scanning conductor line 8 in every row and the other terminals of the above-mentioned thin film transistors T, T, ... are connected to a display conductor line 14 in every column. Output signals at scanning signal output terminals 6, 6, ... of scanning drive LSI's (not shown) and output signals at display signal output terminals 12, 12, ... of display drive LSI's (not shown) are supplied to the scanning conductor lines 8, 8, ... and to the display conductor lines 14, 14, ... In general, an output signal at one scanning signal output terminal 6 of a scanning signal is supplied to one scanning conductor line 8, as shown in Fig. 1. Therefore, for example, when 240 scanning conductor lines 8 are driven, the scanning drive LSI's are required to produce the same number of output signals, i.e., 240 output signals. In this case, when an LSI which produces 60 output signals is selected, four LSIs are to be provided. When an LSI which produces 80 output signals is selected, three LSIs are to be provided. When an LSI which produces 120 output signals is selected, two LSIs are to be provided. Lately, flat type display devices such as a liquid crystal display are in keen underselling competition, and therefore cost reduction has been a serious problem. Among others, the drive LSIs (scanning drive LSIs and display drive LSIs) are expensive to share a great part of the total cost of the liquid crystal display device, and therefore the LSIs are the key parts for cost reduction. In view of the above-mentioned fact, there have been trials to reduce chip size by making finer the pitch of output terminals or to increase the number of output terminals per drive LSI to reduce the number of LSIs for the purpose of reducing the costs of the drive LSIs. However, none of the above-mentioned trials have been able to achieve sufficient cost reduction, rather, it is possible to lower the yield and reliability because a lot of terminals must be connected at a fine pitch on the board of the liquid crystal display device. The above-mentioned fact is common to devices such as a simple-matrix display device where its display signal system is scanned by means of a drive LSI. Fig. 2 shows another display device wherein ramification conductor lines A, B, C, and D, one end of each ramification conductor line being connected to a scanning signal output terminal 6 and the other end thereof ramifying into n branches, are provided, and a three-terminal transistors SW 11 to SW n4 are provided between the branches and corresponding scanning conductor lines G(1) through G(n) (US-A-4 816 819). The corresponding branches of the ramification conductor lines constitute a group, thus n groups of the branches are provided. Fig. 3 shows the waveforms of control signals for the display device shown in Fig. 2. In the display device of Fig. 2, n control conductor lines E(1) through E(n) connected to the gates of the transistors SW 11 - SW n4 are sequentially selected (electric potential is made to be positive (+V)) to sequentially select the groups of the branches, and the ramification conductor lines A, B, C, and D are sequentially selected when each group of the branches is selected to thereby select the corresponding scanning conductor lines in each group as shown in Fig. 3. With the above-mentioned operation, the scanning conductor lines are selected group by group. Meanwhile, when the transistors SW 11 to SW n4 are turned off, transistors P 1 through P N connected to the corresponding scanning conductor lines G(1) through G(N) are turned on (by signals I and J), and a constant voltage is applied to the scanning conductor lines G(1) through G(N) to achieve sufficient discharge of electricity. In the display device shown in Fig. 2, the number of connection terminals of the scanning line drive circuit 5 is smaller than the number of the scanning conductor lines G(1) through G(N). However, since the whole length of the conductor lines arranged between the scanning line drive circuit 5 and a display board 1 are long, the resistance of the conductor lines made of a thin film increases, and therefore the conductor lines may catch noise to cause the malfunction of the display device or the deterioration of the display image quality. Furthermore, in the display device shown in Fig. 2, since the scanning line drive circuit 5 and the signal line drive circuit 2 are arranged in two different directions with respect to the display board 1, two connection steps are required for the connection between the display board 1 and the drive circuit boards. When occasion demands, a junction board 9 for transmitting timing signals and other signals must be provided, and therefore connection between the junction board and the drive circuit boards is required. Fig. 4 shows a display device assembling process where the display device shown in Fig. 2 is coupled with a junction board. First, a connection material (anisotropic conductive film or the like) for connecting display conductor lines S(1) through S(M) on the display board (display panel) 1 with the signal line drive circuit 2 is supplied to either the display board 1 or to a signal line drive circuit board (not shown) (step S1). Then the signal line drive circuit board is placed adjacently along a side of the display board 1, that is, the signal line drive circuit board is aligned with the display board 1 (step S2). Then the signal line drive circuit board is connected with the display board so that the display conductor lines S(1) through S(M) on the display board 1 and output terminals of the signal line drive circuit 2 are connected with each other by way of the connection material (step S3). Then the display board 1 is turned by an angle of 90° (step S4). In the same manner as described above, the ramification conductor lines A, B, C, and D, the control conductor lines E(1) through E(n), and the other conductor lines on the display board 1 are connected with output terminals of the scanning line drive circuit 5 by means of a connection material (steps S5 - S7). Then the display board 1 with the signal line drive circuit board and the scanning line drive circuit board is transferred (step S8). Subsequently, a connection material is supplied to either the junction board or the scanning line drive circuit board (step S9), alignment of both boards is performed (step S10), and conductor lines of the junction board are connected to the input terminals of the scanning line drive circuit 5 (step S11). Then the display board 1 in this condition is transferred (step S12). In the same manner as described above, a connection material is supplied to either the junction board or the signal line drive circuit 2 (step S13), alignment of both boards is performed (step S14), and conductor lines of the junction board are connected to the input terminals of the signal line drive circuit 2 (step S15). As described above, there are problematically many processes for the assembling (note that the alignment process at step S14 can be eliminated). As is evident from the above, the assembling procedure has a lot of steps and is troublesome to result in cost increase. Furthermore, since the conductor lines A-D, E(1)-E(n), and I and J occupy a wide non-display area on the display board and also since the circuit extends in two directions with respect to the display screen, the resulting module disadvantageously has a large size and heavy weight after assembling. Furthermore, since the operation frequencies of the transistors P 1 through P N are high, they possibly operate faultily. Furthermore, the display device shown in Fig. 2 has many intersections of the conductor lines, which results in a reduced yield of the products. The present invention provides a display device comprising: a substrate; a plurality of scanning conductor lines disposed on the substrate; a plurality of signal conductor lines disposed on the substrate, the signal conductor lines intersecting the scanning conductor lines; a plurality of first switching elements, one first switching element disposed at each intersection between a signal conductor line and a scanning conductor line; a scanning conductor line drive element for driving the scanning conductor lines; a signal conductor line drive element for driving the signal conductor lines; and a plurality of second switching elements, one second switching element disposed between each scanning conductor line and the scanning conductor drive element; characterised in that it further comprises a second switching element drive means for putting the second switching elements into an open state in synchronisation with a fall in the level of a video signal applied to the signal conductor lines thereby to electrically disconnect the scanning conductor lines from the scanning conductor line drive element. In a preferred embodiment, the second switching element drive means comprises an interruption signal terminal electrically connected to all the second switching elements; and means for applying an interruption control signal to the interruption signal terminal in synchronisation with a fall in the level of the video signal applied to the signal conductor lines. In a preferred embodiment the display device further comprises an external power input terminal connected, in use, to an external power source having an OFF voltage sufficient to turn off the first switching elements; a plurality of third switching elements, one third switching element associated with each scanning conductor line for selectively connecting the scanning conductor line to the external power input terminal; and third switching element drive means for applying a switching control signal to the third switching elements when the second switching elements are in an open state, so that when the second switching elements are in an open state the OFF voltage is supplied to the associated scanning conductor line so as sufficiently to turn off the first switching elements associated with the scanning conductor line. In a preferred embodiment the third switching element drive means comprises an input terminal connected to all the third switching elements, and means for applying the switching control signal to the input terminal. When the second switching element is put in an open condition, that is, turned off, the signal conductor line is electrically disconnected from the scanning conductor line drive element. Therefore, by temporarily turning off the second switching element in synchronization with the fall in level of the video signal given to the signal conductor line, an instantaneous voltage drop occurring to the signal conductor line due to the fall in level of the video signal from "H" to "L" is not transmitted to the scanning conductor line drive element. As a result, various signals in the scanning conductor line drive element can be maintained in a stable state. Therefore, a possible faulty operation of the scanning conductor line drive element and total breakdown of the element due to a voltage exceeding the endurance voltage of the element can be prevented. Furthermore, a momentary change of voltage in the signals in the scanning conductor line drive element due to the instantaneous voltage drop is avoided, and therefore a great voltage difference between an ON voltage and an OFF voltage of the first switching element can be assured within the range of the endurance voltage of the scanning conductor line drive element. Thus, the display quality can be improved. It is possible that the electric potential at each scanning conductor line become unstable to insufficiently turn off the first switching element when the scanning conductor line is disconnected from the scanning conductor line drive element. Such a possibility can be avoided by a display device further comprising an external power input terminal connected to an external power source having an OFF voltage sufficient to turn off the first switches, and a third switching element provided on each scanning conductor line for switching a signal to be transmitted through each scanning conductor line between a scanning signal supplied from the scanning conductor line drive element and the OFF voltage, wherein the second switching element is in an open state, a switching control signal is given to an input terminal of the third switching element so that the OFF voltage is supplied to the scanning conductor line to sufficiently turn off the first switching element. With this structure, the first switching element is sufficiently turned off to easily solve the above-mentioned problem. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: Fig. 1 is a diagram showing a display section of a conventional active-matrix type liquid crystal display device; Fig. 2 is a circuit diagram showing the construction of another conventional active-matrix type liquid crystal display device; Fig. 3 is a timing chart of control signals of the liquid crystal display device shown in Fig. 2; Fig. 4 is a flow chart of the assembly process of the liquid crystal display device shown in Fig. 2; Fig. 5 is a circuit diagram of an active-matrix display device in accordance with an embodiment of the present invention; Fig. 6 is a timing chart showing the timing of turning on and off various signals in the active-matrix display device shown in Fig. 5; Fig. 7 is a circuit diagram of an active-matrix display device in accordance with an embodiment of the present invention; Fig. 8 is a timing chart showing the timing of turning on and off various signals in the active-matrix display device shown in Fig. 7; and Fig. 9 is a timing chart for explaining a problem caused by the instantaneous voltage drop of a scanning signal. Fig. 5 shows an active-matrix display device in accordance with an embodiment of the present invention. On a transparent insulating board 302 which is made of glass and which has a laterally elongated rectangular shape are provided a plurality of gate lines 307 serving as scanning conductor lines and a plurality of source lines 308 serving as signal lines perpendicularly intersecting the gate lines 307. In more detail, the gate lines 307 are arranged in the horizontal direction, while the source lines 308 are arranged in the vertical direction. In the areas surrounded by the gate lines 307 and the source lines 308 are provided pixels 303 in a matrix form. At the intersections between the gate lines 307 and the source lines 308 are provided thin film transistors 304 serving as switching elements. At a lengthwise end portion of the transparent insulating board 302 is provided a gate line drive element 305 extending in the widthwise direction, where the gate line drive element 305 is connected to an end of each of the gate lines 307. At a widthwise end portion of the transparent insulating board 302 is provided a source line drive element 306 extending in the lengthwise direction, where the source line drive element 306 is connected to an end of each of the source lines 308. It should be noted that the gate line drive element 305 and the source line drive element 306 are practically provided on the transparent insulating board pattern by the COG (Chip On Glass) connection method. The gate line drive element 305 is connected to a plurality of input terminal lines 310. The gate line drive element 305 sequentially scans the gate lines 307 in the vertical direction while the source line drive element 306 applies a video signal to a source line 308 to write the signal line by line to thereby make the active-matrix display device perform an image display operation. Portions of the transparent insulating board 302 except for the portions where the gate line drive element 305 and the source line drive element 306 are provided serve as a display section 301. Between the gate line drive element 305 and the gate lines 307 are provided interruption thin film transistors 309 serving as switching elements to make and break a connection between them. The gate electrodes of the interruption thin film transistors 309 are connected to an interruption signal input terminal 316 formed on the transparent insulating board 302 in a position slightly apart toward an end of the board 302 from the aforementioned input terminal lines 310, and an interruption control signal 313 is input from an external control circuit (not shown) via the interruption signal input terminal 316. In Fig. 6, (a), (b), and (c) show the waveforms of a video signal 311 supplied to the source lines 308, a scanning signal 312 given to the gate lines 307, and an interruption control signal 313 input to the interruption signal input terminal 316 for the interruption thin film transistor 309, respectively. In Fig. 6, (d) shows the waveform of a certain signal inside the gate line drive element 305. In the present case, the interruption thin film transistor 309 is provided to prevent the possible bad influence of an instantaneous voltage drop 315 attributed to the change in level from "H" to "L" of the video signal 311 supplied to the source lines 308 from being exerted on the gate line drive element 305. The following describes the detail of the operation with reference to Fig. 6. As shown in Fig. 6, the interruption control signal 313 having the waveform shown in (c) is given to the interruption signal input terminal 316 for the interruption thin film transistors 309 temporarily in synchronization with the fall from "H" to "L" of the video signal 311 in the present embodiment. While the control signal 313 is given, the interruption thin film transistors 309 are off to open the connection between the gate lines 307 and the gate line drive element 305, i.e., temporarily electrically disconnect the gate lines 307 from the gate line drive element 305. In the time except for the fall time of the video signal 311, the interruption thin film transistor 309 is on to connect the gate line drive element 305 with the gate lines 307. At the time when the video signal 311 falls from "H" to "L", there is generated on the scanning signal 312 the instantaneous voltage drop 315 attributed to coupling noise between the scanning signal 312 and the video signal 311 as shown in (b) in Fig. 6. If the gate lines are always electrically connected with the gate line drive element 305, the instantaneous voltage drop 315 at the gate lines will be transmitted to the gate line drive element 305 and affect various signals (represented by the signal 314) therein so that a momentary change of voltage takes place to the signal 314 as shown in Fig. 9. However, since the gate lines 307 are electrically disconnected from the gate line drive element 305 temporarily in synchronization with the fall from "H" to "L" of the video signal 311 in the present embodiment, the bad influence of the instantaneous voltage drop 315 of the scanning signal 312 can be prevented from being exerted on the gate line drive element 305. As a result, as shown in (d) in Fig. 6, various signals represented by the signal 314 in the gate line drive element 305 are in a stable condition, i.e., kept unchanged without suffering from the bad influence of the instantaneous voltage drop 315. According to the present embodiment, the gate line drive element 305 neither malfunctions nor is subjected to total breakdown due to a voltage exceeding its endurance voltage. Furthermore, since no momentary change of voltage due to the instantaneous voltage drop takes place, a great voltage difference between the "on" voltage and the "off" voltage for the thin film transistor 304 is ensured within the range of the endurance voltage of the gate line drive element 305. The voltage difference is sufficient to easily improve the display quality. Furthermore, if an active-matrix display device having the gate lines always electrically connected with the gate line drive element is formed by the COG connection method, a resistance combined with the connection resistance and the resistance of the patterned conductor lines on the display board will be exerted on the input terminal lines 310 of the gate line drive element 305 and enlarge the momentary change of voltage due to the instantaneous voltage drop 315. However, in the present invention, because the gate lines 307 are designed to be electrically disconnected from the gate line drive element 305 when the instantaneous voltage drop 315 occurs, the instantaneous voltage drop 315 is not transmitted to the gate line drive element 305 and the voltage change of the signal 314 in the gate line drive element 305 as shown in Fig. 9 is avoided. Therefore, even though a large resistance is exerted on the gate line drive element 305, influence of the voltage drop is not amplified, so that in designing the COG connection type display device taking into consideration the operation of the gate line drive element 305 and the display image quality, it is not very necessary to provide against the influence of the resistances added to the input terminal lines 310 of the gate line drive element such as the connection resistance and the resistance of the patterned conductor lines on the display board. As a result, the innate advantages of the COG connection type active-matrix display device, i.e., the fine pitch, low cost, and high reliability can be fully appreciated. Fig. 7 shows an active-matrix display device in accordance with another embodiment of the present invention. The present embodiment takes the measure of avoiding the condition where the thin film transistors 304 are not sufficiently turned off due to the possible instability of the electric potential at the gate lines 307 when the gate lines 307 are electrically disconnected from the gate line drive element 305 by the operation of the interruption thin film transistors 309. The following describes the detail of the construction. It should be noted that the construction of most of the parts is similar to that in Fig. 5, and therefore like components are denoted by the same reference numerals in Figs. 5 and 7 and no further description on those components is provided here. On the transparent insulating board 302 are provided gate line signal switching thin film transistors 317, 317, ... adjacent to the interruption thin film transistors 309, 309, ... The gate electrode of each gate line signal switching thin film transistor 317 is connected to a control signal input terminal 319 provided at a portion adjacent to the interruption signal input terminal 316 on the transparent insulating board 302, and a power switching control signal 320 (see Fig. 8 (e)) is input via the control signal input terminal 319 from an external circuit. The source electrode of each switching thin film transistor 317 is connected to the thin film transistor 304, while the drain electrode thereof is connected to an external power input terminal 318. The external power input terminal 318 is formed in a position slightly apart from the input terminal line 310 at the opposite side from the interruption signal input terminal 316 and control signal input terminal 319, and an OFF voltage signal 321 is input via the external power input terminal 318 from an external power source. In the construction as described above, the signal 320 is given to the gate electrode of the switching thin film transistor 317 at the time as shown in Fig. 8, i.e., when the video signal 311 falls from "H" to "L" and an interruption control signal 314 at an OFF level is input to the interruption thin film transistors 309. At this time, an OFF voltage 321 is given to the thin film transistors 304 via the external power input terminal 318 and the switching thin film transistors 317. According to the present embodiment, an OFF voltage to sufficiently turn transistors off is supplied to the thin film transistors 304 when the gate lines 307 are made to be electrically disconnected from the gate line drive element 305 by the operation of the interruption thin film transistors 309. Therefore, the insufficient turning off of the thin film transistors 304 due to the unstable electric potential at the gate lines 307 can be securely prevented. Although the above description on the embodiments shown in Figs. 5-8 has been made for the case where the present invention is applied to a COG connection type active-matrix display device, the present invention can be also applied to the case where a TAB (Tape Automated Bonding) drive element is formed by the COF (Chip On Film) method or to the case where a drive element is formed on a glass board. The thin film transistor to be used in the present invention is typically an insulated gate field effect transistor using amorphous silicon, polysilicon, Te, or the like as a semiconductor material. Furthermore, the active-matrix display device of the present invention can be applied not only to a monochrome liquid crystal display device but also to a color liquid crystal display device employing a color filter. As is evident from the above, according to the active-matrix display devices shown in Figs. 5 and 7, since the connection between the scanning conductor lines and the scanning conductor line drive element is temporarily undone in synchronization with the fall of the video signal level from "H" to "L", the bad influence of the instantaneous voltage drop of the scanning signal due to the amplitude of the video signal can be prevented from being exerted on the gate line drive element regardless of the assembly structure of the drive element. As a result, the various signals inside the scanning conductor line drive element can be kept stable, and therefore the possible malfunction or breakdown of the element due to a voltage exceeding the endurance voltage of the scanning conductor line drive element can be securely prevented. Furthermore, the momentary change of voltage attributed to the instantaneous voltage drop does not takes place, and therefore a great voltage difference can be assured between the "on" voltage and the "off" voltage of the first switching element within the range of the endurance voltage of the gate line drive element. Thus, the present invention can provide a high-quality, high-reliability active-matrix display device. Furthermore, the active-matrix display device shown in Fig. 8 can further improve the reliability, because the first switching elements 304 are prevented from being insufficiently turned off due to the instability of the electric potential at the gate lines while the connection between the scanning conductor lines and the scanning conductor line drive element is undone. The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims: A display device comprising: a substrate (302); a plurality of scanning conductor lines (307) disposed on the substrate; a plurality of signal conductor lines (308) disposed on the substrate, the signal conductor lines intersecting the scanning conductor lines; a plurality of first switching elements (304), one first switching element disposed at each intersection between a signal conductor line and a scanning conductor line; a scanning conductor line drive element (305) for driving the scanning conductor lines; a signal conductor line drive element (306) for driving the signal conductor lines; and a plurality of second switching elements (309), one second switching element disposed between each scanning conductor line and the scanning conductor drive element; characterised in that it further comprises a second switching element drive means (316) for putting the second switching elements into an open state in synchronisation with a fall in the level of a video signal (311) applied to the signal conductor lines thereby to electrically disconnect the scanning conductor lines (307) from the scanning conductor line drive element (305). A display device as claimed in claim 1 wherein the second switching element drive means comprises an interruption signal terminal (316) electrically connected to all the second switching elements; and means for applying an interruption control signal (313) to the interruption signal terminal (316) in synchronisation with a fall in the level of the video signal (311) applied to the signal conductor lines. A display device as claimed in claim 1 or 2 and further comprising an external power input terminal (318) connected, in use, to an external power source having an OFF voltage sufficient to turn off the first switching elements (304); a plurality of third switching elements (317), one third switching element associated with each scanning conductor line for selectively connecting the scanning conductor line to the external power input terminal; and third switching clement drive means (319) for applying a switching control signal (320) to the third switching elements (317) when the second switching elements are in an open state, so that when the second switching elements are in an open state the OFF voltage is supplied to the associated scanning conductor line so as sufficiently to turn off the first switching elements associated with the scanning conductor line. A display device as claimed in claim 3 wherein the third switching element drive means comprises an input terminal (319) connected to all the third switching elements (317), and means for applying the switching control signal (320) to the input terminal.

Inventors: AKIYAMA HIDEMI, INADA KIYOSHI, NAKASHIMA TAKASHI, SHIMOKAWA HIROSHI, TAGUSA YASUNOBU, YAMAMOTO YUJI, AKIYAMA, HIDEMI, INADA, KIYOSHI, NAKASHIMA, TAKASHI, SHIMOKAWA, HIROSHI, TAGUSA, YASUNOBU, YAMAMOTO, YUJI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1993
Main classification: G02F1
IPC codes: G09G3