An active matrix display apparatus

Abstract: An active matrix display apparatus comprising pixel electrodes arranged in a matrix fashion on an insulating substrate, storage capacity electrodes ar­ ranged opposite to said pixel electrodes, storage capacity lines (8) connected individually to said storage capacity electrodes, a common main line (9) connected to said storage capacity lines, at least one branch line (17) branched from said common main line, and a branch terminal (16) formed at the leading end of said branch line, whereby a signal delay can be minimized.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention: This invention relates to an active matrix display apparatus having a storage capacity. 2. Description of the prior art: An active matrix system of the type having pixel electrodes arranged in a matrix fashion on an insulating substrate so that the pixel.electrodes are independently driven has been employed in display apparatuses using liquid crystals. Such an active matrix system has often been employed especially in large-sized display apparatuses geared for high-density display. Thin film transistor (TFT) apparatuses, MOS transistor devices, HIM (metal - insulator - metal) devices, diodes, varistors, and the like have been used as switching devices for selectively driving the pixel electrodes. An active matrix drive system affords high contrast display and indeed it has been put into practical use in various areas of application, including liquid crystal television, word processors, and terminal display units for computers. Figure 4 shows a plan view of a conventional active matrix display apparatus, which includes an active matrix board 1 and a counter substrate 2 placed upon the board 1. In this display apparatus, TFTs are used as switching devices. A display medium, such as a liquid crystal, is contained in the space between the active matrix board 1 and the counter substrate 2 to thereby form the display apparatus. Figure 5 schematically illustrates the active matrix board 1 shown in Figure 4. The active matrix board 1 comprises gate bus lines 7, source bus lines 9, intersecting the gate bus lines 7, and storage capacity lines (i.e., addition capacity lines) 8 arranged in a parallel relation to the gate bus lines 7. All the storage capacity lines 8 are connected to a common main line 6 for storage capacities. As shown in Figure 4, in portions of the active matrix board 1 which are not covered by the counter substrate 2 placed on the board 1 there are arranged source signal terminals 3a, 3b, gate signal terminals 4, and common line terminals 5a, 5b connected to the common main line 6. Figure 2 is a schematic diagram showing a rectangular area surrounded by source bus lines 9, a gate bus line 7 and a storage capacity line 8 as are shown in Figure 5. A gate electrode 21 of a TFT 10 is connected to the gate bus line 7, and a gate electrode 22 of the TFT 10 is connected to one of the source bus lines 9. A drain electrode 23 is connected to a pixel electrode 11. A storage capacity 12 is formed between a storage capacity electrode (i.e., an addition capacity electrode) 24 connected to the storage capacity line 8 and the pixel electrode 11. In this display apparatus, when an ON signal is applied to the gate bus line 7, the resistance of the TFT 10 is lowered and a data signal output to the one source bus line 9 is written to the pixel electrode 11. Upon completion of writing of the data, an OFF signal is applied to the gate bus line 7 and the resistance of the TFT 10 becomes higher. The data signal written is held by the storage capacity 12 between the pixel electrode 11 and the storage capacity electrode 24 and also by a pixel capacity between the pixel electrode 11 and a counter electrode (not shown) on the counter substrate 2. The data signal is retained in place until the next writing takes place. Each gate bus line 7, each source bus line 9, and each storage capacity line 8 are made of metal or other conductive materials and respectively have electric resistances R (G), R (S), and R (Cs). These lines 7, 9 and 8 respectively have capacities C (G), C (S), and C (Cs) formed between the individual lines 7, 9 and 8 as one part and other individual intersecting lines and counter electrodes as the other part. Therefore, on the respective lines 7, 9 and 8 there will occur signal delays corresponding to time constants τ (G), τ (S), τ (Cs) represented by products of the respective resistances and the respective capacities. Because of such signal delay, a signal applied to the terminal of each respective line will delay as it advances toward the leading end of the line. The magnitude of such a signal delay depends upon time constants τ (G) and τ (S) on the gate bus line 7 and source bus line 9 respectively, where a signal delay on the storage capacity line 8 depends on the value of τ (Cs) plus τ (Cs ₀) on the common main line 6. Since all storage capacity lines 8 are connected to the common main line 6, the value of τ (Cs ₀) is an enormous one. Therefore, a signal applied to common line terminals 5a and 5b will delay on the common main line 6 and further delay on the storage capacity line 8. In the active matrix board shown in Figure 5, a signal delay on the common main line 6 is greatest at a central part of the line 6 which is remotest from the common line terminals 5a and 5b. A signal delay on the storage capacity line 8 is largest at a portion of the line 8 which is remotest from the common main line 6. In the example shown in Figure 5, therefore, a signal delay is largest at a central portion of the board at the right end thereof. Data signals cannot satisfactorily be written to the pixel electrode 11 connected to the portion of the storage capacity line 8 at which a large signal delay occurs while an ON signal is being applied to the gate bus line 7. Thus, a display irregularity due to the signal delay occurs on the display screen. As the display screen becomes larger, line resistance and line capacity become greater and accordingly the above-mentioned problems will more conspicuously occur. Similarly, as the display screen becomes more refined, a greater number of lines are involved and accordingly such problems will become more conspicuous. For instance, a trial calculation can be made with respect to a liquid crystal display apparatus having a diagonal of the order of 14 inches. Assuming that the material of the common main line 6 is Ti metal (with a specific resistance of 10⁻⁴ Ω cm) and that the line 6 is 4000 Å in thickness, 2 mm in width, and 200 mm in length, the resistance over the entire length of the common main line 6 is about 250 Ω. Since the capacity of the common main line 6 is more than 0.2 µ F, the time constant at the center portion of the common main line 6 at which the signal delay is greatest is more than 12.5 µ sec. In a display apparatus in which 480 bus lines are subjected to non-interlace scanning, the required write time for a data signal is about 30 µ sec. It can be understood from this that the above-mentioned time constant value is unacceptably large. Therefore, the display apparatus is subject to considerable display irregularities. For a further discussion of the prior art see GB-A-2 173 628. SUMMARY OF THE INVENTION The active matrix display apparatus of this invention as defined in claim 1, which overcomes the above-discussed and numerous other disadvantages and deficiencies of the prior art, comprises an active matrix display apparatus comprising pixel electrodes arranged in a matrix fashion on an insulating substrate, storage capacity electrodes arranged opposite to said pixel electrodes, storage capacity lines connected individually to said storage capacity electrodes, a common main line connected to said storage capacity lines, at least one branch line branched from said common main line, and a branch terminal formed at the leading end of said branch line. In one embodiment, the common main line is connected to one end of said storage capacity line. In one embodiment, the common main lines are connected to respective opposite ends of said storage capacity lines. The matrix display of this invention, as defined in claim 4, which is of the type in which a common electrode is coupled via respective conductors to pixels in respective lines of the matrix, the common electrode having terminals adjacent each end thereof, is characterised in that the common electrode has a further terminal connected thereto at an intermediate position thereof. Thus, the invention described herein makes possible the objectives of (1) providing an active matrix display apparatus having storage capacity lines which are less likely to involve a signal delay; and (2) providing an active matrix display apparatus that has a branch line branched from a common main line, and a branch terminal formed at the leading end of the branch line, and according a signal delay if any, can be minimized on the storage capacity lines, whereby the display apparatus of the invention provides a higher level of image quality and is capable of meeting the requirements for larger size construction and a higher degree of sophistication for such a display apparatus. BRIEF DESCRIPTION OF THE DRAWINGS 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: Figure 1 is a schematic diagram showing an active matrix board of a display apparatus of this invention. Figure 2 is a schematic diagram showing an enlarged portion of the active matrix board shown in Figures 1 and 5. Figure 3 is a schematic diagram showing another active matrix board employed in the display apparatus of this invention. Figure 4 is a plan view showing a conventional active matrix display apparatus. Figure 5 is a schematic diagram showing an active matrix board employed in the display apparatus of Figure 4. DESCRIPTION OF THE PREFERRED EMBODIMENTS The active matrix display apparatus of this invention has, in addition to common line terminals at both ends of a common main line, a branch terminal provided at the leading end of a branch line branched from the common main line. The branch terminal functions as a signal input in the same manner as the common line terminals, and accordingly the common main line is divided at the point at which the branch line is branched from the common main line. Therefore, the respective divisional portions of the common main line are reduced in resistance and capacity and thus the problem of signal delay can be effectively solved. For example, one branch line is provided at a median point of the common main line and a branch terminal is provided at the leading end of the branch line, whereby the common main line is equally divided into two parts. Accordingly, the resistance and capacity of the common main line are halved between the respective half portions of the common main line so divided. Therefore, the time constant which represents a signal delay on the common main line is reduced to one quarter thereof. Similarly, where two branch lines are provided at two points at which the common main line is divided into three parts, and branch terminals are provided at the respective leading ends of the branch lines, the time constant on the common main line is reduced to one ninth. By increasing the number of branch lines in this way, it is possible to significantly reduce a possible signal delay. Example 1 Figure 1 is a schematic diagram showing one example of an active matrix board 1 employed in the display apparatus according to the invention. A fragmentary enlarged view of the board in Figure 1 is such as that shown in Figure 2. The active matrix display apparatus in this example comprises pixel electrodes 11 arranged in a matrix fashion on an insulating substrate, storage capacity electrodes 24 arranged opposite to the pixel electrodes 11, storage capacity lines 8 connected to the storage capacity electrodes 24, a common main line 6 connected to the storage capacity lines 8 at one end thereof, a branch line 17 branched from the common main line 6, and a branch terminal 16 formed at the leading end of the branch line 17. Gate bus lines 7 parallel to each other are arranged between the individual pixel electrodes 11, and source bus lines 9 are arranged in an intersecting relation with the gate bus lines 7. The gate bus lines 7 are parallel to the storage capacity lines 8. As shown in Figure 2, a gate electrode 21 of a TFT 10 is connected to one gate bus line 7, and a source electrode 22 of the TFT 10 is connected to one source bus line 9. A drain electrode 23 of the TFT 10 is connected to one pixel electrode 11. A storage capacity 12 is formed between the storage capacity electrode 24 connected to the storage capacity line 8 and the pixel electrode 11. In the present example as shown in Figure 1, source bus lines 9 each having a source signal terminal 3a on one side of the substrate 1 and source bus lines 9 each having a source signal terminal 3b on the other side of the substrate 1, are arranged in an alternate relation. A gate signal terminal 4 is provided at one end of each gate bus line 7. The common main line 6 is provided on that side of the substrate 1 which is opposite to the side on which each gate signal terminal 4 is provided. The common main line 6 is connected to all the storage capacity lines 8. Common line terminals 5a and 5b are provided respectively at opposite ends of the common main line 6. The branch line 17 is branched from a median-point of the common main line 6. The branch terminal 16 is provided at the leading end of the branch line 17. In the active matrix display apparatus of this example, the common line terminals 5a, 5b and the branch terminal 16 are used as signal inputs and, therefore, the common main line 6 is divided into two equal parts at the point at which the branch line 17 is branched from the common main line 6. The resistance and capacity of two equal half portions each of the common main line 6 correspond to one half values of those of a common main line having no such a branch line 17. In the display apparatus of the present example, therefore, the time constant on the common main line 6 is one fourth of that on a common main line having no branch line, a possible signal delay being thus considerably reduced. Example 2 Another example of the active matrix board employed in the display apparatus of this invention is shown in Figure 3. The present example represents a case where the invention is applied to a large-size display apparatus. In this example, the gate bus lines 7 are divided into three blocks of gate bus lines 7a, 7b, 7c. The individual gate bus lines 7a, 7b, 7c each have gate signal terminals 4a and 4b provided at both ends thereof. At both ends of each source bus line 9 there are provided source signal terminals 3a and 3b. In this example, therefore, scan signals are applied from both ends of the individual gate bus lines 7a, 7b, 7c and data signals are applied from both ends of the individual source bus lines 9. In this example, the storage capacity lines 8 are also divided into three blocks of storage capacity lines 8a, 8b, 8c. Common main lines 6a and 6b are connected respectively to opposite ends of each storage capacity line 8. In this example, therefore, signals are input from both ends of each storage capacity line 8. At both ends of the common main lines 6a and 6b there are provided common line terminals 5a and 5b and 5c and 5d. Branch lines 17a and 17b, and 17c and 17d are provided at points at which the respective common main lines 6a and 6b are divided into three equal parts. At ends of the respective branch lines 17a, 17b, 17c, 17d are provided branch terminals 16a, 16b, 16c, 16d. In the active matrix apparatus of the present example, the common line terminals 5a, 5b, 5c, 5d and the branch terminals 16a, 16b, 16c, 16d are all used as signal inputs. The common main line 6a at points at which the branch terminals 16a and 16b is divided into three equal parts, and accordingly the resistance and capacity of each of the three equal parts correspond to one third of the resistance and of the capacity of the entire common main line 6a. In the display apparatus of the present example, the time constant on the common main line 6a is one ninth of that on a common main line having no branch line 17a, 17b. Similarly, the time constant on the common main line 6b is one ninth of that on a common main line having no branch line 17c, 17d. Thus, a possible signal delay can be considerably reduced. 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 of this invention as defined in the appended claims.

Claims: An active matrix display apparatus comprising pixel electrodes (11) arranged in a matrix fashion on an insulating substrate, storage capacity electrodes (24) arranged opposite to said pixel electrodes, storage capacity lines (8) connected individually to said storage capacity electrodes, a common main line (6) connected to said storage capacity lines, characterized by at least one branch line (17) branched from said common main line (6), and a branch terminal (16) formed at the leading end of said branch line. An active matrix display apparatus according to claim 1, wherein said common main line is connected to one end of each said storage capacity line. An active matrix display apparatus according to claim 1, wherein common main lines are connected to respective opposite ends of said storage capacity lines. A matrix display of the type in which a common electrode (6; 6a, 6b) is coupled via respective conductors (8; 8a, 8b, 8c) to pixels in respective lines of the matrix, the common electrode having terminals (5a, 5b; 5a to 5d) adjacent each end thereof, characterised in that the common electrode-has a further terminal (16; 16a to 16d) connected thereto at an intermediate position thereof.

Inventors: IMAYA AKIHIKO, KATAYAMA MIKIO, KATO HIROAKI, NAKAZAWA KIYOSHI, IMAYA, AKIHIKO, KATAYAMA, MIKIO, KATO, HIROAKI, NAKAZAWA, KIYOSHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1990
Main classification: G09G3
IPC codes: G09G3, G02F1