Display apparatus and assembly of its driving circuit

Abstract: A display apparatus and an assembly of a driving circuit for the display apparatus having a display device, a film carrier with a lead to transfer a signal for driving the display device, and a bus board to supply a signal to the lead. A dummy lead is provided along the outside of each of an input side outer lead and an output side outer lead of the film carrier. A predetermined voltage is applied to the dummy leads. The predetermined voltage value is set to a value so as not to stationarily cause a DC bias for the voltage of the outer lead on the outside.

Description: BACKGROUND OF THE INVENTION Field of the Invention The invention relates to a display apparatus which is used for a television receiver, a view finder of a video camera, a light bulb of a video projector, a display of a computer, or the like and, more particularly, to an assembly of a driving circuit of such a display apparatus. Related Background Art As a recent display apparatus, attention is paid to a display apparatus having a display device such as liquid crystal device, plasma light emitting device, electrochromic device, electron emitting device, or the like in place of a CRT. Each of those display devices has a number of pixels and has an electrode to apply an electric signal to decide an optical state of each pixel. Further, in order to apply the electric signal to the electrode of the display device, a rigid wiring board called a bus board or a driver board and made of glass epoxy or the like and a flexible wiring board such as a TAB film or the like are used. Such a structure has been disclosed in U.S. Patents US 5,019,201 or US 5,360,943. A case where a liquid crystal display panel is used as a display device and where a film carrier having a driving IC chip is used as a flexible wiring board will now be described hereinbelow. Hitherto, in case of connecting the film carrier on which a driving circuit is mounted to a display panel and a bus board, there is a drawback such that outer leads locating at both ends among the outer leads on the input side and output side are likely to be damaged by a stress from the outside due to a shock, a vibration, a thermal stress, or the like. To solve such a drawback, therefore, as shown in Fig. 6, dummy leads 8 and 9 which are electrically not related to the driving for an image display are arranged on both outsides of outer leads 6 and 5 on the input side and output side among the leads of the film carrier, thereby preventing damage of the outer leads at both ends due to an external stress. In Fig. 6, reference numeral 1 denotes a display panel; 2 a driving circuit such as an IC chip or the like; 3 a bus board; 4 a film carrier; and 7 a soldering land for the dummy leads. There is a case where quality of an image which is displayed in a display area is improved by always holding the outside (i.e. a frame or border) of the display area to display the image in either a white state (bright state) or a black state (dark state). Such a method is called frame driving by the Applicant but, to distinguish this method from the technique of driving the display apparatus to display a plurality of sequential frames giving the impression of a moving image, the phrase "border driving" will be used hereafter. Fig. 7 shows an example of a display apparatus. Four electrodes 99 for border driving are provided for the display panel 1 and are connected to leads for border driving of film carriers 4 and 4' arranged at both ends of two bus boards 3. Reference numerals 201 and 201' denote scanning line driving circuits of the film carriers 4 and 4'. One of the methods for border driving is a method whereby film carriers and driving circuits of substantially the same construction are used as all of the film carriers 4 and 4' and all of the driving circuits 201 and 201' and the apparatus is driven so as to supply a signal to the electrode 99 for always holding the pixels (border pixels) connected to the border driving electrodes 99 in a white or black state. Another method is a method invented by the present inventors et al. , namely, a method whereby the structure of the film carriers 4' at both ends of the bus board 3 is made different from a structure of the other film carriers 4. Fig. 8 shows a structure of the film carrier 4 which is used in such another method. It is also possible to construct in a manner such that an input side and an output side of dummy leads 8' and 9' are electrically short-circuited and connected in order to supply a signal for, for example, always displaying in white to the frame driving electrodes 99 on the outside of a display area of the display panel 1, the input side being connected to the bus board 3, and the output side is being connected to the terminal of the border driving electrodes of the display apparatus, thereby directly supplying the signal from the bus board 3 to the display apparatus without passing through the driving circuit 201' and driving the border area. In case of the above structure, the same IC chip as that of another driving circuit 201 can be used as a driving circuit 201'. There is no need to give a special construction or special driving control for border driving to the IC chip. In case of a structure shown in Fig. 6, since the dummy lead 9 electrically enters a floating state, it functions as . an antenna and radiation noises are generated. To avoid such a problem, when the dummy leads 8 and 9 are short-circuited to the adjacent outer lead 5, the dummy leads 8 and 9 cannot be used for border driving of the display panel 1. In the case where, for example, the dummy leads 8 and 9 are held to a ground and are fixed to a grounding voltage in order to avoid the electrical floating state, a stationary DC voltage difference occurs between the dummy leads 8 and 9 and the outer leads 5 and 6 which are adjacent thereto. At this time, particularly, in a connecting portion between the display panel 1 and the outer lead 9 on the output side, a failure such as disconnection or the like due to electric corrosion is likely to be caused. Devices of the type described above are disclosed in EP 0601869 and in US 5,285,301. In EP 0601869 the arrangement is used with a TFT (thin film transistor) type display panel, and lines in the position of dummy leads are used for supplying TFT control signals for controlling turning on and turning off of TFTs. In US 5,285,301 some of the dummy leads are connected to an unspecified voltage, whilst others are connected to a ground potential. SUMMARY OF THE INVENTION It is an object of the invention to provide a display apparatus with minimised erroneous operation due to noises. Another object of the invention is to provide a display apparatus having a driving circuit assembly which can be also applied to a display panel which needs border driving. Still another object of the invention is to provide a display apparatus in which disconnection due to electric corrosion is reduced. A further object of the invention is to provide a display apparatus having an assembly of a driving circuit in which a generality is high and costs are low. The invention is made in consideration of the above circumstances and there is provided a driving circuit assembly for a display apparatus, which as known from EP 0601869, comprises: a driver IC provided on a tape substrate for providing drive signals to a display device; a bus board for supplying a voltage signal to the driver IC; a plurality of input leads arranged on said tape substrate and connected between said bus board and said driver IC; a plurality of output leads arranged on said tape substrate and connected to said driver IC for connecting said driver IC to the display device; a dummy input lead connected between said bus board and said tape substrate and positioned between an edge of the tape substrate and the input lead closest to that edge; and a dummy output lead provided between said edge of said tape substrate and the output lead closest to that edge for connection to the display device, wherein said dummy input lead and said dummy output lead are electrically connected and the bus board is arranged to supply a predetermined voltage signal to said dummy output lead. This driving circuit assembly is, in accordance with the present invention, characterised in that: the bus board is arranged to supply, as said predetermined voltage, a voltage having a DC average which is equal to or approximately equal to that of a drive signal supplied to said output edge lead such that the potential difference between the dummy output lead and the output edge lead averaged over one frame period is equal to or approximately equal to zero. According to the invention, by applying a voltage signal to the dummy output lead so as to provide a near zero DC bias between the dummy output lead and the outer lead on the output side, the movement of ions in a connecting portion between the outer lead and the display apparatus can be suppressed and failure such as disconnection or the like due to electric corrosion can be prevented. Since the dummy output lead is not in the electrical floating state, there is no fear of radiation noise from the dummy output lead. On the other hand, since the dummy input and output leads are electrically connected by arbitrarily setting the voltage to be applied to a land for the dummy lead on the input side, a voltage can be also directly applied to the display apparatus from the bus board through the film carrier without passing through the driver IC. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A to 1C are diagrams showing an assembly according to the first embodiment of the invention; Fig. 2 is a diagram showing voltage waveforms according to the first embodiment of the invention; Fig. 3 is a diagram showing voltage waveforms according to the second embodiment of the invention; Fig. 4 is a diagram showing an assembly of the second embodiment of the invention; Fig. 5 is a diagram showing a control system of a display apparatus of the invention; Fig. 6 is a diagram showing a conventional assembly; Fig. 7 is a diagram showing a conventional display apparatus; Fig. 8 is a diagram showing an example of an assembly; and Fig. 9 is a diagram showing a comparison of voltage waveforms of the display apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1A is a plan view showing an assembly of a driving circuit according to a preferred embodiment of the invention. Fig. 1B is a cross sectional view taken along the line 1B-1B in Fig. 1A. Fig. 1C is a cross sectional view taken along the line 1C-1C in Fig. 1A. The structure of Fig. 1A can be used in place of all of the film carriers 201 and 201' in the diagrams. Reference numeral 1 denotes the display panel as a display device. The display panel 1 has a number of pixels. Reference numeral 2 denotes the IC chip as a driving circuit; 3 the bus board; 4 the film carrier; 5 the outer lead on the output side; 6 the outer lead on the input side; 8 the dummy lead on the input side; 9 the dummy lead on the output side; 16 a soldering land as a connecting terminal on the bus board side; 18 a soldering land for the dummy lead; and 15 an electrode of the display panel. The electrodes 15 are connected as scanning electrodes or information electrodes to the pixels. Reference numeral 19 denotes a dummy lead of the display panel. In case of performing the frame driving, the dummy lead 19 functions as an electrode for the frame driving and is extended into the display panel. However, in the other cases, as shown in Fig. 1A the dummy lead 19 is not extended. AD denotes a connecting member such as soldering, anisotropic conductive adhesive agent, or the like. The driving circuit 2 applies a voltage necessary for driving the display panel 1 to the display panel 1 through the outer lead 5 on the output side. Such a voltage supply is performed on the basis of a power source and a signal which are supplied from the group of input side outer leads 6 electrically connected to the bus board 3. The voltage that is applied to the display panel 1 through the output side outer lead 5 in this instance is shown in Fig. 2. In case of using the driving circuit 2 as a circuit for driving the scanning electrodes, a waveform A in Fig. 2 is time-sequentially output to the driving electrodes 15. In case of using the driving circuit 2 as a circuit for driving the information electrodes, a waveform B in Fig. 2 according to an image to be displayed is output to the driving electrodes 15. A reference voltage VC is applied to the dummy electrode 19 of the display panel 1 from the land 18 of the wiring of the bus board without passing through the driving circuit 2. When the scanning side waveform shown as a voltage waveform example A of the outer lead 5 on the output side is seen with respect to only a period within the period of time shown in Fig. 2, although its average voltage is a plus voltage, a duty of the scanning side waveform is small and is equal to or less than 1/400. Therefore, it can be regarded as almost zero DC voltage for the reference voltage VC when it is seen as a voltage in, for example, one frame period. In the information side waveform example shown as a voltage waveform example B of the output side outer lead, an average voltage value in one horizontal scanning period is equal to zero for VC. The dummy lead 9 on the output side is electrically connected to the dummy lead 8 on the input side and the dummy lead on the input side is connected to the land 7 for the dummy lead on the bus board 3 by soldering, so that the voltage of the output side dummy lead 9 can be arbitrarily set. At this time, for example, assuming that the voltage of the output side dummy lead 9 is set to VC as shown in Fig. 2, a voltage difference between the output side dummy lead 9 and the output side outer lead (in case of an example on the information side) on the outermost side that is adjacent to the dummy lead 9 is equal to zero as a DC level at least in one horizontal scanning period. An instantaneous voltage difference is also equal to |V + - VC| at most, the movement of ions can be suppressed to a low level. Thus, leading electrodes of the display panel 1 made of a thin metal (for example, aluminum) film having a thickness within a range from tens of nm to hundreds of nm (hundreds of Å to thousands of Å) can be protected from damage due to electric corrosion. A case where, for example, a voltage waveform shown in Fig. 3 is applied to the output side dummy lead will now be described. That is, it is used for applying a voltage waveform to the electrodes 99 in a border driving area arranged on the outside of a display area of the display panel 1 as shown in Fig. 4 through the dummy lead 9 arranged on the panel outside of the driver IC 201' on the outermost side in the group of driver ICs 2 mounted on, for instance, a TAB film shown in Fig. 7 and for always fixing the border driving portion to a white display or a black display. At this time, an alternating voltage difference whose DC component is equal to zero is caused between the output side dummy lead 9 and the output side outer lead (corresponding to the electrode on the information side) on the outermost side that is adjacent to the dummy lead 9 only for one horizontal scanning period during which the voltage for the border driving is applied. For the other period of time in one frame, even an alternating voltage difference doesn't occur between the dummy lead and the outer lead and the DC component is equal to zero and a DC level is equal to zero. The instantaneous voltage difference is equal to 2|V + - VC| at most. When it is compared with the case where the voltage of the output side dummy lead is fixed to VC, although the voltage difference is larger, its level is almost equal to a voltage difference between the output side outer leads. The output side dummy lead portion is not weak particularly for the electric corrosion. In each of embodiments shown in the diagrams, the voltage which is applied to the dummy electrode of the display panel or the electrode for border driving through the dummy lead has been directly supplied to the display panel from the bus board through the film carrier without passing through the driver IC. However, a form in which such a voltage is applied through the driver IC can be also used. In case of the form of applying the voltage through the driver IC, since the number of terminals of the driver IC increases, the form shown in the diagram is more preferable in an apparatus which preferentially intends to realize low costs. The driver IC is not necessarily the film carrier package IC but may be also a driver IC provided as a COG (chip-on-glass) on the display panel. It is sufficient not to cause a DC component in the voltage difference between the dummy lead and the lead adjacent thereto for a period of time longer than one horizontal scanning period. However, it is preferable that there is no DC component in one horizontal scanning period. It is, therefore, sufficient to decide a voltage to be applied to the dummy lead so as to eliminate the DC component. As the number of dummy leads, it is sufficient that at least one dummy lead is provided on each of the right and left sides of the group of outer leads per one film carrier. More preferably, two to four dummy leads are arranged on each of the left and right sides. As a display panel which is used in the invention, a liquid crystal device, a plasma light emitting device, an electrochromic device, an electron emitting device, or the like can be mentioned. Particularly, the invention is effective for a non-active matrix type device using a chiral smectic liquid crystal or a chiral nematic liquid crystal or an active matrix type device using a TN liquid crystal. An image display apparatus using the liquid crystal devices in the embodiments described above will now be explained. Fig. 5 is a block diagram of a control system of the image display apparatus. Reference numeral 200 denotes a display panel in which a polarization plate is combined to the foregoing liquid crystal device and which has a back light source as necessary. Reference numeral 201 denotes the scanning line driving circuit including a decoder and a switch; 202 an information line driving circuit including a latch circuit, a shift register, a switch, and the like; 203 a voltage generating circuit for generating a number of reference voltages which are supplied to both of the driving circuits 201 and 202; 204 a control circuit which includes a CPU and a RAM to store image information and which outputs signals to be supplied to the outer lead and the dummy lead; and 210 an image signal source such as image sensor for inputting an image, computer which executes an application program, or the like. The invention is applied to an assembly with at least any one of the display panel 200, scanning line driving circuit 201, and information line driving circuit 202. As compared with the case where the output side dummy lead 9 is fixed to the GND level as shown in Fig. 9, a fact that the apparatus of the invention is strong several times for the electric corrosion has been confirmed by the following reliability tests. As for the structure of Fig. 6, a pitch of the output side outer leads (= leading electrode pitch of the display panel) is set to 100 µm and a leading electrode construction is formed by the electrodes in which ITO (1000 Å) and aluminum (3000 Å) are laminated on a soda glass. VC is set to 20 V, V + is set to 26 V, and V - is set to 14 V. Under an environment of 45°C/95 %RH, the output side dummy lead is fixed to the GND voltage and driven as in the conventional apparatus shown in Fig. 9. In this case, a disconnection due to the electric corrosion occurs in the leading electrode portion connected to the output side dummy lead 9 and in the leading electrode portion adjacent to the above leading electrode after the elapse of 168 hours. On the other hand, in the case where the output side dummy lead is fixed to the VC voltage and is driven as shown in Fig. 2, it has been confirmed that no failure due to the electric corrosion occurs even after the elapse of 1000 hours. As described above, according to a particular embodiment, the input side dummy lead and the output side dummy lead of the film carrier on which the driving circuit is mounted are commonly constructed and the voltage of the dummy lead is set to be almost zero for the voltage waveform of the output side outer lead, so that the movement of ions in the connecting portion of the outer lead and the display apparatus can be suppressed. The failure such as a disconnection due to the electric corrosion or the like can be prevented. Since the dummy lead is not in the electrical floating state, there is no fear of radiation noises from the dummy lead. On the other hand, since in the preferred embodiments the input side dummy lead and the output side dummy lead are commonly constructed, by arbitrarily setting the voltage to be applied to the land for the input side dummy lead, the voltage can be also directly applied to the display panel without passing through the film carrier.

Claims: A driving circuit assembly for a display apparatus, comprising: a driver IC (2) provided on a tape substrate (4) for providing drive signals to a display device (1); a bus board (3) for supplying a voltage signal to the drive IC (2); a plurality of input leads (6) arranged on said tape substrate (4) and connected between said bus board (3) and said driver IC (2); a plurality of output leads (5) arranged on said tape substrate (4) and connected to said driver IC (2) for connecting said driver IC to the display device (1); a dummy input lead (8) connected between said bus board (3) and said tape substrate (4) and positioned between an edge of the tape substrate and the input lead (6) closest to that edge; and a dummy output lead (9) provided between said edge of said tape substrate (4) and the output lead (5) closest to that edge for connection to the display device (1), wherein said dummy input lead (8) and said dummy output lead (9) are electrically connected and the bus board (3) is arranged to supply a predetermined voltage signal to said dummy output lead (9); characterised in that : the bus board (3) is arranged to supply, as said predetermined voltage, a voltage having a DC average which is equal to or approximately equal to that of a drive signal supplied to said output edge lead (5) such that the potential difference between the dummy output lead (9) and the output edge lead (5) averaged over one frame period is equal to or approximately equal to zero. A display apparatus comprising: a driving circuit according to claim 1; a display device (1) comprising a group of electrodes (15) for supplying signals to pixels, said electrodes being electrically connected to respective ones of said output leads; and wherein said bus board (3) comprises a group of electrodes (16), said bus board electrodes being electrically connected to respective ones of said input leads. An apparatus according to claim 2, wherein said predetermined voltage is a centre voltage (V c ) and said apparatus is arranged to apply through said output edge lead (5) a waveform as one of the drive signals to said display device electrodes (15), said waveform comprising voltages (V + , V - ) greater than and less than said centre voltage by a predetermined magnitude. An apparatus according to claim 3, wherein said centre voltage (V c ) is a scanning non-selection voltage and said applied voltages (V + , V - ) are scanning selection voltages. An apparatus according to claim 2, wherein said predetermined voltage supplied to said dummy output lead is a fixed voltage. An apparatus according to any of claims 2 to 5, wherein said bus board which is arranged to supply said predetermined voltage, is arranged to supply a first signal to said dummy output lead (9) for one horizontal scanning period and to further supply a second signal to said dummy output lead (9) for a time period equal to the difference between one frame period and one horizontal scanning period. An apparatus according to any of claims 2 to 6, wherein said bus board is arranged to apply a voltage signal for border driving to said dummy lead, said voltage signal for border driving being a signal for driving a border area arranged on the outside of the display area of the display device. An apparatus according to any of claims 2 to 7, wherein said dummy output lead (9) is electrically connected to a dummy lead (19) of said display device (1). An apparatus according to claim 8, wherein said dummy output lead (9) and said display device (1) dummy lead (19) are connected by an anisotropic conductive adhesive agent. An apparatus according to claim 8 or 9, wherein said display panel dummy lead (19) is connected to an electrode (99) in a border driving area arranged on the outside of a display area of the display device for border driving. An apparatus according to any of claims 2 to 10, wherein said display device (1) is any one of a liquid crystal device, a plasma device, an electrochromic device, and an electron emitting device. An apparatus according to any of claims 8 to 11, wherein said bus board is further arranged to supply said predetermined voltage to said dummy leads (9; 19) for at least one frame period. An apparatus according to claim 2, wherein said dummy input lead (8) and said dummy output lead (9) are directly short-circuited. An apparatus according to claim 2, wherein said dummy input lead (8) and said dummy output lead (9) are electrically not related to the driving of an image display, and are connected via the driver IC (2). An apparatus according to claim 2, wherein each drive signal comprises a scanning selection voltage and scanning non-selection voltage, and said predetermined voltage is the scanning non-selection voltage.

Inventors: MORI HIDEO, NIIBORI KENJI, OHSAWA SEIJI, TAKAHASHI MASANORI, YOKOMIZO HIROYUKI, MORI, HIDEO, NIIBORI, KENJI, OHSAWA, SEIJI, TAKAHASHI, MASANORI, YOKOMIZO, HIROYUKI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1996
Main classification: G09G3
IPC codes: G02F1, H05K1, H05K3, G09G3