JPH087341B2 - Display device and driving method thereof - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention drives a display material such as a liquid crystal by using an active matrix array substrate having a matrix of switching elements such as thin film transistors, pixel electrodes and capacitors for holding image signals. And a driving method thereof.

2. Description of the Related Art A typical liquid crystal display device will be described below with reference to the drawings.

FIG. 4 is a circuit diagram of a main part of a liquid crystal display device using an active matrix array substrate having an image signal holding capacitor (first capacitor). A large number of pixels (portion surrounded by broken line 1) are repeatedly arranged in a matrix in the display area surrounded by broken line ABCD. Each pixel has a thin film transistor (hereinafter referred to as TFT) 2 as a switching element.
, The gate electrode of the TFT is connected to the scanning signal wiring 7, the source electrode is connected to the image signal wiring 8, and the drain electrode is connected to the image signal holding capacitor 3 (first
Cs) and the pixel electrode 4. The image signal holding capacitor 3 is also connected to the common wiring 9. The liquid crystal 6 is driven by being sandwiched between the counter electrode 5 and the pixel electrode 4. Of the constituent elements of these pixels, the elements other than the counter electrode 5 and the liquid crystal 6 are formed on the active matrix array substrate. The counter electrode 5 is composed of one wide electrode over the entire display portion and is built on the counter substrate (note that the number of wirings is small in the drawing, but in the case of TV display etc., the scanning signal is used). The number of wirings 7 and image signal wirings 8 exceeds several hundreds).

A portion surrounded by a broken line EFGH is a display panel having an active matrix array substrate, a counter substrate and liquid crystal as main constituent elements, and by mounting this on drive circuits 10, 11 and 12, a liquid crystal display device is constituted. FIG. 5 shows an example in which the display panel is mounted on a part of the drive circuit.
Reference numeral 13 is an active matrix array substrate, and 14 is a counter substrate, and liquid crystal is enclosed between these two substrates to form a display panel. Reference numeral 15 is a mounting electrode built on the active matrix array substrate, and using this mounting electrode, the display panel is mounted on the printed circuit board 18 via the flexible substrate 16 on which the driving IC 17 is mounted. 19
Is a connector for connecting to an external circuit.

The liquid crystal display turns on the TFT by sequentially applying scanning signals to the scanning signal wiring 7 in FIG.
The image signal applied to the pixel electrode 4 is transmitted to the pixel electrode 4
This is performed by driving the liquid crystal 6 with a voltage difference between the counter electrode 5 and the counter electrode 5. The image signal holding capacitor 3 (first capacitor) is used for holding signals during the OFF period of the TFT 2 and for applying various modulation signals in liquid crystal driving. Is suitable for high quality image display.

FIG. 6 shows a typical example of drive signals applied to the display panel section of the display device of the type shown in FIG. The figure shows a signal related to one pixel on the screen. Figure 6 (a)
Is called common constant drive, and the potential Vc of the counter electrode 5 is kept constant and the signal including the image signal Vsig
Display is performed by applying Vs to the image signal wiring 8 and applying the scanning signal Vg to the scanning signal wiring 7. It should be noted that the signal Vs includes, in time series, image signals to all pixels connected to the scanning signal line of interest. Although not shown in the figure, the potential of the common wiring 9 is always kept constant. FIG. 6 (b) is called common inversion drive, which is a method of changing the potential Vc of the counter electrode 5 at a constant cycle (normally in one field or one frame for NTSC TV signals). The potential of the wiring 9 is kept at the same potential as Vc. The advantage of the common inversion drive is that the dynamic range of the signal Vs can be reduced, power consumption can be reduced, and low-cost IC with low withstand voltage can be used.

For the above, see, for example, Shoichi Matsumoto; Electronic Display Device (1984) Ohmsha, Inc., and Television Academic Journal Vol. 42, No. 1 (1988).

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention As described above, the display device provided with the image signal holding capacitor is suitable for performing high quality image display such as TV display by the function of the signal holding capacitor. In addition, power consumption can be reduced by performing common inversion driving.

However, when the display device of the type shown in FIG. 4 is driven as shown in FIG. 6, the brightness gradient (same image signal
Even if Vsig is input, the brightness of the screen changes continuously from the top to the bottom of the screen). For example, Fig. 7 shows a liquid crystal display panel of normally black display (liquid crystal display panel for TR-3LT1 manufactured by Matsushita Electric Industrial Co., Ltd .; 89280 pixels, size of 1 pixel 160 µm × 190 µm, capacity of signal holding capacitor per pixel). Cs = 1pF, amorphous Si TFT (W
/L=5.5) is used) shows the experimental results of investigating the relationship between the image signal and the screen brightness when the common driving is performed. The figure shows the difference in brightness depending on the display location, and the same image signal Vsig1
It has been measured that there is a brightness difference (brightness slope) of T1 between the top and bottom of the screen even when is applied. Furthermore, the exact same liquid crystal display panel is driven by common inversion (opposite electrode potential Vc
Fig. 8 shows the results when the amplitude was 5.5v). In this case, the brightness gradient is even larger, and the same image signal Vsig2
On the other hand, the brightness difference of T2 occurs at the top and bottom of the screen. This brightness gradient impairs the sense of uniformity on the screen and is a serious obstacle to high-quality display.

The brightness gradient is a manifestation of a change in the image signal holding characteristic due to a difference in scanning timing due to a difference in location.
Therefore, in principle, it is possible to correct by performing a complicated signal correction (at each location) according to the timing, but not only the circuit becomes complicated, but also the development of a dedicated IC, the addition of a power supply circuit, and the power consumption increase. It leads to an increase and is not a good idea.

The present invention provides a display device having a signal holding capacitor (first capacitor), which has a simple structure and which is driven by a common inversion drive that consumes less power than the conventional one. It is an object of the present invention to improve the uniformity of image quality and realize higher quality image display.

Means for Solving the Problems A second capacitor is provided between the counter electrode and the common wiring, and a signal applied to the common wiring is transmitted to the counter electrode via the second capacitor.

Effect According to the present invention, it is possible to supplement the loss component of the image signal when the potential of the common wiring changes, while performing the drive according to the common inversion with low power consumption by the simple means of installing the second capacitor. As a result, it is possible to realize a uniform image with little brightness inclination. It is very effective in practice because it can be driven by a conventional drive circuit without the need for adding a complicated circuit or power supply circuit or developing an IC.

Specific means for installing the second capacitor are as follows: 1) The capacitor is mounted on the drive circuit (printed circuit board or the like) or directly on the display panel.

2) A capacitor is formed in the counter substrate during the process of forming the counter substrate.

3) Capacitors are built in the active matrix array substrate during the process of making the active matrix array substrate.

4) When assembling the display panel, capacitors are built using both the counter substrate and the active matrix array substrate.

And so on. Each method has its advantages, and which method to use can be selected according to the display performance and manufacturing process to be realized.

Embodiment Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 (a) is an explanatory view of the main part of the liquid crystal display device of the first embodiment of the present invention, and corresponds to the invention described in claim 1. The second capacitor 21 is directly mounted on the display panel 20 similar to the conventional one.

A partial cross-sectional view of the portion where the capacitor 21 is mounted is shown in FIG. 22 is a glass substrate for an active matrix array substrate, 23 is a glass substrate for a counter substrate, 24 is a liquid crystal sandwiched between them, and 25 is a sealing material for sealing the liquid crystal (the alignment film is complicated because the drawing becomes complicated. (Not shown). One end of the second capacitor 21 is connected to the electrode 26 connected to the common wiring, and the other end is connected to the wiring 29 connected to the counter electrode via the electrode 27 and the conductive paste 28.

In this embodiment, the structure of the display unit itself is exactly the same as the one obtained by measuring the conventional results shown in FIGS. 7 and 8, and the second capacitor (10000 pF) is directly mounted on the display panel. Then, this is further mounted in the same common inversion drive circuit as in the conventional case to form a display device. At this time, the conventional signal was applied to the common wiring, and the signal was applied to the counter electrode via the second capacitor. The characteristics in this case are shown in FIG. The characteristics at the top and bottom of the screen were almost the same, the brightness gradient was not visible, the uniformity of the image was increased, and the display performance was improved.

FIG. 3 shows an equivalent circuit centered on the capacitance component for one pixel (the same components as those in FIG. 4 are designated by the same reference numerals, and the description thereof will be omitted). Although not shown in FIG. 4, there are actually many capacitive components,
Based on this, let us consider the principle of the present invention. The capacitance of the first capacitor for signal holding is Cs, the capacitance between the pixel electrode and the counter electrode is Clc, the capacitance between the pixel electrode and the scanning signal line is Cgd, the capacitance between the pixel electrode and the image signal line is Csd, and the counter electrode is The capacitance between the scanning signal wiring and the scanning signal wiring is defined as Ccg, and the capacitance between the counter electrode and the image signal wiring is defined as Ccs. Further, 30 is a component per pixel of the second capacitor, and the capacitance value is C2. Regarding the potential relationship, the scanning signal wiring potential is Vg, the image signal wiring potential is Vs, the pixel electrode potential is Vd, and the counter electrode potential is Vc '.
And At the time of driving, the common wiring 9 and one end 31 of the second capacitor 30 are provided with a signal Vc to the counter electrode at the time of the conventional common inversion driving.
(See FIG. 6) will be applied.

When the common inversion drive is performed in this circuit network and the value of Vc changes by .DELTA.V, the potential (| Vc'-Vd |) applied to the liquid crystal due to the coupling of the capacitance decreases by the value X in the following equation.

X = ΔV [Cs (Ccs + Ccg) -C2 (Cgd + Csd)] / Y However, Y = Clc (Ccs + Ccg + C2 + Cgd + Csd + Cs) + (Ccs + Ccg + C2) ・ (Cgd + Csd + Cs) (In this formula, when C2 = common, the potential loss is 0, common is 0 and common is 0. It represents the increase of the brightness gradient when inverted.) Therefore, Cs (Ccs + Ccg) -C2 (Cgd + Csd) = 0
If C2 is installed, the brightness gradient generated during common inversion drive will be canceled out. Furthermore, if C2 is set to an optimum value, it is considered that the amount of brightness gradient at the time of constant common driving is also canceled. In the system of the present embodiment, the value of X is calculated when the second capacitor is attached with 10000 pF during common inversion drive and when it is not attached (the design value is used as each capacitance value; Cl per pixel).
c = 0.24, Cs = 1.00, Ccs = 0.0019, Ccg = 0.006, Cgd = 0.10, C
sd = 0.001, C2 = 0.11 or 0; unit pF), and the value with C2 is about 1/3 of the value without. Actually, the experimental result (Fig. 2) cannot be explained completely due to the deviation from the drive circuit side capacity estimation and the design value, but by adding the second capacitor as described above, the It can be inferred that the drive signal loss becomes small and the brightness gradient is alleviated.

FIG. 9 is an explanatory diagram of a main part of a liquid crystal display device according to a second embodiment of the present invention, which corresponds to the invention described in claim 3, and a part of the display part (a region over several pixels) is shown. It represents.

When performing liquid crystal display, a light shielding layer (black matrix) may be provided to prevent light leaking from the non-display area between the adjacent pixel electrodes. In this embodiment, a second capacitor is installed by using this black matrix. FIG. 9 (a) shows the shape of the black matrix 32 built in the counter substrate, and FIG. 9 (b) is a sectional view of the display panel at that portion. The counter substrate is formed by forming a black matrix 32 of metal Cr on a glass substrate 33 and forming a counter electrode 34 via a dielectric layer 35. Again 37
Is a glass substrate for active matrix array substrate, 39
Is a pixel electrode and 38 is a liquid crystal (components not directly related to the present invention are omitted in the drawing). In this structure, the capacitor 36 is formed between the counter electrode and the black matrix. In this embodiment, the capacitor 36 is used as the second capacitor, and the display panel having this structure is mounted in the same common inversion drive circuit as in the conventional case to form a display device. Then, when the conventional common inversion drive Vc was applied to the common wiring (not shown) and the black matrix 32 for driving, the characteristics of the upper part and the lower part of the screen were almost the same as in the first embodiment. The uniformity was increased and the display performance was improved. In the structure of this embodiment, since the dye layer or the like of the color filter can be used for the dielectric layer 35, it is possible to carry out the process without increasing the number of manufacturing steps and constituent materials.

FIG. 10 is an explanatory view of the main part of the liquid crystal display device of the third embodiment according to the present invention, which corresponds to the invention described in claim 4, and shows the same parts as in FIG. 1 (b). ing.
40 is a glass substrate for an active matrix array substrate,
41 is a glass substrate for a counter substrate, 48 is a liquid crystal sandwiched between them, and 49 is a sealing material. The electrode 42 and the electrode 44 connected to the common wiring sandwich the dielectric layer 43 to form a second capacitor.
Forming 47. The electrode 44 is connected to the wiring 46 connected to the counter electrode via the conductive paste 45. The display panel having this structure is mounted on the same common inversion drive circuit as the conventional one to form a display device. In this embodiment, as in the first embodiment, when the conventional common inversion driving Vc was applied to the electrode 42 connected to the common wiring for driving, the uniformity of the image was increased as in the first embodiment. Performance improved. In this embodiment, the dielectric layer 43 is formed at the same time as the gate insulating layer of the TFT, and the electrodes 44 and 42 are also formed at the same time when the gate electrode and the source electrode of the TFT are formed respectively. It has the feature that it can be implemented without it.

FIG. 11 is an explanatory diagram of a main part of a liquid crystal display device according to a fourth embodiment of the present invention, which corresponds to the invention described in claim 5, and shows the same part as FIG. 9 (b). ing.
50 is a glass substrate for the active matrix array substrate,
51 is a glass substrate for a counter substrate, and 52 sandwiched between them is a liquid crystal. 53 is a black matrix, 54 is a dye layer for a color filter, and 55 is a counter electrode. 56 is a pixel electrode,
57 is an electrode connected to the common wiring. With this configuration, by assembling the display panel, a capacitor 58 is formed between the counter electrode 55 formed on the counter substrate side and the electrode 57 formed on the active matrix substrate side, and the capacitor 58 is formed on the second substrate.
Used as a capacitor. The display panel having this structure is mounted on the same common inversion drive circuit as the conventional one to form a display device. Then, as in the first embodiment, common wiring and electrodes
When Vc of the conventional common inversion drive was applied to 57 for driving, the image uniformity was increased and the display performance was improved as in the first embodiment. The present embodiment is characterized in that the electrode 57 can be formed at the same time as a TFT light shielding layer (not shown) or the like and can be carried out without increasing the number of steps and constituent materials. Further, conventionally, it was necessary to directly connect the electrode on the counter substrate side to the outside with a conductor by some method (for example, use an electrically conductive paste to establish an electrical connection between the two substrates), but in this embodiment, The potential of the counter substrate side is determined by the action of the second capacitor, and there is no need to directly connect it to the outside with a conductor, and the manufacturing process is simpler than in the past.

EFFECTS OF THE INVENTION According to the present invention, a uniform image with a small luminance gradient can be realized by a very simple implementation means of adding a capacitor and a driving method according to common inversion with low power consumption. It is very effective in practice because it does not require the development of complicated ICs or the addition of correction circuits and power supply circuits, and can be implemented by applying conventional drive circuits.

[Brief description of drawings]

FIG. 1 (a) is a perspective view showing the structure of a liquid crystal display panel in the first embodiment of the present invention, FIG. 1 (b) is an enlarged sectional view showing the main part thereof, and FIG. FIG. 3 is a diagram showing a relationship between an image signal and screen brightness in the embodiment, FIG. 3 is an equivalent circuit diagram of an essential part for explaining the operation of the present invention, and FIGS. 4 and 5 are conventional liquid crystal displays. 6 is a circuit diagram of a main part of the device and a perspective view showing the structure of the main part. FIGS. 6 (a) and 6 (b) are drive signal waveform diagrams of the main part in a conventional liquid crystal display device, and FIGS. FIG. 9 (a) is a diagram showing the relationship between the image signal and the screen brightness when the display is performed by the conventional liquid crystal display device.
And (b) are a plan view and a sectional view, respectively, showing the structure of the main part of the display panel of the second embodiment of the present invention, and FIG. 10 is the main part of the display panel of the third embodiment of the present invention. FIG. 11 is a sectional view showing the structure, and FIG. 11 is a sectional view showing the structure of the main part of the display panel in the fourth embodiment of the present invention. 21,36,47,58,30 …… Second capacitor, 5,34,55 …… Counter electrode, 29,46 …… Wiring connected to counter electrode, 9 …… Common wiring, 26,42,57 …… Electrodes connected to common wiring.

Claims (6)

[Claims] 1. An active matrix array substrate having, for each pixel, a first capacitor for holding an image signal, which is connected to a common wiring, an opposing substrate having an opposing electrode formed therein, and sandwiched between the two types of substrates. In a display device in which a display panel made of the above display material is mounted in a drive circuit, a second capacitor is mounted between the wiring connected to the counter electrode and the wiring connected to the common wiring. 2. The display device according to claim 1, wherein the second capacitor is directly mounted on the display panel. 3. An active matrix array substrate having, for each pixel, a first capacitor for holding an image signal connected to a common wiring, an opposing substrate having an opposing electrode formed therein, and sandwiched between the two types of substrates. In a display device in which a display panel made of the display material is mounted in a drive circuit, the counter electrode is connected to the common wiring via a second capacitor formed on the counter substrate. Display device. 4. An active matrix array substrate having, for each pixel, a first capacitor for holding an image signal, which is connected to a common line, an opposing substrate having an opposing electrode formed therein, and sandwiched between the two types of substrates. In a display device in which a display panel made of a display material is mounted in a drive circuit, the counter electrode is connected to the common wiring via a second capacitor formed on the active matrix substrate. And display device. 5. An active matrix array substrate having, for each pixel, a first capacitor for holding an image signal, which is connected to a common line, an opposing substrate having an opposing electrode formed therein, and sandwiched between the two types of substrates. In a display device in which a display panel made of a display material is mounted in a drive circuit, the counter electrode is connected to the common wiring via a second capacitor formed between the active matrix substrate and the counter substrate. A display device characterized by being. 6. A method of driving a display device according to claim 1, 3, 4 or 5, wherein a signal is applied from a drive circuit to a counter electrode via a second capacitor.