JPH0766245B2 - Liquid crystal display - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a display device, particularly a matrix type display device.

2. Description of the Related Art An active matrix type liquid crystal display device in which a display unit is provided at an intersection of a column electrode bus bar and a row electrode bus bar is actively researched because it can be flattened. FIG. 4 is a principle diagram of the active matrix. As shown in FIG. 4, an electrode bus bar (hereinafter, referred to as “X selection line”) 1 that transmits a display signal and an electrode bus line (hereinafter, referred to as “Y selection line”) 2 that transmits a scanning signal are intersected with each other. The display unit 3 is formed. Using the horizontal driver circuit 4 and the vertical driver circuit 5, the m-th Y selection line 2 and the n-th X selection line 1 are used to identify the mn addresses (showing m rows and n columns) indicated by diagonal lines in the drawing. The address of the display unit 3a is designated.

FIG. 5 is a circuit diagram of the display unit 3, 6 is a switching element composed of, for example, an insulated gate field effect transistor (hereinafter referred to as “MOS • FET”), 7 is a liquid crystal, and 8 is a storage capacitor. When a scanning signal for turning on the switching element 6 is input from the Y selection line 2, the display signal transmitted to the X selection line 1 is supplied to the liquid crystal 7. When the switching element 6 is formed of a thin film transistor having a slow response, a method is used in which a display signal for one row is temporarily stored in a memory and a display signal for one row is simultaneously transmitted within a scanning signal for one row. There is. This is called a line sequential method.

Problems to be Solved by the Invention Even when the display signal is transmitted to the display unit 3 by the line-sequential method, when displaying a television signal, the time for one line is required.
1H is 60 μsec, and the display signal must be given to the liquid crystal 7 within this time.

In order to explain such a tendency, the liquid crystal voltage when a scanning signal voltage and a display signal voltage as shown in FIG. 6 are applied will be considered. That is, as shown in FIG. 7B, the display signal 12V is applied at the first 1H (60 μsec) and the next 1H (60 μse).
The display signal 0V is given in c), and the scanning signal 18V is given during this 2H as shown in FIG. Corresponding liquid crystal 7
The voltage response is as shown in FIG. 6 (C). However, the parameter in FIG. 6C is the ratio W / L of the channel width W and the channel length L, and it can be seen that the rising and falling times become shorter as W / L becomes larger.

Therefore, when thin film transistors having the same carrier mobility μ (hereinafter referred to as “TFT”) are used, it can be understood that a structure having a large W / L may be used. However, since L is determined by the minimum line width of pattern accuracy, W is increased. However, when W is increased, the area of the TFT is increased, the proportion of the TFT occupied in the display unit area is increased, the effective display area of the liquid crystal television receiver is decreased, and the light transmittance is decreased.

Also, as W increases, the parasitic capacitance 9 due to the overlap of the gate drain increases. This parasitic capacitance 9
Becomes larger, the liquid crystal voltage changes in proportion to the change ΔV ₆ of the gate voltage. The so-called pull-out effect increases. This overshoot voltage is It is represented by. Where ΔV ₆ is the change in gate voltage, C ⁶⁰
Is the gate-drain parasitic capacitance, C _add is the auxiliary capacitance, C _LC
Is the liquid crystal capacitance.

From the above two points, there is a limit in increasing V, and there is a problem that the usable range is limited in the case of a TFT having low mobility.

A more specific description will be given. With the overall configuration as shown in Fig. 4,
A liquid crystal display panel having the display unit 3 shown in FIG. 5 was formed on a Corning 7059 glass substrate. Switching element 6
Is a TFT continuously formed by a semiconductor made of 1000 Å a-Si (amorphous silicon) and an insulator made of 4000 Å SiN. The capacitance of the liquid crystal 7 is 0.23 pF, and the auxiliary capacitance 8 is 1.47 pF. The TFT that constitutes the switching element 6 has an electron mobility of 0.5 cm ₂ / V · sec, a channel width W of 80 μm, and a channel length L of 8 μm.

In order to transmit a display signal voltage of 12V or 0V to this liquid crystal display panel, conventionally, a gate voltage (that is, a scanning signal voltage) as shown in FIG. 7A and a 12V voltage as shown in FIG.
And 0 V signal voltage for 60 μsec each for 1 frame time (1
6.7 msec) is given at intervals.

By doing so, even if the liquid crystal voltage immediately before the surging effect by turning off the gate voltage is 11.7 as shown in FIG. 7 (C).
6V, which does not reach the desired signal voltage of 12V. On the other hand, when the signal voltage is 0V, the liquid crystal voltage quickly reaches the predetermined 0V. Such a result shows that, for example, as seen in FIG. 6C, it takes a long time to reach the applied voltage during the period for charging the liquid crystal which is the capacitive component,
It is also clear from the tendency that the time to reach the discharge potential during the discharging period is very short. This comes from the basic characteristics of MOS-FET (gate-drain voltage-drain current characteristics) and is an essential problem.

Means for Solving the Problems In order to solve the above problems, the liquid crystal display device of the present invention is
A first electrode busbar group for transmitting a display signal, a second electrode busbar group for transmitting a scanning signal, and an intersection of the first electrode busbar group and the second electrode busbar group are arranged in correspondence with each other. A display pixel electrode, and a gate insulating transistor is arranged for each display pixel electrode, and the first electrode bus bar is provided at the source (or drain) of the gate insulating transistor.
The drain (or source) is connected to the display pixel electrode, and the gate is connected to the second electrode bus. The gate insulation transistor is turned on by the scan signal, and the display is performed from the first electrode bus. An active matrix type liquid crystal display device for transmitting a signal to the display pixel electrode, wherein the first electrode bus bar group first has a reserve larger than an average value of absolute values of the display signal during a conduction period of the gate insulation type transistor. A signal supply means for applying a desired display signal after applying the signal is provided.

If the working switch is composed of MOS FETs, the falling edge shows a much faster response than the rising edge. In other words, the characteristics of the switching element do not match at the rising edge and the falling edge. Therefore,
In order to reach the vicinity of the rising display signal voltage (predetermined voltage), it can be achieved by setting the rising voltage application time longer than the falling voltage application time.

However, the voltage application time is the maximum for television signals.
It is 60 μsec, and if it does not reach within this period, the voltage difference will appear in the image as it is. Therefore, in the present invention, in order to improve the rising, first, a preliminary signal larger than the absolute value of the average value of the display signal voltage in the frame period is given for a predetermined time, and a predetermined display signal is given in the remaining period, so that The predetermined display signal value is reached more quickly than if the display signal were applied throughout.

Embodiment An embodiment of the present invention will be described below with reference to FIGS.

First, the principle of the present invention will be described with reference to FIG.
In this embodiment, as shown in FIG. 1 (B), a constant preliminary signal (a) having a value larger than the constant signal voltage, regardless of the signal voltage, was given for a constant time. Specifically, the same figure (A)
Gate voltage application time of 18V gate voltage as shown in 60
Of the μsec, 12V as the display signal voltage (b) and 0V as (c) were applied for 30μsec. In addition, the preliminary signal (a)
As a result, 14 V and 30 μsec each were applied prior to each display signal. The period between the preliminary signals is one frame period.
Repeated every 16.7 msec.

The time characteristic of the voltage actually applied to the liquid crystal when a signal voltage having such a preliminary signal component is applied is shown in FIG.

As is clear from comparison between FIG. 7C and FIG. 7C, the actual voltage applied to the liquid crystal has a predetermined value by giving a preliminary signal larger than the signal voltage which is the gist of the present invention. It can be seen that the voltage reaches (12V) for a period of 60 μsec.

Specifically, in the case of this embodiment, that is, the backup signal 14V, 30
If μsec is given, the application time of the display signal of 12V is 22μ
sec is sufficient, and the liquid crystal voltage reaches a predetermined liquid crystal display signal voltage of 12 V in a total of 52 μsec, and there is a margin of 8 μsec.

That is, a desired signal voltage can be applied as the liquid crystal voltage without changing the size of the TFT. In other words, even if the TFTs have the same characteristics, the desired display signal can be transmitted at a higher speed by giving the preliminary signal (a) in advance. From these facts, the same display signal can be transmitted at the same time. It is possible to reduce the W of the TFT formed of a material having the characteristics, improve the display characteristics of the liquid crystal display device, and reduce the shadowing effect.

Next, as another embodiment of the present invention, the behavior of the open / close switch TFT provided between the liquid crystal display panel and the signal bus bar will be described.

That is, the basic configuration of the open / close switch TFT of this embodiment is
The number of connections between the liquid crystal display panel and the signal busbars of the external circuit of the liquid crystal display panel is reduced to 1 / N, and accordingly, the transmission timing pulse is generated so that the video input becomes an image signal grouped by the degree of overlap N. And control the opening and closing by the control line.

FIG. 2 is a circuit diagram of a liquid crystal display device in the case where N is set to 3 as one embodiment of the above-mentioned embodiment. The TFT drive liquid crystal display panel 11 is mainly composed of amorphous silicon and silicon nitride by a normal plasma CVD method. It is composed of TFT as an ingredient. However,
In this liquid crystal display panel 11, an open / close switch TFT 14 is simultaneously formed at the end of the signal side bus bar 13 in addition to the display element TFT 12.

The video input to the signal sampling circuit 15 which constitutes the horizontal driver is sampled and enters the n memories 16 corresponding to 1H of picture elements. The transmission timing pulses P, Q, R obtained by dividing 1H into three and the transmission gate lines Ta, Tb, Tc can be arbitrarily combined by the transmission timing switcher 17. Each of the image signals collected for each degree of overlap 3 is
Output amplifier 18 in time serial for every time 1H is divided into 3
Sent from.

That is, in the embodiment shown in FIG. 2, the number of connections between the signal line panel and the external circuit of the panel is reduced to 1/3.

On the other hand, on the array side of the TFT 12, an open / close switch TFT14 is connected to one end of each of the signal side busbars 13. These are control lines Ga, Gb, Gc connected to the inputs P, Q, R to the acceptance timing switcher 19 and the S signal corresponding to the period of the preliminary signal.
The opening and closing is controlled by.

Note that when switching between sending and receiving timing, Ta-Ga, T
b-Gb and Tc-Gc are synchronized with the timing of P, Q, and R signals. Reference numeral 20 is a liquid crystal, 21 is a vertical driver circuit, 22 is a scanning side bus bar, and 23 is an auxiliary capacitor. With this configuration, the number of connections with external circuits can be reduced, but the switch period is 60/3 (=
20) Since it is less than μsec, the operational margin of the TFT 14 decreases with the number of N.

Specifically, the gate signals (VG1, VG2, VG3) applied to the display signals VSIG, TFT14, the source (or drain) of the TFT14.
Voltage (A ₁ , B ₁ , C ₁ ) and gate signal V applied to the TFT12
The GL applies a voltage according to the timing chart shown in FIG.

The preliminary signal (a) is output from the signal sampling circuit 15, and at this time, all the opening / closing switches TFT14 are turned on by the reception timing switcher 19.

Here, the capacitance of the liquid crystal 20 in the screen is 0.04 pF, and the auxiliary capacitance 23 is
0.31pF, TFT12 W / L = 24μm / 8μm, open / close switch T
When the channel length of FT14 is fixed at 8 μm and the channel width is changed, 60 of the liquid crystal 20 in the screen through the signal side bus bar 13 is obtained.
The voltage after μsec is shown in the table.

However, the preliminary signal voltage application time was 12 μsec, the display signal voltage value was kept constant with 12 V, and the preliminary signal voltage value and the display signal voltage application time were changed. The voltage values in the table are
Since it is the value after 60 μsec, it is after the voltage drops due to the sticking effect.

Further, in the conventional configuration similar to this embodiment, for example, the circuit diagram of the liquid crystal display device shown in FIG. 8 is different from the circuit diagram of the liquid crystal display device of the present invention shown in FIG. There is no spare signal output from the circuit 15 and there is no S signal corresponding to it, and the timing chart of the transmission gate line and control line is different, and the other operational relations of FIG. It is similar to FIG.

That is, the display signals VSIG, TFT of the present invention shown in FIG.
Gate signal applied to the 14 (VG1, VG2, VG3) , the voltage of the source of the TFT 14 (or _{drain) (A 1, B 1,} C 1) and TFT12
FIG. 9 shows a timing chart of each voltage in the conventional configuration for comparison with each timing chart of the gate signal VGL applied to the.

As is clear from the table, when the preliminary signal voltage value is equal to or lower than the display signal voltage value, the source (or drain) voltage of the TFT14 is low even when the display signal voltage is applied for a long time. Need to be higher than.

In addition, the display signal voltage is applied after applying the 14 V preliminary signal for 12 μsec.
It can be seen that a higher source (or drain) voltage of the TFT 14 can be obtained by comparing the configuration of the present invention that provides 12 μsec with the conventional configuration that provides only the display signal for 20 μsec.

In the configuration of the present embodiment, the total application time of the preliminary signal voltage 14V and the display signal voltage 12V is 24μsec, considering the longer than the conventional configuration of applying the display signal voltage 12V 20μsec, the preliminary signal voltage 12V Compared with the example in which the display signal voltage 12V is applied for 12 μsec after the application for 12 μsec (total application time 24 μsec), since the source (or drain) voltage of the TFT 14 is higher in this embodiment, the display signal voltage of the present invention is higher. The effect of applying the preliminary signal higher than that is remarkable.

That is, the timing charts of A ₁ , B ₁ and C ₁ in FIGS. 2 and 8 draw loci as shown in FIGS. 3 and 9, respectively.

However, the essence of the present invention is to make the source signal or drain signal of the TFT reach a predetermined value quickly, and the applied voltage of the preliminary signal can be arbitrarily selected at least in a range higher than the display signal voltage, and the applied time Of course, it can be arbitrarily selected.

Further, in the above-described embodiment, the case where one kind of rectangular wave is used as the display signal has been described, but the actual display signal has a large number of levels and is basically alternating current, and the essence of the present invention is the display signal voltage. The purpose is to provide a preliminary signal voltage higher than the average of absolute values.

As described above, according to the present invention, in the liquid crystal device using the MOS FET as the switching element, the preliminary signal larger than the average value of the absolute values of the display signals is applied, so that the liquid crystal display signal voltage It is possible to shorten the startup time. As a result, the liquid crystal display voltage can be sufficiently charged up to a predetermined signal voltage, and a good image without unevenness can be realized. Further, if this improvement in performance is directed toward the reduction of the area of the transistor, the transistor of the display unit in the screen can improve the aperture ratio of the panel, that is, the brightness can be increased. Further, if applied to the transistor of the open / close switch, the panel size can be reduced. Further, in both cases, the area of the transistor is reduced, so that it is possible to reduce the influence of the parasitic effect due to parasitic capacitance.

[Brief description of drawings]

FIG. 1 is a diagram for explaining the principle of the display device of the present invention, FIG. 2 is a circuit diagram of the display device in one embodiment of the present invention, and FIG.
One timing chart of the circuit shown in FIG. 4, FIG. 4 is a configuration diagram of a general matrix type display device, FIG. 5 is a circuit diagram of a display unit of the same matrix type display device, and FIG. 6 is a conventional line sequential system. FIG. 7 is a signal waveform diagram of each part of the display device, FIG. 7 is a diagram explaining the principle of the conventional display device, FIG. 8 is a circuit diagram of the conventional display device, and FIG. 9 is a timing chart of the circuit shown in FIG. 11 …… TFT drive LCD panel, 12 …… TFT, 13 …… Signal side busbar, 14 …… Open / close switch TFT, 15 …… Signal sampling circuit, 20 …… Liquid crystal, 22 …… Scanning side busbar, 23 …… Auxiliary capacity.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Seiichi Nagata 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) Reference JP-A-59-210420 (JP, A) JP-A-59- 57217 (JP, A) JP-A-52-115197 (JP, A) Actually developed 59-111332 (JP, U)

Claims (1)

[Claims] 1. A first electrode bus bar group for transmitting a display signal,
A second electrode busbar group for transmitting a scanning signal, display pixel electrodes arranged corresponding to respective intersections of the first electrode busbar group and the second electrode busbar group, and each of the display pixel electrodes A gate-insulated transistor is disposed, the source (or drain) of the gate-insulated transistor is the first electrode bus, the drain (or source) is the display pixel electrode, and the gate is the second electrode. An active matrix type liquid crystal display device, wherein bus lines are connected to each other, the gate insulating transistor is made conductive by the scanning signal, and the display signal is transmitted to the display pixel electrode from the first electrode bus line, During the conduction period of the insulation type transistor, first, a preliminary signal larger than the average value of the absolute values of the display signals is given to the first electrode bus bar group, and then the desired display signal is given. The liquid crystal display device that includes a signal supply means for providing a.