JPH1020280A - Driving method of liquid crystal display element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a driving method for driving a memory type liquid crystal at high speed using a cholesteric/nematic phase transition type liquid crystal and a cholesteric/high-molecular composite type liquid crystal. SOLUTION: Plural scanning electrodes G1 -Gl and plural signal electrodes S1 -Sk are formed in a matrix manner and a liquid crystal showing the memory property is interposed between active matrix type liquid crystal display elements prepared by using a substrate provided with switching elements at the respective intersections of the electrodes G1 -Gl . The liquid crystal having the memory property is not a surface stabilized ferroelectric liquid crystal and the switching elements provided in the respective liquid crystal display elements are made to be turned on three times for writing displaying information once.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for driving a liquid crystal display device. More specifically, the present invention relates to a method for driving a liquid crystal display element formed by combining a liquid crystal having a memory property with an active matrix substrate.

[0002]

2. Description of the Related Art At present, liquid crystal display elements are used in a wide range of fields, such as clocks, calculators, OA equipment such as word processors and personal computers, and pocket televisions. As a recent trend, a high-quality liquid crystal display device capable of large-capacity display has been demanded. It is generally accepted that a high-capacity display with the highest display quality can be obtained. Twisted nematic (hereinafter abbreviated as TN) liquid crystal is applied to an active matrix substrate having thin film transistors (hereinafter abbreviated as TFT) arranged on a matrix. It is a liquid crystal display element combined.

A conventional example in which a liquid crystal having no memory property is actively driven will be described with reference to FIGS. FIG. 3 is an equivalent circuit diagram of a liquid crystal display element. In each display pixel, a liquid crystal element (denoted by LC in the figure) and a TFT are arranged. Next, the operation will be described. FIG. 4 is a diagram showing a display state of the liquid crystal display element. In this figure, (G
1, (S1) will be described as an example.

First, a voltage V ₁ is applied to the gate electrode G ₁ for a period of t ₀ to turn on the TFT. T within this period of t ₀
Since the FT is ON, the voltage applied to G1 becomes zero and the TFT turns OFF at the same time. And
After a period of t ₁ to again send a period signal of t _0, ON the TFT
To

A signal corresponding to the display state of the pixel to be displayed is transmitted from the signal electrode in synchronization with the period during which t ₀ is applied. Since the voltage of V ₂ is applied to the period from S1, will be a voltage of V ₂ is applied to the pixel (G1, S1), even after the voltage applied to the gate electrode is zero, the pixel (G1, S1) to continue consuming V _2. After a period of t _1, when the V ₁ is applied again to the gate electrode, the voltage of -V ₂ is applied from the signal electrodes S ₁ correspondingly. Therefore, the pixel (G1, S1) would -V ₂ is applied.

As described above, the pixel (G1, S1) has t ₁
The will be V ₂ and -V ₂ are alternately applied to as a cycle, because this voltage is applied, the liquid crystal is turned ON.

On the other hand, regarding (G1, S2), G
During the period when the TFT 1 is ON, 0V is applied from S2.
Is applied to the pixel (G1, S2).
V will continue to be applied. As a result, the liquid crystal becomes O
It becomes FF.

However, a normal TFT-TN type liquid crystal display element is a transmissive type having a backlight behind, and has a problem that power consumption increases due to the backlight. Therefore, recently, research on a reflective liquid crystal display that does not use a backlight has been actively conducted.

However, in the reflection type liquid crystal display using the above-mentioned TFT-TN type liquid crystal display element, a voltage must be continuously applied during the display, so that the power consumption is sufficiently reduced. Difficult to do.

One means for solving the problem is a method in which no voltage is applied when the display is not rewritten. For this purpose, the liquid crystal itself needs to have a memory property. Examples of the liquid crystal having a memory property include a ferroelectric liquid crystal, a bistable nematic liquid crystal, a cholesteric / nematic phase transition type liquid crystal, and a cholesteric / polymer composite liquid crystal.

Of these, ferroelectric liquid crystal and bistable nematic liquid crystal usually perform display by combining a polarizing plate. Since the transmittance of the polarizing plate is 50% or less, bright display cannot be obtained with these liquid crystals, and it cannot be said that the liquid crystal is preferable as a reflection type. On the other hand, the cholesteric / nematic phase transition type liquid crystal and the cholesteric / polymer composite type liquid crystal can perform display without using a polarizing plate and are preferably used as a reflection type.

Here, a conventional example in which a liquid crystal having a memory property is driven by a simple matrix will be described with reference to FIGS.

In FIG. 6, electrodes Row1 to Row4 on the scanning side are shown.
An example of driving of a simple matrix type liquid crystal display in which the electrodes Col1 and Col2 on the signal side are combined in an XY matrix form is shown. In the following description, a specific voltage value will be described, but this is merely a typical example. As shown in FIG. 7, a voltage (50 V) having a frequency of 2 kHz is applied for 60 msec.
After applying the voltage for 9 msec, 9 V and-
Apply a voltage of 9V.

Thus, the scanning electrode is selected during this period. A voltage waveform as shown in FIG. 8 is applied to the pixels P11 and P21 by a signal from the signal electrode corresponding to this period. In the case of P11, the voltage applied during the selected period is ± 13 V, and the liquid crystal is turned on. On the other hand, at P21, the voltage applied during the selected period is ± 5 V, and thus the liquid crystal is turned off. In the case of a liquid crystal having a memory property, the display content written here is kept as it is, so that P11 is ON and P21 is O
The display state of the FF is maintained.

[0015]

When a display is performed using these memory type liquid crystals, the time t ₄ required to display one screen is t ₄ = t ₅ · m in consideration of simple matrix driving. expressed. Here, t ₅ is required to write a one scan line time (line address time), m is the number of scanning lines.

A ferroelectric liquid crystal can respond at high speed, and t
₅ can be set to 50 μsec or less. if,
When m = 1000 displays are considered, the time t ₄ required to display one screen is t ₄ = t ₅ · m = 50 μsec · 1000 = 50 msec, and there is not much problem in display.

However, the response speed of a cholesteric / nematic phase transition type liquid crystal or a cholesteric / polymer composite type liquid crystal is much slower, for example, about 20 msec.
If m = 1000 displays are considered, the time t ₄ required to display one screen is t ₄ = t ₅ · m = 20 msec · 1000 = 20 sec, and the display speed is slow as a display, which is problematic. .

The present invention has been made under such circumstances, and provides a driving method for driving a memory type liquid crystal using a cholesteric / nematic phase transition type liquid crystal or a cholesteric / polymer composite type liquid crystal at a high speed. Things.

[0019]

According to the present invention, there is provided an active matrix type electrode formed by using a substrate on which a plurality of scanning electrodes and a plurality of signal electrodes are formed in a matrix and a switching element is provided at each intersection of the electrodes. A liquid crystal having a memory property is sandwiched between liquid crystal display elements, and a liquid crystal having a memory property is not a surface-stabilized ferroelectric liquid crystal.

As a first process of the driving method, a signal is sent from the scanning electrode to turn on the switching element, and in synchronization with this, a positive, negative or zero voltage corresponding to a desired display is applied from the signal electrode. feeding, as a second process, after a predetermined time t _1, the switching element in the oN state sends a signal again from the scanning electrode, in synchronization with this, sends the voltage signal electrodes corresponding to the display for determining, in the second The polarity of the voltage sent from the signal electrode during processing is the first
In the processing of (3), the polarity is opposite to that of the voltage sent from the signal electrode and the absolute value is the same.
₍₁₎ After that, a driving method is provided in which a signal is sent again from the scanning electrode to turn on the switching element, and in synchronization with this, a signal is applied from the signal electrode so that the voltage applied to the liquid crystal becomes zero. You.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to one scan electrode G ₁ , G ₂ ,. . . , G _n−1 , G _n , G _{n + 1} ,
G _{n + 2,.} . . , G _l−1 , G _l and k signal electrodes S ₁ ,
S ₂ ,. . . , S _m , S _{m + 1,.} . . , S _k−1 , and S _k are formed in a matrix, and a liquid crystal display element as shown in FIG. 1 will be described in which a memory liquid crystal is combined with an active matrix substrate in which thin film transistors (TFTs) are arranged at respective intersections.

The gate electrode of the TFT at each intersection is connected to the scanning electrode, and the source electrode is connected to the signal electrode.
P _1/1 , P _{1/2 ,.} . . P _{1 / m} , P _{1 / m + 1,.} . . P _{n / 1} ,
P _{n / 2,.} . . P _{n / m} , P _{n / m + 1,.} . . And the like indicate pixels connected to the drain electrode of the TFT formed at each intersection.

Here, the case where the display of the pixel connected to G _n is performed will be considered. FIG. 2 shows the scanning electrode _Gn and the signal electrode.
The signal waveform shown in FIG. At this time, the voltage waveform applied to the pixel is as shown in FIG.

In this figure, a signal G _n indicates a voltage waveform applied to the n-th scanning electrode. This signal G _n is
a pulse voltage of period t ₀ is applied, repeat this in a cycle of period t _1.

The signal S _m illustrates the voltage waveform applied to the signal electrode of the m-th. The signal S _m applies a voltage corresponding to the state to be displayed in pixels.

The signal S _{m + 1} indicates a voltage waveform applied to the (m + 1) th signal electrode. This signal S _{m + 1} applies a voltage according to the state of the pixel to be displayed.

The signal S _{m + 2} indicates a voltage waveform applied to the (m + 2) th signal electrode. The signal _{Sm + 2} applies a voltage according to the state of the pixel to be displayed.

The signal P _{n / m} is supplied to a thin film transistor (TF) located at the intersection of the n-th scanning electrode and the m-th signal electrode.
3 shows a voltage waveform applied to the pixel electrode via T). The signal P _{n / m} is the period in which the voltage to the scan electrode G _n is applied, the voltage applied to the signal electrode S _m is the period of t _1, it will be applied to the pixel electrode.

The signal P _{n / m + 1} is the signal of the _n-th scanning electrode
5 shows a voltage waveform applied to a pixel electrode via a thin film transistor (TFT) located at the intersection of a + 1st signal electrode. The signal P _{n / m + 1} is obtained by applying the voltage applied to the signal electrode S _{m + 1} during the period when the voltage is applied to the scan electrode G _n to the pixel electrode during the period t _1. become.

The signal P _{n / m + 2} is output from the n-th scanning electrode
The voltage waveform applied to the pixel electrode via the thin film transistor (TFT) located at the intersection of the +2 signal electrode is shown. This signal P _{n / m + 2} is that the voltage applied to the signal electrode S _{m + 2} during the period when the voltage is applied to the scan electrode G _n is applied to the pixel electrode during the period t _1. become. In this case, during the period when the voltage is applied to the scan electrode _Gn ,
Since the voltage applied to the signal electrode S _{m + 2} is zero, no voltage is applied to the pixel electrode.

Firstly, as a first processing, time t _0, to turn on the TFT sends a signal from the scanning electrode G _n. In synchronization with this, a zero or positive voltage corresponding to the display state of the connected pixel is applied from the signal electrode.

Next, as a second process, after a certain time t ₁ ,
A signal is sent again from the scanning electrode G _{n for} a time t ₀ to turn on the TFT. In synchronization with this, a zero or negative voltage according to the display state of the connected pixel is applied from the signal electrode.

Further, as a third process, after a certain time t ₁ , a signal is sent again from the scan electrode G _n to the time t ₀ to turn on the switching element, and in synchronization with this, the voltage applied to the liquid crystal is changed. Is sent from the signal electrode.

In this case, the polarity of the voltage sent from the signal electrode in the second processing needs to be opposite in polarity to the voltage sent from the signal electrode in the first processing and have the same absolute value. This is necessary to prevent bias of the polarity of the voltage applied to the liquid crystal. In the case of this specific example, the voltage waveforms applied to the pixels are as shown in _{Pn / m} , _{Pn / m + 1} , and _{Pn / m + 2} in FIG. In other words, the time t _1, a positive or zero voltage is applied, the time subsequent t ₁ to a voltage of negative or zero is applied, then the voltage is not applied.

As described above, the driving method of the present invention is characterized in that the switching element is turned on three times in order to write the display content once.

The time t ₀ is a time required for charging the electric charge input from the signal electrode between the electrodes sandwiching the liquid crystal by the TFT, and a value such as 16 μsec can be used.

The time t ₁ is not particularly limited, but is 16.7.
It is particularly preferable to set the time to msec or less. This is because it is said that the frequency at which human eyes perceive flicker is 60 Hz or less. Therefore, 60 Hz drive, ie,
By setting t ₁ <16.7 msec, flicker does not occur.

Now, assuming that the response speed of the liquid crystal is t ₂ ,
If t ₂ <2t ₁ , there is no problem. Conversely, if t ₂ > 2t ₁ , the liquid crystal cannot sufficiently respond during the voltage application period, and an appropriate display cannot be obtained. In this case, the problem can be solved by repeating the first processing to the third processing a plurality of times.

Assuming that the number of repetitions is n, t ₂ <2t _1.
It is necessary that n hold. With this driving method,
Unlike the simple matrix type, it can display at high speed.
That is, a drive signal for one screen can be sent in a period of t ₀ × 3 × m. For example, t ₀ = 16 μsec and m =
Assuming 1000 lines, t ₀ × 3 × m = 16 μsec × 3 × 1000 = 48 ms
ec. After the signal is sent, the liquid crystal responds by the voltage of the electric charge held between the electrodes. For example, if the response time of the liquid crystal is shorter than 16 msec, the response time for obtaining one screen display is 16 msec. Becomes When the response speed of the liquid crystal is longer than 16 msec, the response time of the liquid crystal is the display response time. Therefore, the display can be performed much faster than the response time (t ₄ ) t ₄ = t ₅ · m = 20 msec · 1000 = 20 sec as shown in the example of the simple matrix.

As the switching element provided at each intersection of the scanning electrode and the signal electrode, various elements are possible. For example, a-Si (amorphous silicon) or p-type
A thin film transistor (TFT) using poly-Si (polysilicon), a plasma-addressed element, and the like are possible.

According to the driving method of the present invention, it is of course possible to continuously send a signal. This is not preferable from the viewpoint of power consumption. From the viewpoint of power consumption, for example, a partial rewriting method in which a signal is transmitted only to a portion of the screen where display content is to be rewritten is preferable. In this case, the signal may be sent only to the scanning electrode and the signal electrode connected to the pixel whose display content is to be rewritten.

Alternatively, when the display state changes,
There is also a method in which the first to third processes are performed line-sequentially for all pixels.

Also, when the memory property of the liquid crystal is not sufficiently long, regardless of the change of the display state, every first time t ₃ or when an instruction is issued, the first processing is performed on the entire element. The processing up to the third processing may be performed in a line-sequential manner. t ₃ is, for example, one minute or more.

It is also possible to adjust the voltage applied to the liquid crystal by applying a voltage to the counter electrode Vcom.

The driving method of the present invention cannot be applied to a surface-stabilized ferroelectric liquid crystal. In the surface-stabilized ferroelectric liquid crystal, switching is controlled by the polarity of the voltage, so that an appropriate display cannot be obtained by the driving method of the present invention.

[0046]

According to the present invention, it is possible to obtain a liquid crystal display device capable of high-speed display with low power consumption.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an active matrix type liquid crystal display device of the present invention.

FIG. 2 is a diagram showing waveforms of various signals according to the present invention.

FIG. 3 is an equivalent circuit diagram of a conventional liquid crystal display element when a liquid crystal having no memory property is actively driven.

FIG. 4 is a diagram showing a display state of a liquid crystal display element when a conventional liquid crystal having no memory property is actively driven.

FIG. 5 is a drive signal waveform diagram when a conventional liquid crystal having no memory property is actively driven.

FIG. 6 is a circuit diagram of a liquid crystal display element when a liquid crystal having a memory property is driven by a simple matrix in a conventional example.

FIG. 7 is a driving signal waveform diagram in a conventional example in which a liquid crystal having a memory property is driven by a simple matrix.

FIG. 8 is a waveform diagram of a voltage applied to a liquid crystal display element when a liquid crystal having memory properties is driven by a simple matrix in a conventional example.

[Explanation of symbols]

S _1, ..., S _k signal electrodes G _1, ..., G _l scan electrodes _{P 1/1, ..., P l /} k pixels

Claims (8)

[Claims] 1. An active matrix type in which a plurality of scanning electrodes and a plurality of signal electrodes are formed in a matrix and a switching element is provided at each intersection of the electrodes, and a liquid crystal exhibiting a memory property is sandwiched. As a first process, a signal is sent from the scanning electrode to the liquid crystal display element to turn on the switching element, and in synchronization with this, a positive, negative or zero voltage corresponding to the desired display is sent from the signal electrode. As a second process, after a certain period of time, a signal is again sent from the scanning electrode to turn on the switching element, and in synchronization with this, a voltage corresponding to a desired display is sent from the signal electrode, and a voltage of the voltage sent from the signal electrode is sent. The polarity is opposite in polarity to the voltage sent from the signal electrode in the first processing and has the same absolute value. A switching element which is turned on by sending a signal from the scanning electrode and a scanning electrode, and in synchronism with this, sends a signal from the signal electrode so that the voltage applied to the liquid crystal becomes zero. 2. The method according to claim 1, wherein the steps from the first processing to the third processing are repeated a plurality of times. 3. The method according to claim 1, wherein the predetermined time is shorter than 16.7 msec. 4. When the number of repetitions from the first processing to the third processing is n and the response speed of the liquid crystal is t ₂ , t ₂ <2t ₁ · n (where t ₁ is the predetermined time) 4. The relationship according to claim 1, wherein
4. A method for driving a liquid crystal display element according to any one of the first to third aspects. 5. The method according to claim 1, wherein when the display state is changed, the first processing to the third processing are performed line-sequentially for all pixels. 1
4. A method for driving a liquid crystal display element according to any one of the first to third aspects. 6. The method according to claim 1, wherein the first processing is performed on the pixels on the scanning electrodes to which the pixels having the change in the display state are combined.
The driving process is not performed on the pixels on the scanning electrodes to which the pixels having no change in the display state are combined after performing the processing of (1) to (5). A method for driving a liquid crystal display element. 7. Regardless of whether or not the display state has changed,
At regular intervals of 1 minute or more, or when a command is issued,
The method according to any one of claims 1 to 4, wherein the first processing to the third processing are performed line-sequentially for all pixels. 8. The driving method of a liquid crystal display device according to claim 1, wherein the liquid crystal is a cholesteric liquid crystal / polymer composite bistable liquid crystal.