JPH0469394B2 - - Google Patents

Description

[Detailed Description of the Invention] <Technical Field> The present invention relates to a color liquid crystal display device, and in particular includes a mosaic colored body corresponding to a matrix-type display pixel pattern for displaying television images, etc. The present invention relates to a method for driving a matrix type liquid crystal display device in which a switching transistor is added to a matrix type liquid crystal display device.

<Prior art> Matrix-type liquid crystal display devices using switching transistors incorporate switching transistors into the liquid crystal display panel, allowing high-contrast display even when performing multiplex drive with a small duty ratio, i.e., multiple lines. This is a display device that makes it possible, and is generally represented by an equivalent circuit as shown in FIG. 11 in Figure 1
is a switching transistor, and a field effect transistor using a single crystal, polycrystal, or amorphous semiconductor such as Si, Te, or CdSe is used. 12
1 is a row electrode, and 13 is a column electrode, which are connected to the gate electrode and source electrode of the transistor 11, respectively. 14 is the capacitance of the liquid crystal layer sandwiched between the display picture element electrode and the counter electrode, and 15 is a storage capacitor provided when the charge storage ability of the liquid crystal layer is insufficient.

A color liquid crystal display panel has a colored body such as a striped or mosaic color filter disposed corresponding to a large number of pixel electrodes arranged in a dot matrix, and each pixel has a color corresponding to it. By applying a video signal according to
Using the same principle as a CRT (cathode ray tube), it can display any image containing intermediate colors that are additively mixed.

Usually, additive primary colors are selected as the colors of the colored bodies. As the colored body, a polarizing plate, an interference filter, a filter made of an inorganic or organic dye or pigment, etc. are used. The colored body may be provided on the outer surface or the inner surface of the substrate constituting the liquid crystal display panel. In the latter case, it may be provided above or below the picture element electrode or the common electrode.

There are two types of arrangement of colored bodies made of color filters: stripes and mosaics.
The vertical stripe type shown in Figure A, the horizontal stripe type shown in Figure 2B, the latter includes the 9-pixel staircase type shown in Figure 2C, the vertical 6-pixel type shown in Figure 2D, and the horizontal stripe type shown in Figure 2E. There are horizontal 6-pixel types as shown, 4-pixel types as shown in Figure 2F, etc.
There are also variations of them. These various patterns are periodic both in the vertical and horizontal directions.
The vertical period n, which is an important element in the present invention, is the second
Figure A has one picture element, Figures B, C, and D have three picture elements, and Figures E and F have two picture elements.

The operating principle of the liquid crystal display device will be explained using the equivalent circuit shown in FIG. 1 and the drive signal waveform shown in FIG. 3.

Figure 3 A and B are i-th and (i+
1) It is a scanning pulse applied to the row electrode 12, and when the total scanning time is T and the number of scanning lines is N,
A waveform that turns on the transistors for a time of H=T/N out of T is applied to each row electrode 12 so that each row is turned on one after another as shown in the figure. FIG. 3C shows the data signal waveform applied to the j-th column electrode 13. A voltage waveform corresponding to the voltage applied to the picture elements in each row is applied to the column electrode B in synchronization with the transistors in each row of the j-th column being sequentially turned on. FIG. 3C shows a case where a voltage of V volts is applied to the picture element in the i-th row of the j-th column, and a voltage of zero volts is applied to all other picture elements. Note that the data signal waveform is applied with its polarity inverted for each scan in order to drive the liquid crystal with alternating current.

First, consider the picture element in the i-th row and j-th column in FIG. When the transistor 11 turns on, charge is charged from the column electrode to the liquid crystal and the storage capacitor through the on-resistance R _ON of the transistor, and the potential of the display pixel electrode becomes +V, which is the same as the data signal.
becomes. Next, when the transistor is turned off, the charged charge is held as it is, so the potential of the display picture element electrode is held at +V. When the transistor is turned on again next time, charging is performed in the same way so that the potential of the display picture element electrode becomes -V.
Retains its value during the next off state. As a result, a nearly rectangular voltage waveform is applied to the display pixel electrode as shown in Figure 3d, and when the potential of the counter electrode is set to zero volts and the value of V is set above the threshold of the liquid crystal. The liquid crystal is turned on.

Next, considering the pixel in the (i+1)th row and jth column, in this case, the display pixel electrode is charged and held at zero volts as shown in Figure 3e, so no voltage is applied to the liquid crystal. It is in the off state.

As described above, in the liquid crystal display device described above, even though multiplex driving is performed, a voltage equivalent to static driving is applied to the liquid crystal, so that a high contrast display can be obtained.

In addition, in the above explanation, for the sake of simplicity, the data signal is ±
Although the case where V volts and the counter electrode are grounded is shown, there is also a method of applying a rectangular wave or a ramp wave whose polarity is reversed for each field to the counter electrode. This method has the following advantages.

(1) Since the voltage amplitude of the analog signal is small, the power consumption of the analog signal processing circuit is reduced.

(2) Contrast and bias (for CRT)
(equivalent to BRIGHTNESS) can be adjusted independently, making image adjustments easier. In particular, the fact that the bias can be adjusted independently is effective for adjusting the viewing angle.

In order to explain the above driving method, FIG. 4 shows the relationship between applied voltage and transmittance in a twisted nematic (TN) type liquid crystal display device. The solid line in the figure indicates the case where the polarization axes of the two polarizing plates installed on the front and rear sides of the liquid crystal display panel are set parallel, and the broken line indicates the case when they are set perpendicular to each other. The former will be explained below as an example, but in the latter case, the meaning will be exactly the same if the white level and black level are reversed. As shown in FIG. 4, the transmittance hardly changes when the applied voltage is below Vth, but changes greatly between Vth and Vsat, and approaches saturation when it exceeds Vsat. TN
For the type, Vth is approximately 2V and Vsat is approximately 3V. Therefore, by setting the black level to about 2V and the white level to about 3V, halftones can be reproduced almost faithfully. Since the polarity is reversed to drive the liquid crystal with AC, the data signal can be up to ±3V in the method where the counter electrode is grounded.
= 6V _PP amplitude. Generally, in analog circuits, the power supply voltage must be greater than the maximum value of the signal voltage handled, and power consumption is roughly proportional to the square of the power supply voltage. Therefore, the power consumption in this case is much larger than that in a method in which a rectangular wave or a ramp wave is applied to the counter electrode, which will be described later. Furthermore, in this method, when the gain of the circuit that handles the data signal is changed, the white level and black level change in conjunction with each other, that is, the contrast and bias change in conjunction with each other. In order to adjust contrast and bias independently, it is necessary to take measures such as introducing a level shift circuit. In contrast, in a method that applies a voltage to the counter electrode, the white level of the signal is (Vsat-Vth)/2, and the black level is -(Vsat-
Vth)/2, and the counter electrode is -(Vsat+
Apply a voltage of Vth)/2. In the case of TN, the voltages are 0.5V, -0.5V, and -2.5V, respectively, and a voltage of 3V at the white level and 2V at the black level is applied to the liquid crystal.
All polarities are reversed for each field for AC drive. In this way, the amplitude of the data signal is ±
0.5V = 1V _PP , which is 1/6 of the counter-grounding type.
Additionally, power consumption is reduced to approximately 1/36. Furthermore, in this method, the amplitude of the data signal corresponds to the contrast, and the voltage applied to the opposing electrode corresponds to the bias, so both can be adjusted independently.

Note that the method of applying a ramp wave is difficult to use when the off-resistance of the TFT is insufficient, or when viewing from close to the LCD panel, the viewing angle changes depending on the vertical position of the LCD panel. This is a driving method intended to correct unevenness.

In the above driving method, the charging time constant R _ON・C _L
(R _ON is the on-resistance of the transistor, C _L is the parallel capacitance of the liquid crystal and the storage capacitor) is set so that it is sufficiently short compared to the width H of the scanning pulse, and the potential of the display pixel electrode is set to the voltage of the data signal waveform. It is desirable to charge the battery sufficiently until it becomes equal to V. If the time constant is not a sufficiently small value compared to H and the display picture element electrodes are not sufficiently charged, a voltage of V is applied to the column electrodes as shown in Figure 5. This is because, in reality, only a voltage of V ₁ is applied to the liquid crystal. Furthermore, in such a state, the voltage V ₁ applied to the liquid crystal varies depending on the value of R _ON・C _L , so R _ON is applied to each pixel in the display device.
If there are variations in the values of CL and _CL , these will be reflected in the display contrast, which will be a major hindrance in displays that require intermediate tones, such as television images.

In the above driving method, the width H of the scanning pulse is H=
It is calculated as (total scanning time: T)/(number of scanning lines: N), but since T cannot be increased due to the problem of flickering of the liquid crystal, there is a limit to increasing H. It is difficult to reduce the capacitance C _L from the viewpoint of charge retention function. Regarding on-resistance, for example, in transistors using semiconductors with low conductivity such as amorphous Si, it may not be possible to reduce the value sufficiently, and in such cases, the above-mentioned problems may occur. occurs.

<Object of the Invention> The present invention has been made in view of the above-mentioned problems in the conventional driving method of a matrix type liquid crystal display device. It is an object of the present invention to provide a new and useful method for driving a liquid crystal display device that can obtain good display contrast.

<Basic Principle of the Invention> A feature of the driving method of the present invention is that when the vertical period of the color pattern is n picture elements, prior to charging with the original data signal, the driving method of the present invention Precharging with a data signal has the same effect as substantially widening the width of the scanning pulse, and FIG. 6 shows its driving waveform. The figure shows, as an example, the case where a scanning pulse is applied twice in a nine-pixel pattern (FIG. 2C) in which the color period in the vertical direction is three pixels. In FIG. 6, A to D are the waveforms of scanning pulses applied to the (i-3) to i-th row electrodes, respectively, and E is a data signal waveform applied to the j-th column electrode. In comparison with the drive waveform F of the conventional method, the drive waveform G according to the present invention
In this case, the width of the scanning pulse is substantially widened by precharging with the data signal of the same color arrangement of n rows before. The principle of operation will be explained by taking the picture element in the i-th row and j-th column as an example.

G represents the potential of the display pixel electrode in the i-th row and the j-th column, and Vi-n and Vi represent the potential of each (i-th
-n) row, the data voltage corresponding to the i-th row.
First, at the beginning of each field, each picture element electrode is charged to the opposite polarity in the previous field. Next, Hi-n
When the transistor is on during the period of
The display pixel electrodes in the row and jth column are pre-charged toward the data voltage Vin of the nth row before. Then, during the Hi-n+1 to Hi-1 period, it is once turned off, and during the next Hi period, it is turned on again, and charging is performed toward the original data voltage Vi, resulting in the charging shown in Figure 6 G. It becomes a curve, and the sixth
Charging is performed to a higher voltage than in the conventional method shown in Figure F. Vi− like a TV image display
If n and Vi are data signals of the same color and have a high correlation, by applying the scanning pulse twice as described above, R _ON・_CL can be reduced to 1/2 without increasing the width. You can get the same effect.

When the width of _the scanning pulse is widened as described above, _Vi
An error occurs due to the difference between −n and Vi, but if the correlation between Vi−n and Vi is high and the number of scan pulses applied is not too large compared to the total number of scans, as in a normal video signal, the value will be It is sufficiently small compared to the driving voltage of the liquid crystal and does not pose a problem.

In the above example, the case where the vertical tone period is 3 pixels has been described, but it goes without saying that the present invention is also applicable to cases where the color period is 2 pixels or other pixels. Furthermore, the number of pulses is not limited to two.

<Embodiment> FIGS. 7A and 7B are a block diagram and a waveform diagram of a drive circuit illustrating an embodiment of the present invention. In the liquid crystal panel 21, matrix electrodes are formed by row electrodes and column electrodes, and switching transistors are added at the intersections of the matrix electrodes. Examples of switching transistors include thin film transistors using amorphous silicon and transistors formed on silicon substrates.
MOSFET is used. 22 is a column electrode driver consisting of a shift register, sample/hold, etc., which samples and holds the data signal D sent from the data control section 23 in synchronization with the clock φ ₁ from the signal control section 24, and in synchronization with φ ₂ . and output it to each column electrode. Each column electrode is connected to a source electrode of a switching transistor. Reference numeral 25 denotes a row electrode driver consisting of a shift register, which outputs a scanning pulse to each row electrode by sequentially shifting the basic scanning pulse S sent from the signal control section 24 using a clock φ ₂ . Each row electrode is connected to the gate electrode of a switching transistor to control on/off of the switching transistor. At this time, the number of scanning pulses can be changed by making the basic scanning pulse S into a plurality of pulse trains synchronized with the color period. The configuration of the present drive circuit is almost the same as that of the conventional drive method, and can be easily applied to the present drive method by changing the number of basic scanning pulses S in the conventional drive circuit. The pulse applied to the row electrode to which each gate of the switching transistor is connected is calculated as follows: H = field time ÷ number of row electrodes.
The pulse width is H and consists of a series of multiple pulses having intervals of (n-1)×H.

<Effects of the Invention> As described above, when the vertical period of the color pattern is n picture elements, the data of the picture elements of the same color arrangement in the previous n rows is detected using the original data signal prior to charging. Since preliminary charging is possible using a signal, it is possible to reduce voltage drop and deterioration of display characteristics caused by insufficient charging of the display pixel electrode through the switching transistor.
This is an effective driving method and is extremely useful for driving large-capacity XY matrix type liquid crystal display devices.

[Brief explanation of drawings]

FIG. 1 is an equivalent circuit diagram of a matrix type liquid crystal display device to which switching transistors are added. FIG. 2 is an explanatory diagram illustrating a coloring pattern. FIG. 3 is a signal waveform diagram of each electrode in a conventional driving method. Figure 4 shows applied voltage vs. transmittance characteristics in a TN type liquid crystal display. FIG. 5 is a charging curve showing the voltage error when the display picture element electrode is insufficiently charged. FIG. 6 shows one of the driving methods of the present invention.
FIG. 3 is a signal waveform diagram of each electrode showing an example. FIG. 7 is a block diagram of a circuit configuration and its signal waveform diagram for explaining one embodiment of the present invention. 11...Switching transistor, 12...
Row electrode, 13... Column electrode, 21... Liquid crystal panel,
22... Column electrode driver, 25... Row electrode driver.

Claims (1)

[Scope of Claims] 1. A liquid crystal display device for color image display, in which a switching transistor is added to each picture element arranged in a matrix, and a mosaic coloring means corresponding to each picture element is provided, The color period in the vertical direction of the mosaic coloring means is n
If the picture element is field time divided by the number of row electrodes = H, then the pulse applied to the row electrode to which each gate of the switching transistor is connected has a pulse width of H and an interval of (n-1) x H. , and the application timing of the plurality of pulses is set to be earlier than the timing of the data signal of the corresponding picture element applied to the column electrode, so that the same color array n rows before A method for driving a liquid crystal display device, characterized in that each display pixel electrode is precharged with a data signal of a pixel.