JP2002175057A - Liquid crystal display, and drive method for the liquid crystal display - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a liquid crystal display which has superior moving-image display quality by preventing 'ghosts' where a display of the previous frame is visible as an after-image. SOLUTION: In this line sequential type drive method, which displays an image of one frame by writing a signal to all pixels by sequentially selecting respective gate lines, each gate line is selected at least twice, within a period of one frame and an erasure voltage for placing the respective pixels in the same state and a gradation voltage corresponding to an image to be displayed are each written once, to each pixel connected to a gate line.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device, and more particularly to a method for driving a liquid crystal display device having a switching element in each pixel.

[0002]

2. Description of the Related Art FIG. 7 shows a conventional liquid crystal display device.

[0005] The liquid crystal display device 10 includes a liquid crystal panel 11 serving as an image display unit and a driving circuit such as a gate driver 31 and a source driver 32. The liquid crystal panel 11 has a large number of pixels 1 arranged in a matrix of vertical and horizontal directions.
2, each pixel 12 has a pixel transistor 1
3 is provided. Further, the liquid crystal panel 10 is provided with a plurality of gate lines 21 arranged in parallel with each other, and a plurality of source lines 22 orthogonal to the gate lines 21 and arranged in parallel with each other. The gate electrode of the pixel transistor 13 is connected to the gate line 21, and the source electrode is connected to the source line 22.

Next, the operation of the liquid crystal display device will be described.

A source driver 32 sets the potential of each source line 22 to a predetermined value based on an input image signal. The gate driver 31 selects a certain (for example, n-th) gate line 21 according to the input start pulse. The selection is performed by increasing the potential of the gate line 21. All the pixel transistors 13 connected to the (n-th) gate line 21 are turned on, and the potential of each source line 22 is set to the respective value of the n-th row. The data is written to the pixel 12.

Next, the source driver 32 sets the potential of each source line to a new potential based on the image signal, and the gate driver 31 selects the (n + 1) th gate line according to the start pulse. All the pixel transistors connected to the (n + 1) th gate line are turned on, and the potential of each source line is written to each pixel in the (n + 1) th row.

In this manner, by sequentially selecting a plurality of gate lines 21, an arbitrary potential can be written to all the pixels 12 of the liquid crystal panel 11. The liquid crystal of each pixel is driven by a potential difference between the potential written to each pixel and the potential of a separately provided common electrode, and the transmittance of light passing through the liquid crystal is changed, so that a desired display can be obtained.

[0008] Liquid crystal display devices are widely used because they are small and light, have low power consumption, and can provide precise display. However, it has been pointed out that the display quality of moving images is low. The reason will be described with reference to FIG.

Generally, in many display devices including a liquid crystal display device, the frame rate is 60 Hz, that is, 6 frames per second.
The display image is rewritten 0 times. Therefore, the time required for rewriting the display image is about 16.6 ms.
This is called one frame period F. General TN
In a (twisted nematic) liquid crystal, to change the state of the liquid crystal from one transmittance to another transmittance, 6
A time of 0 ms or more is required. Therefore, when the frame rate is 60 Hz, about 1 in one frame period
During 6.6 ms, the liquid crystal cannot respond sufficiently and cannot reach the desired transmittance.

The pixel A in which the voltage V ₁ is written and in the state of the transmittance T ₁ and the pixel B in which the voltage V ₂ is written and in the state of the transmittance T ₂ are provided with a new transmittance T ₃ . Voltage V ₃
Is written. The liquid crystal of the pixels A and B is driven by the voltage V ₃ , but the response of the liquid crystal is slow, so that the transmittance T ₃ does not reach during one frame period. Therefore, the same voltage V ₃
Is applied, the transmittance of the pixel A and the transmittance of the pixel B at the end of the frame period are different. This means that the display image of the current frame is affected by the display image of the previous frame, and the display of the previous frame is visually recognized as a “ghost” that remains as an afterimage, resulting in a decrease in display quality. I have.

[0011]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and a liquid crystal display device which is excellent in display quality of moving images by preventing "ghost" in which the display of the previous frame is visually recognized as an afterimage. I will provide a.

[0012]

In the liquid crystal display device according to the present invention, a source driver for supplying a signal to a source line is provided with a gradation voltage corresponding to an image to be displayed and a state of each pixel. A signal switching board for alternately supplying an erasing voltage is provided.

Further, the display area of the liquid crystal display device is divided into a plurality of areas, lamps are provided corresponding to the respective areas, and these lamps can be turned off independently of each other.

A method for driving a liquid crystal display device according to the present invention comprises:
A line-sequential driving method in which signals are written to all pixels to display an image of one frame by sequentially selecting each gate line, wherein each gate line is selected at least twice within one frame period, An erasing voltage for adjusting the state of each pixel and a gradation voltage corresponding to an image to be displayed are written at least once to each pixel connected to the gate line.

Further, a backlight which can be turned off in a divided manner is provided.
The backlight is not illuminated until a predetermined time elapses after the writing of the gradation voltage.

Further, when the period of one frame is F and the number of gate lines is Ng, the erase voltage and the gray scale voltage are alternately applied to the source line at time {F / (2 × Ng)}. It is characterized by being applied.

When the period of one frame is F and the number of gate lines is Ng, the selection period of each gate line is F / (2 × Ng), and the time 書 き 込 み (F / Ng) .times.b + F / (2.times.Ng): b is a positive integer.epsilon.

The erasing voltage is preferably a gradation voltage that minimizes the light transmittance of the pixel.

The gradation voltage may be determined as a voltage at which an image to be displayed can be obtained approximately one frame period after the application of the erase voltage.

[0020]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

Embodiment 1 FIG. 1 shows a liquid crystal display device according to the present embodiment.

The liquid crystal display device 10 includes a liquid crystal panel 11
A driving circuit such as a gate driver 31 and a source driver 32 is provided. The liquid crystal panel 11 includes a large number of pixels 12 arranged in a matrix of vertical and horizontal directions, and each pixel 12 includes a pixel transistor 13.
Further, the liquid crystal panel 10 is provided with a plurality of gate lines 21 arranged parallel to each other and a plurality of source lines 22 orthogonal to the gate lines 21 and arranged parallel to each other.

In the liquid crystal display device of the present embodiment,
In order to prevent ghost caused by the slow response of the liquid crystal,
First, an erasing voltage for adjusting the state of each pixel is applied,
Thereafter, a gradation voltage based on the image signal is written to each pixel. As the erasing voltage, a gradation voltage (hereinafter, referred to as a blackening voltage) that minimizes the transmittance of the liquid crystal is used.

For this purpose, the source driver 32 is provided with a signal switching board 33 for switching between a gradation voltage and a blackening voltage based on an image signal and supplying the same to the source line 22. The signal switching board 33 is switched at a time interval that is half the time F / Ng obtained by dividing one frame period F by the number Ng of gate lines, that is, at a period of F / (2 × Ng).

The gate driver 31 has a shift register, sequentially selects each gate line 21 and outputs a potential V
_On is applied to turn on the pixel transistor 13. This shift register can perform a gate line selecting operation at a time interval that is half the time F / Ng obtained by dividing one frame period F by the number Ng of gate lines, that is, at a period of F / (2 × Ng).

It should be noted that the gate driver 31 and the source driver 32 operate in synchronization, and signal switching and gate line selection are performed at the same timing.

The operation of the liquid crystal display device according to the present embodiment will be described with reference to FIGS. 3, 4, and 5. FIG. 3 shows the potentials of the n-th, (n + 1) -th, and (n + b) -th gate lines (denoted as GL potentials in the figure). FIG. 4 shows the potential of the m-th source line (denoted as SL potential in the drawing) and the voltage applied to the pixel connected to the n-th gate line and the m-th source line (in the drawing, , N × m-th pixel electrode). FIG. 5 shows the light transmittance of the pixels connected to the n-th gate line and the m-th source line (in the figure, the transmittance of the n × m-th pixel) and the luminance of the backlight. Have been.

First, a black start pulse V _SB is input to the gate driver 31 to select the n-th gate line. The selection period is half the time F / Ng obtained by dividing one frame period F by the number Ng of gate lines, that is, F / Ng.
(2 × Ng), and during this selection period, the potential of the n-th gate line is the ON potential V _on of the pixel transistor.

[0029] At this time, in synchronization with the selection of the gate lines, the time F / (2 × Ng) during the blackening voltage by all signal switching board 33 to the source line 22 V _b (or, -V _b) Is applied.

[0030] Therefore, n-th blackening voltage to all the pixels connected to the gate lines V _b (or, -V _b) is written, as shown in the upper part of FIG. 5, the transmittance of the pixel Changes to black display (small transmittance).

Subsequently, at intervals of time F / Ng, n
The + 1st and subsequent gate lines are selected. As described above, the signal switching board 33 has the period F / (2 ×
Ng) in blackening voltage V _b (or are switched alternately -V _b) and the gradation voltages. Thus, for the n + 1 -th gate line, similarly to the n-th gate line, all the black pixel of the voltage V _b (-V _b) is written, each pixel is changed to black display.

When the display is changed to a black display, the response of the liquid crystal is relatively fast, and one frame period F
The response is almost complete in a fraction of the time
A black display is obtained. Therefore, when the pixel connected to the n-th gate line becomes almost black display, the image signal writing start pulse _VSP is input to select the n-th gate line again, and the pixel based on the image signal is selected. Write the adjustment voltage.

At this time, the signal switching board 33 must be switched to the image signal (gradation voltage) side so that the blackening voltage is not written again. Therefore, n by the image signal write start pulse V _SP
The selection of the gate line is made by the black start pulse V _SB
｛(F / N)
g) × b + F / (2 × Ng)}.
Here, b is a positive integer, and (F / Ng) × _b is referred to as a blackening advance time Tb.

[0034] Continuing, for the (n + 1) th and subsequent gate line, time from writing of blackening voltage T _b + F /
After the lapse of (2 × Ng), writing of a gradation voltage based on an image signal is performed.

As described above, in this embodiment, before writing the gradation potential based on the image signal, first, the blackening voltage is written to each pixel. For this reason,
Regardless of the display of the previous frame, the liquid crystal of each pixel is once set to the same state, and thereafter, a gradation voltage is applied, and the liquid crystal is driven. Therefore, it is possible to prevent a “ghost” caused by a different display state of the previous frame for each pixel, and to obtain a favorable moving image display.

In this embodiment, each gate line is selected twice during one frame period F, that is, when writing the blackening voltage and when writing the gradation voltage. Therefore, each selection time is about 1/2 of the conventional time.
It is conceivable that the potential cannot be sufficiently written.

[0037] In such a case, select gate lines again by his second image signal writing start pulse V _SP, may be to perform a write again gradation voltages. In the example shown in FIG. 3, the time from selection by the start pulse V _SP in (2 × F / Ng) after, it is performed selection by his second start pulse V _SP.

When writing the gradation voltage twice, two gate lines are selected at the same time. For example, in the example shown in FIG. 3, the (n-2) th gate line is selected at the same time as the first selection of the nth gate line, and the (n + 2) th gate line is selected at the same time as the second selection of the nth gate line. A gate line is selected. In this case, when the gate line is selected for the second time, the gradation voltage to be displayed on the pixel of the gate line is preferably written. At the time of the first selection, the gray scale voltage to be displayed is written to the pixels of the other gate lines. However, the time between the first selection and the second selection is short, and the response of the liquid crystal is slow. Therefore, the display is not disturbed.

Similarly, for the blackening voltage, it is preferable to select the gate line again by the second blackening start pulse V _SB and write the blacking voltage again. Each pixel can be more reliably and blackening voltage V _b (-V _b).

The polarity of the blackening voltage applied to the pixel is changed for each frame in order to prevent the liquid crystal from cracking.
Alternatively, it may be reversed every several frames.

In the present embodiment, as shown by the SL potential in FIG. 4, a so-called dot inversion drive in which voltages applied to adjacent pixels have opposite polarities is performed. Therefore, after a lapse of time (2 × F / Ng) from the selection by the first start pulse V _SP , the second start pulse V _SP
By performing the selection by _SP, when selected by the selection time and the second time of the start pulse V _SP by 1 time of the start pulse V _SP, a voltage of the same polarity are to be applied to the pixel electrode. Therefore, the interval between the first start pulse V _SP and the second start pulse V _SP is
(2 × F / Ng) to a limited _{without, {(n e × F /} Ng):
_ne may be a positive even number｝.

Similarly, the first blackening start pulse V _SB
About the interval between the second time blackening start pulse V _SB, not limited to (2 × F / Ng), {(n e × F / N
g): _ne may be a positive even number｝.

[0043] Furthermore, blackening prior time T _b is desirably about 1～4Ms, among them approximately 1~3ms is desirable. Generally, a TN liquid crystal has a transmittance of 0 (black display) by applying a voltage.
The response when changing to the state is fast, 1-4 ms
A black display state is sufficiently obtained in a short time, the image of the previous frame is erased, and the occurrence of an afterimage due to a delay in liquid crystal response can be suppressed.

On the other hand, when the frame rate is 60 Hz, 1
Frame period F is about 16.6 ms, if too taking longer blackened prior time T _b, the application of the gradation voltage is slow, there is a possibility that the liquid crystal can not be sufficiently respond within one frame. In the present embodiment, since the state of the liquid crystal of each pixel is made uniform by the blackening voltage and then the gradation voltage is applied, no ghost occurs even when the liquid crystal cannot respond sufficiently. However, since the liquid crystal cannot respond sufficiently, the transmittance is reduced, and the entire display image is displayed darker.

Since it is necessary to secure both the response time to the blackening voltage and the response time to the gradation voltage, the preceding blackening time Tb is determined. In a normal TN type liquid crystal, the cell gap (the thickness of the liquid crystal layer ₎ is determined. ), The temperature, the viscosity of the liquid crystal, the blackening voltage, the potential of the gradation voltage, and the driving method, but generally 1 to 5 ms.

Embodiment 2 As shown in FIGS. 6A and 6B, in the conventional driving method of a liquid crystal display device, an image signal designating a transmittance T ₄ is given as a display of a certain pixel. If had a gradation voltage for applying a voltage V ₄ such that the transmittance T ₄ when sufficient time has elapsed to complete the response of the liquid crystal in the pixel.

[0047] Thus, as already mentioned, if the pixel is not in the state close to the transmittance T ₄ in the previous frame, it is impossible to obtain a display of the transmittance T ₄ in one frame period, the previous frame This causes "ghost" in which the afterimage is visually recognized, and also causes the contrast to not increase.

The image signal (or gradation voltage) of the previous frame is stored and compared with the image signal to be newly displayed, so that the voltage in consideration of the state of the pixel of the previous frame is converted to the gradation voltage. A method of applying a voltage to a pixel is also considered. However, this method requires a storage device for storing the image signal of the previous frame,
Further, a comparison table and an operation means for comparing the image signal of the previous frame with the new image signal are required.

On the other hand, in the present invention, as already described in the first embodiment, since the state of the liquid crystal of each pixel is made uniform by the blackening voltage, the liquid crystal is not applied when the gradation voltage based on the image signal is applied. Is known and constant. Therefore, it is possible to determine a voltage to be applied to a pixel without using a storage device or a comparison table, and to obtain a display with high contrast without “ghost”.

As shown in FIGS. 6 (c) and 6 (d), in the present invention, a blackening voltage is applied at the beginning of one frame period, and after a lapse of time T _b + F / (2 × Ng), an image signal is generated. Is applied. Therefore, time F-
By applying a voltage V ₅ as displayed to _{{T b + F / (2} × Ng)} desired transmittance T ₄ from the black display in changes to the pixel, the transmittance of the pixels at the end frame period T ₄
It can be.

According to the present embodiment, it is possible to speed up the response of the liquid crystal without using a storage device or a comparison table, and to obtain a display with high contrast.

Embodiment 3 Many liquid crystal display devices include a backlight as a light source. The present embodiment is characterized in that, as shown in FIG. 2, the backlight 40 includes a plurality of lamps, and each lamp can be turned off independently of each other. In FIG. 2, the backlight 40 includes eight cold cathode tubes 41 as lamps, and can be individually turned on by an inverter 42 and a changeover switch 43.

The liquid crystal display panel 11 of the liquid crystal display device 10
Is also divided into eight regions including a plurality of gate lines, and the cold cathode tubes 41 are provided corresponding to these regions.

Each cold-cathode tube 41 is turned on after a predetermined time elapses after a gradation voltage is applied to each pixel in the area.
Thereafter, since the blackening voltage is applied to each pixel, the cold cathode tube 41 is turned off prior to the application of the blackening voltage. In the subsequent frame, the grayscale voltage is applied again, and the cold-cathode tube 41 remains off until a predetermined time has elapsed.

As described above, by turning off the cold-cathode tube 41 during the period when the light transmittance is low after the application of the blackening voltage and the application of the gradation voltage, unnecessary power consumption is prevented, and
It is possible to improve power consumption and efficiency.

Further, since the state where the light transmittance is low after the application of the blackening voltage and the application of the gradation voltage is not visually recognized,
It is possible to obtain a display with high contrast.

Further, since the display of each pixel is visually recognized only during the lighting period of the cold-cathode tube, the display becomes quasi-pulsed, and a hold-type display in which display is always performed like a conventional liquid crystal display device. Can be improved. The blur width of the moving image blur is improved in proportion to the lighting ratio of the cold cathode fluorescent lamp. The cold-cathode tube is set to 1 /
When the lighting is performed during the period of 3, the blur width is reduced to 1/3.

In the present embodiment, the number of cold cathode tubes is eight. As the number of cold cathode tubes increases, the number of gate lines corresponding to one cold cathode tube decreases, and luminance unevenness caused by different signal application timings for each gate line is reduced. The cost required for an inverter, a changeover switch, etc. also increases. Therefore, the number of cold cathode tubes is preferably 4 to 10, more preferably 6 to 8.

In this embodiment, an example in which a cold cathode tube is used as a lamp has been described. However, a display area of a liquid crystal display panel is divided into a plurality of areas corresponding to gate lines.
Any light source such as a hot cathode tube, EL, or LED may be used as long as these regions can be turned off and on independently.

[0060]

According to the present invention, it is possible to prevent "ghost" in which the display image of the previous frame is visually recognized as an afterimage, and it is possible to obtain a favorable moving image display.

Further, it is possible to speed up the response of the liquid crystal without using a storage device or a comparison table, and to obtain a display with high contrast.

Further, by limiting the lighting period of the backlight lamp, it is possible to reduce the power consumption, to obtain a display with high contrast, and to improve the motion blur caused by the hold type display. It is possible.

[Brief description of the drawings]

FIG. 1 is a view showing a liquid crystal display device according to the present invention.

FIG. 2 is a view showing a backlight of the liquid crystal display device according to the present invention.

FIG. 3 is a diagram illustrating a driving method of the liquid crystal display device according to the present invention, and is a diagram illustrating selection of a gate line.

FIG. 4 is a diagram illustrating a driving method of the liquid crystal display device of the present invention, which illustrates a potential of a source line and a voltage written to a pixel.

FIG. 5 is a diagram illustrating a driving method of the liquid crystal display device of the present invention, and is a diagram illustrating a transmittance of a pixel and a lighting period of a backlight.

FIG. 6 is a diagram illustrating a method of determining a gray scale voltage according to a conventional technique and a method of determining a gray scale voltage according to the present invention.

FIG. 7 is a diagram illustrating a liquid crystal display device according to a conventional technique.

FIG. 8 is a diagram illustrating the reason for the occurrence of ghost in a conventional liquid crystal display device.

[Explanation of symbols]

10 liquid crystal display device, 11 liquid crystal panel, 12 pixels,
13 pixel transistor, 21 gate line, 22
Source line, 31 gate driver, 32 source driver, 33 signal switch board, 40 backlight, 41 cold cathode tube, 42 inverter, 43 switch.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G09G 3/20 623 G09G 3/20 623R 641 641R 641C 3/34 3/34 J (72) Inventor Shin Tabata 2-3-2 Marunouchi, Chiyoda-ku, Tokyo Mitsubishi Electric Corporation (72) Inventor Toshio Tobita 2-3-2 Marunouchi, Chiyoda-ku, Tokyo Mitsubishi Electric Corporation F-term (reference) 2H093 NA16 NA33 NA51 NC03 NC16 NC34 NC42 ND04 ND12 ND39 ND54 5C006 AA01 AA16 AF33 AF44 BB16 BB29 BC03 BC12 BF03 EA01 FA29 5C080 AA10 BB05 DD05 DD06 EE19 EE29 FF11 JJ02 JJ04 JJ05

Claims (8)

[Claims] A plurality of gate lines, a plurality of source lines, pixel transistors connected to the gate lines and the source lines, a gate driver for sequentially selecting the gate lines, and a signal applied to the source lines. And a source driver that supplies a pixel signal. When a gate line is selected, all the pixel transistors connected to the gate line are turned on, and a signal of the source line is written to each pixel connected to the gate line. A display device, wherein the source driver has a signal switching function for alternately supplying a gradation voltage corresponding to an image to be displayed and an erase voltage for aligning the state of each pixel. Liquid crystal display device. 2. The liquid crystal according to claim 1, wherein the plurality of gate lines are divided into a plurality of groups, and a backlight which can be turned off and on independently is provided for each group. Display device. 3. A semiconductor device comprising: a plurality of gate lines; a plurality of source lines; and a pixel transistor connected to the gate line and the source line, and connected to the gate line by selecting a gate line. In a liquid crystal display device in which all the pixel transistors are turned on and the signal of the source line is written to each pixel connected to the gate line, the signal is written to all the pixels by sequentially selecting each gate line, and one frame is written. A gate line is selected at least twice within one frame period, and the state of each pixel is aligned with each pixel connected to the gate line. The erase voltage and the gradation voltage corresponding to the image to be displayed are written at least once each. Method of driving a liquid crystal display device which. 4. A backlight comprising: a backlight; a plurality of gate lines; a plurality of source lines; and a pixel transistor connected to the gate line and the source line. In a liquid crystal display device in which all pixel transistors connected to a line are turned on and a signal of a source line is written to each pixel connected to the gate line, by sequentially selecting each gate line, a signal is applied to all pixels. And a line-sequential driving method for displaying an image of one frame, wherein each gate line is selected at least twice within a period of one frame, and each pixel connected to the gate line is provided with a state of each pixel. The erase voltage and the grayscale voltage corresponding to the image to be displayed at least once each Together we are, none of the pixels, the writing of the erase voltage, the during until the predetermined time elapses after the writing of the gradation voltages, driving method of a liquid crystal display device characterized by not illuminated by the backlight. 5. When one frame period is F and the number of gate lines is Ng, the erasing voltage and the gray scale voltage as signals on the source line substantially correspond to the time ΔF /
5. The method according to claim 3, wherein (2 * Ng) is applied alternately. 6. When one frame period is F and the number of gate lines is Ng, the period of each selection is approximately F.
/ (2 × Ng), and after the writing of the erasing voltage, the time {(F / Ng) × b + F / (2 × Ng): b is a positive integer} elapses and the writing of the gradation voltage is performed. 6. The method for driving a liquid crystal display device according to claim 3, wherein: 7. The erasing voltage according to claim 3, wherein the erasing voltage is a gradation voltage at which the light transmittance of the pixel is minimized.
7. The method for driving a liquid crystal display device according to 5 or 6. 8. A gray scale voltage corresponding to an image to be displayed is determined as a voltage at which a transmittance to be displayed is obtained after approximately one frame period has elapsed from the application of the erase voltage. 8. The method for driving a liquid crystal display device according to 3, 4, 5, 6, or 7.