KR101158570B1 - Liquid crystal display device - Google Patents

Abstract

The present invention relates to a liquid crystal display device for improving image quality, comprising: first and second substrates bonded to each other so as to maintain a constant cell gap, and having a liquid crystal layer in the cell gap; A plurality of pixels arranged on the first substrate, each pixel including a pixel electrode; An image display unit composed of a combination of the pixels and displaying an image; A common electrode formed on the second substrate and applying an electric field to the liquid crystal layer together with the pixel electrode; A temperature sensor for sensing an external temperature and outputting a control signal according to the temperature; A power supply unit forming at least two common voltages; And a multiplexer for selectively applying one of the common voltages applied from the power supply unit to the common electrode according to the control signal, and correcting the magnitude of the common voltage according to temperature.

Description

[0001] LIQUID CRYSTAL DISPLAY DEVICE [0002]

1 is an equivalent circuit diagram of a unit liquid crystal cell of a general liquid crystal panel.

2 is a voltage waveform diagram of a unit pixel to which an inversion driving method is applied.

FIG. 3A is a view showing an image displayed on a liquid crystal display at room temperature; FIG.

Fig. 3B is a view showing an image displayed on the liquid crystal display in a low temperature state.

4 is a view showing a liquid crystal display device according to the present invention;

5 is a voltage waveform diagram of a unit pixel according to the present invention;

*** Description of the symbols for the main parts of the drawings ***

210: liquid crystal panel 215: image display unit

230: power supply 235: multiplexer

240: temperature detection unit CS11: control signal

Vcom1 to Vcomk: Common Voltage

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device. More particularly, the present invention relates to a liquid crystal display device which suppresses deterioration of image quality by maintaining a constant voltage difference between a common voltage and a pixel voltage even under various temperatures. will be.

In general, a liquid crystal display device is a display device in which an image signal corresponding to an image signal is individually supplied to pixels arranged in a matrix form so as to display a desired image by adjusting light transmittance of the pixels. .

Accordingly, the liquid crystal display includes a liquid crystal panel in which pixels, which are the smallest unit for implementing an image, are arranged in an active matrix form; A driving unit for driving the liquid crystal panel is provided.

The liquid crystal panel is bonded so that a color filter substrate and a thin film transistor array substrate opposing each other maintain a constant cell-gap, and the liquid crystal is disposed in the space between the cell-gap. It is produced by filling the layers.

Common electrodes and pixel electrodes are formed on opposite inner surfaces of the color filter substrate and the thin film transistor array substrate to apply an electric field to the liquid crystal layer. In this case, the pixel electrode is formed for each pixel on the thin film transistor array substrate, while the common electrode is integrally formed on the entire surface of the color filter substrate. Therefore, by controlling the voltage applied to the pixel electrode in a state where a voltage is applied to the common electrode, the light transmittance of the pixels can be adjusted individually.

A plurality of data lines are formed on the thin film transistor array substrate to be uniformly spaced in the longitudinal direction, and a plurality of gate lines are formed to be regularly spaced in the lateral direction. The gate lines and the data lines vertically cross each other to define a plurality of rectangular areas, which are defined as pixels. The pixels are electrically connected to the gate line and the data line to receive various control signals and image signals supplied from the driver to the gate lines and the data lines. That is, the driver sequentially supplies scan signals to the gate lines so that pixels arranged in a matrix form are sequentially selected by one horizontal line, and the data from the driver is included in the pixels of the selected one line. The image signal is supplied through the lines.

As described above, in order to control the voltage applied to the pixel electrode for each pixel, a thin film transistor used as a switching element is formed in each pixel, and a scan signal is applied to the gate electrode of the thin film transistor through the gate lines. In the supplied pixels, a conductive channel is formed between the source electrode and the drain electrode of the thin film transistor, wherein an image signal supplied to the source electrode of the thin film transistor is transmitted through the data lines. The light transmittance of the pixel is adjusted as it is supplied to the pixel electrode via the drain electrode.

Referring to the driving of the general liquid crystal display as described above in more detail as follows.

First, the driver includes a gate driver for applying a scan signal to the gate lines, and a data driver for applying an image signal to the data lines.

The common voltage is supplied to the common electrode which is integrally formed on the front surface of the color filter substrate, and the scan signal is sequentially supplied to the gate lines by the gate driver so that the pixels are sequentially selected in units of gate lines. Since the scan signals supplied to the pixels of the selected gate line are applied to the gate electrodes of the thin film transistors respectively provided in the pixels, a conductive channel is formed between the source electrode and the drain electrode of the thin film transistor.

In addition, an image signal is supplied to the pixels of the selected gate line through the data lines from the data driver, and the image signal is applied to the source electrode of the thin film transistor. Therefore, the image signal supplied to the source electrode of the thin film transistor is supplied to the drain electrode through the conductive channel during the period in which the scan signal is applied.

The image signal supplied to the drain electrode is supplied to the pixel electrode connected to the drain electrode to apply an electric field to the liquid crystal layer together with the common voltage supplied to the common electrode.

When an electric field is applied to the liquid crystal layer, the liquid crystal is rotated by dielectric anisotropy to transmit light emitted from the backlight toward the color filter substrate through the pixel electrode, the liquid crystal layer, and the common electrode from the thin film transistor array substrate. At this time, the intensity of the electric field is adjusted according to the voltage magnitude of the image signal applied to the pixel electrode, and the light transmittance of the liquid crystal layer is controlled by the strength of the electric field.

On the other hand, the voltage value of the image signal is charged in the storage capacitor provided in each pixel during the period in which the scan signal is applied. The voltage value of the image signal charged in the storage capacitor is supplied to the pixel electrode during the turn-off period of the thin film transistor to which the scan signal is not applied, thereby maintaining the driving of the liquid crystal.

In addition, when an electric field in a constant direction is continuously applied to the liquid crystal layer, the liquid crystal is deteriorated, and the afterimage occurs in the liquid crystal panel by the DC voltage component. Therefore, in order to prevent deterioration of the liquid crystal and to remove the DC voltage component, the voltage value of the image signal is applied such that the positive and negative polarities are repeated with respect to the common electrode. It is called the method.

The driving of the liquid crystal display as described above will be described in detail with reference to the accompanying drawings.

1 is an equivalent circuit diagram of a unit liquid crystal cell of a general liquid crystal panel.

Referring to FIG. 1, a unit pixel has a thin film transistor 100 having a gate electrode connected to a gate line 101, a source electrode connected to a data line 103, and a drain electrode of the thin film transistor 100. An inversion driving method of a liquid crystal panel having a liquid crystal capacitor 102 and a storage capacitor 104 connected in parallel between voltages Vcom and having an equivalent circuit of such unit pixels will be described with reference to the voltage waveform diagram of FIG. 2. It explains in detail.

1 and 2, the common voltage Vcom is applied to the common electrode, the voltage Vdata of the image signal is applied to the source electrode of the thin film transistor 100 through the data line 103, and is scanned. The signal Vg is applied to the gate electrode of the thin film transistor 100 through the gate line 101 every frame.

Therefore, first, in the turn-on period of the thin film transistor 100 to which the scan signal Vg of the nth frame is applied at high potential, a positive image signal voltage value Vdata is transferred from the source electrode to the pixel electrode through the drain electrode. Supplied to and drives the liquid crystal, and is charged to the storage capacitor 104. At this time, the positive image signal voltage value Vdata applied to the pixel electrode is gradually charged due to the influence of the liquid crystal capacitor 102 and the storage capacitor 104 in the turn-on period of the thin film transistor 100. As shown in FIG. 2, the waveform is represented by a pixel voltage Vp.

When the scan signal Vg transitions from a high potential to a low potential and the thin film transistor 100 is turned off, the gate electrode is formed due to parasitic capacitance due to the overlap between the gate electrode and the drain electrode of the thin film transistor 100. Since the voltage variation of the pixel electrode affects the pixel electrode connected to the drain electrode, a voltage drop occurs from the charged pixel voltage Vp, which is referred to as a change in pixel voltage DELTA Vp.

Meanwhile, in the turn-off period of the thin film transistor 100 to which the scan signal Vg is applied at low potential, the pixel voltage Vp charged in the storage capacitor 104 is continuously supplied to the pixel electrode to drive the liquid crystal. Will be maintained.

On the other hand, in the n + 1th frame, since the inversion driving method described above is applied, a negative image signal voltage value Vdata is supplied from the source electrode to the pixel electrode through the drain electrode, and the storage capacitor 104 is provided. Is charged.

Accordingly, the pixel voltage Vp of the n + 1th frame is ideally based on the common voltage Vcom in the turn-on, transition, and turn-off periods of the thin film transistor 100. Must show a voltage waveform that is symmetrical with That is, although the polarity of the negative and positive polarities of the pixel voltage Vp is changed based on the common voltage Vcom, the pixel voltage Vp remains the same based on the common voltage Vcom. If the difference between the pixel voltage Vp and the common voltage Vcom is different in the nth frame and the n + 1th frame, the voltage difference does not cancel each other and some voltages remain to deteriorate the liquid crystal. Reduce image quality. Therefore, a voltage difference having the same magnitude should be set based on the common voltage Vp.

Recently, liquid crystal panels have been applied to various products ranging from liquid crystal televisions, monitors, cellular phones, or personal digital assistants (PDAs). In particular, small devices such as mobile phones and personal terminals should be designed to be resistant to various temperature ranges because they are not used in certain spaces such as liquid crystal televisions or monitors, but are used in various places such as moving or inside and outside of buildings. . That is, the permittivity of the liquid crystal filled in the liquid crystal panel is a factor that reacts sensitively to changes in temperature, so that the image displayed on the liquid crystal panel when used in an office at a room temperature or in a cold outdoor environment is used. The quality can vary greatly.

The above problems are common in general liquid crystal display devices, but in particular, the red, green, and blue light sources are fixed at a certain time through red, green, and blue light lamps for one frame of an image without applying a color filter. In a field sequence color liquid crystal display device which displays color images by being turned on at intervals, image quality deterioration is large.

Therefore, an example of the image quality at various temperature conditions in the time-division type liquid crystal display showing a large difference in image quality according to the temperature will be described with reference to the accompanying drawings.

3A is a view showing an image displayed on the liquid crystal display at room temperature, and FIG. 3B is a view showing an image displayed on the liquid crystal display at low temperature.

Referring to the drawings, the image is displayed accurately in color and luminance in the normal temperature state, but the image is not clearly displayed in the low temperature state. As described above, the dielectric constant of the liquid crystal changes with the change of temperature. The change in the dielectric constant of the liquid crystal affects the liquid crystal capacitance, and the liquid crystal capacitance is inversely related to the change in the pixel voltage (ΔVp). That is, the change in the pixel voltage is changed to ΔVp due to the external temperature change. In this way, when the variation ΔVp of the pixel voltage changes, the pixel voltage waveform also changes on the basis of the common voltage, so that the voltage difference between the pixel voltage and the common voltage changes to bring about a change in the gray level of the image displayed on the screen. Therefore, the image is generally displayed darker or lighter than the original gradation. In particular, when displaying a low gradation image displayed by a minute voltage difference between a common voltage and a pixel voltage, that is, a black image, the voltage difference between the pixel voltage and the common voltage is changed according to the change of the pixel voltage, so that the accurate low gradation image is displayed. Since it cannot be displayed, the overall contrast ratio (C / R) falls.

Accordingly, the present invention has been devised to solve the above-described problems, and an object of the present invention is to change the level of the common voltage according to temperature, thereby compensating for the change of the pixel voltage waveform, thereby preventing deterioration in image quality. It is to provide a liquid crystal display device.

According to an aspect of the present invention, there is provided a liquid crystal display device comprising: first and second substrates which are bonded so that a constant cell gap is maintained and a liquid crystal layer is provided at the cell gap; A plurality of pixels arranged on the first substrate, each pixel including a pixel electrode; An image display unit composed of a combination of the pixels and displaying an image; A common electrode formed on the second substrate and applying an electric field to the liquid crystal layer together with the pixel electrode; A temperature sensor for sensing an external temperature and outputting a control signal according to the temperature; A power supply unit forming at least two common voltages; And a multiplexer for selectively applying one of the common voltages applied from the power supply unit to the common electrode according to the control signal, and correcting the magnitude of the common voltage according to temperature.

The liquid crystal display device includes a liquid crystal panel having a first and second substrates bonded to each other in a constant cell gap, and having a liquid crystal layer in the cell gap, and a driving unit for driving the liquid crystal panel.

In the liquid crystal panel, data lines arranged to be spaced apart in the vertical direction and gate lines arranged to be spaced apart in the horizontal direction cross each other, and pixels are defined at each intersection. The pixels are electrically connected to the gate line and the data line to receive signals from the gate line and the data line.

The driver includes a timing controller, a power supply, a gate driver, and a data driver. The timing controller performs a function of supplying data supplied from an external graphic card to the gate driver and the data driver in accordance with timing. The control unit applies a control signal to the gate driver, and a control signal and an image signal to the data driver. Apply.

The power supply unit receives various external powers to form various driving voltages necessary for the liquid crystal display. For example, a high voltage, a low voltage and a common voltage of a scan signal are formed and supplied to the corresponding parts.

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

In FIG. 4, among the many components constituting the liquid crystal display, the main elements related to the features of the present invention are shown, and the rest of the components are omitted.

Referring to FIG. 4, the liquid crystal display includes a liquid crystal panel 210 in which the first and second substrates facing each other are joined, a temperature sensing unit 240 for sensing an external temperature to form a control signal CS11, and a plurality of liquid crystal panels 210. The power supply unit 230 generates the common voltages Vcom1 to Vcomk and the common voltages Vcom1 to Vcomk are applied from the power supply unit 230 to the control signal CS11 of the temperature sensing unit 240. Accordingly, a multiplexer 235 selects one common voltage Vcom1 to Vcomk and applies it to the liquid crystal panel 210.

The liquid crystal panel 210 is formed by bonding the first substrate and the second substrate to maintain a constant cell gap and filling liquid crystal in the cell gap. In general, the first substrate is a thin film transistor array substrate, and the second substrate is a color filter substrate. Pixels are provided on the first substrate as a minimum unit for displaying an image, and the pixels are arranged on the first substrate in a matrix form. The first substrate is arranged such that data lines and gate lines intersect with each other at regular intervals in a vertical and horizontal direction, and a plurality of regions where the data lines and gate lines cross each other are defined as pixels, and the pixels are formed by gathering. As an area, an image display portion 215 is provided which is an area for actually displaying an image on the first substrate.

The pixels are provided with a thin film transistor which performs a function of a switching element, and the thin film transistor is electrically connected to a gate line and a data line, respectively.

The temperature detection unit 240 is made of a temperature sensor that changes the temperature to an electrical signal by reacting sensitively to the external temperature. The temperature sensing unit 240 may be installed in any portion of the liquid crystal display, but is not formed in the image display unit 215 to prevent a reduction in the aperture ratio of the image display unit 215, and the image display unit 215 It will be desirable to form the outer perimeter. On the other hand, it is formed together in the first substrate manufacturing process. The temperature detector 240 senses a temperature outside the liquid crystal display and forms a control signal CS11 for controlling the multiplexer 235 in response to the detected temperature.

The power supply unit 230 conventionally forms only one common voltage and applies it to the common electrode of the second substrate. On the other hand, the power supply 230 in the present invention forms a plurality of common voltage (Vcom1 ~ Vcomk), and supplies the formed common voltage (Vcom1 ~ Vcomk) to the multiplexer (235). The multiplexer 235 selectively outputs one common voltage among the common voltages Vcom1 to Vcomk supplied from the power supply unit 230 according to the control signal CS11 applied by the temperature sensing unit 240. . The plurality of common voltages Vcom1 to Vcomk formed by the power supply 230 are voltages to be applied to the common electrode of the second substrate according to respective temperatures, and have various levels. In addition, the temperature detector 240 applies a control signal CS11 to the multiplexer 235 so as to accurately select common voltages Vcom1 to Vcomk suitable for external temperature. Accordingly, the multiplexer 235 serves to switch the common voltages Vcom1 to Vcomk according to the control signal CS11. As such, the common voltages Vcom1 to Vcomk output from the multiplexer 235 are applied to the common electrode provided in the liquid crystal panel 210.

Although not shown in the figure, the liquid crystal display includes a timing controller. The timing controller receives external data and applies image signals and control signals to the data driver and the gate driver. The gate driver sequentially supplies a scan signal among the control signals applied from the timing controller to the gate lines, and the data driver outputs an image signal among the image signals and control signals applied from the timing controller. To the fields. That is, the timing controller performs a function of applying the data including the image signal and various control signals to the corresponding devices at timing so that the liquid crystal display device is driven. When a scan signal is applied to the thin film transistor of the pixel through the gate line, the thin film transistor is turned on and an image signal is applied to the thin film transistor through the data line during the turn-on period. As such, the image signal applied to the thin film transistor is supplied to the pixel and applied to the pixel electrode.

In addition, a plurality of lamps are provided on the rear surface of the liquid crystal panel 210 to supply light to the liquid crystal panel 210. In this case, an image is displayed by various transmittances of the liquid crystal layer provided in the cell gap of the liquid crystal panel 210. In a typical liquid crystal display device, a plurality of lamps emitting white light are arranged, and in a time-division type liquid crystal display device, lamps emitting red, blue, and green light are provided, respectively, to the liquid crystal panel 210 sequentially at regular time intervals. It will supply light.

The multiplexer 235 may be provided on the liquid crystal panel 210, but may be provided in the timing controller. Alternatively, the power supply 230 receives a control signal CS11 included in the power supply unit 230 and output from the temperature sensing unit 240 to output one common voltage from the power supply unit 230. You can also design. As described above, the multiplexer 235 may be formed separately on the liquid crystal panel 210 or included in the timing controller or the power supply 230 according to the manufacturer's decision.

The relationship between the common voltages Vcom1 to Vcomk and other signals output through the multiplexer 235 will be described in detail with reference to the accompanying drawings.

5 is a voltage waveform diagram of a unit pixel according to the present invention;

Referring to FIG. 5, first, in a turn-on period of a thin film transistor to which a scan signal Vg is applied at high potential, a positive image signal voltage value Vdata is supplied to a pixel electrode to drive a liquid crystal, and then into a pixel. The storage capacitor is charged and shown as a pixel voltage (Vp) waveform as shown.

When the scan signal Vg transitions from a high potential to a low potential and the thin film transistor is turned off, the voltage variation of the gate electrode is changed by the parasitic capacitance due to the overlap between the gate electrode and the drain electrode of the thin film transistor. The voltage drop is generated from the charged pixel voltage Vp by influencing the pixel electrode connected to the pixel electrode, which is referred to as the variation ΔVp of the pixel voltage.

In the turn-off period of the thin film transistor to which the scan signal Vg is applied at low potential, the pixel voltage charged in the storage capacitor is continuously supplied to the pixel electrode to maintain the driving of the liquid crystal.

Since the inversion driving method is applied, the voltage value Vdata of the negative image signal is applied to the pixel electrode of the pixel in the next frame, and is charged to the storage capacitor.

When the image signal is applied according to the inversion driving method, the image signal is alternately applied to the negative polarity and the positive polarity in each frame, but the absolute value of the absolute value of the liquid crystal driving voltage Vcel for driving the liquid crystal based on the common voltage Vcom is applied. The size must be the same. However, the variation ΔVp of the pixel voltage is changed according to the external temperature change, and the magnitude of the liquid crystal driving voltage Vcel is changed every frame to which the positive image signal and the negative image signal are applied. Therefore, as illustrated in FIG. 4, as the variation ΔVp of the pixel voltage changes, the magnitude of the common voltage Vcom also changes according to the change of the waveform of the pixel voltage Vp. If the variation ΔVp of the pixel voltage is smaller than before when the positive image signal is applied, the common voltage Vcom having a high level is applied, so that the positive image signal and the negative image signal are applied. In this case, the liquid crystal driving voltage Vcel is equally adjusted.

As described above, the method of applying the level of the common voltage Vcom in various sizes corresponding to the temperature does not need to be used only in the inversion driving method, and may be used regardless of the method. For example, in the general driving method in which only a positive image signal is applied instead of an inversion method in which a positive image signal and a negative image signal are alternately applied every frame, a change in pixel voltage ΔVp does not appear constant. When irregularly changed according to an external temperature, the voltage difference between the pixel voltage Vp and the common voltage Vcom is increased or decreased on average. In this case, an abnormality in image quality occurs such that an image displayed on the screen is generally displayed in dark or bright. Accordingly, the magnitude of the common voltage Vcom may be changed to maintain a constant voltage difference from the pixel voltage Vp.

As described above, the magnitude of the common voltage Vcom to be applied according to the external voltage varies depending on the area or model of the liquid crystal display. Therefore, the data value is calculated through actual measurement according to each model to determine the magnitude of the common voltage to be formed in the power supply unit, and to correspond the common voltages to the control signal formed in the temperature sensing unit in the multiplexer. will be.

As described above, if the magnitude of the common voltage is flexibly applied according to the temperature, an accurate gradation according to the image signal can be displayed on the screen, so that the contrast ratio can be increased to realize clear image quality.

As described above, the liquid crystal display according to the present invention selectively applies a common voltage in response to a pixel voltage changing according to an external temperature, thereby maintaining a constant voltage difference between the pixel voltage and the common voltage. It is possible to improve the contrast ratio and to improve the image quality.

Claims (10)

A frame or dot inversion driving liquid crystal display device, First and second substrates bonded to each other to maintain a constant cell-gap, and having a liquid crystal layer formed therein; A plurality of pixels arranged on the first substrate, each pixel including a pixel electrode; An image display unit configured of a combination of the pixels and displaying an image; A plurality of lamps supplying light to the first and second substrates; A common electrode formed on the second substrate and applying an electric field to the liquid crystal layer together with the pixel electrode; A temperature sensing unit disposed outside the image display unit to sense an external temperature and output a control signal according to the temperature; A power supply unit generating a plurality of common voltages having various levels, each reflecting a voltage variation of the pixel according to a temperature change; And And a multiplexer for selectively applying any one of the plurality of common voltages applied from the power supply unit to the common electrode according to the control signal, wherein the selected common voltage level is a value of the pixel voltage applied to the pixel electrode. In correspondence with the polarity, And the temperature sensing unit is formed outside the image display unit in the first substrate manufacturing process. delete delete The liquid crystal display of claim 1, wherein the multiplexer is formed on the first substrate. The liquid crystal display device according to claim 1, further comprising a timing controller electrically connected to the first substrate to apply a scan signal and an image signal to the first substrate. 6. The liquid crystal display of claim 5, wherein the multiplexer is provided in the timing controller. The liquid crystal display of claim 1, wherein the multiplexer is formed to be included in the power supply unit. The liquid crystal display of claim 1, wherein the lamps comprise red, green, and blue lamps. 9. The liquid crystal display device according to claim 8, wherein the red, green, and blue lamps are driven in a field sequence color method which is sequentially turned on at a predetermined time interval. delete

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