KR20000039816A - Liquid crystal display device and method for driving the same - Google Patents

Abstract

PURPOSE: A liquid crystal display device and method for driving the same are provided to prevent liquid crystal display device from being degenerated due to voltage deviation of common electrode voltage. CONSTITUTION: A liquid crystal display device includes a liquid crystal display device panel(100), a gate driver(200), a data driver(300), and a common electrode voltage generator(400). The liquid crystal display device panel(100) includes a plurality of data lines, a plurality of thin film transistors, and a pixel electrode. The gate driver(200) applies gate signal for turning on/off the thin film transistor on the gate line. The data driver(300) applies data voltage representing image signals on the data line. The common electrode voltage generator(400) implies a common electrode voltage on the common electrode of the panel. The common electrode voltage includes at least one step in the range of 0.6-0.8 of the high voltage during switching from a lower voltage to the high voltage.

Description

Liquid Crystal Display and Driving Method of Liquid Crystal Display

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display (hereinafter referred to as LCD) and a driving method thereof. In particular, a thin film transistor liquid crystal display (thin film) for improving display quality by applying a common electrode voltage in a step form transistor LCD, hereafter abbreviated TFT-LCD) and its driving method.

A TFT-LCD is a display device that obtains a desired image signal by applying an electric field to a liquid crystal material having an anisotropic dielectric constant injected between two substrates, and controlling the amount of light transmitted through the substrate by adjusting the intensity of the electric field.

On the substrate of such a TFT-LCD, a plurality of gate lines parallel to each other and a plurality of data lines insulated from and intersecting the gate lines are formed, and an area surrounded by the gate lines and the data lines defines one pixel. TFTs are formed at portions where the gate lines and the data lines of each pixel cross each other.

1 shows an equivalent circuit for a unit pixel in a typical TFT-LCD.

As shown in Fig. 1, the gate electrode g, the source electrode s, and the drain electrode d of the TFT 10 are connected to the gate line Gn, the data line Dm, and the pixel electrode P, respectively. Connected. A liquid crystal material is formed between the pixel electrode P and the common electrode Com, which is equivalently represented as a liquid crystal capacitor Clc. The storage capacitor Cst is formed between the pixel electrode and the front gate line Gn-1, and the parasitic capacitance Cgd is generated between the gate electrode and the drain electrode due to misalignment. The liquid crystal capacitor Ccl and the storage capacitor Cst act as loads that the TFT-LCD should drive.

The operation of the TFT-LCD will be described as follows.

First, the TFT 10 is conducted by applying a gate-on voltage to the gate electrode connected to the gate line Gn to be displayed, and then a data voltage representing an image signal is applied to the source electrode s to drain the data voltage. It is applied to the electrode (d). Then, the data voltage is applied to the liquid crystal capacitor Clc and the storage capacitor Cst through the pixel electrode P, respectively, and an electric field is formed by the voltage difference between the pixel electrode Cp and the common electrode Com. In this case, since the liquid crystal deteriorates when an electric field in the same direction is continuously applied to the liquid crystal material, the LCD panel drives the polarity of the common electrode with respect to the image signal to be positive and negative in order to prevent deterioration of the liquid crystal. The driving method is called an inverting driving method.

Hereinafter, a conventional inversion driving method will be described with reference to FIGS. 2 and 3.

FIG. 2 illustrates a common electrode voltage output from a common voltage generator in a conventional inversion driving method, and FIG. 3 illustrates a case in which white is displayed when the common electrode voltage shown in FIG. 2 is applied to a panel of a liquid crystal display. And waveforms of the common electrode voltage measured on the actual liquid crystal display panel when displaying black.

As shown in FIG. 2, the common electrode voltage has a square wave shape that switches a low voltage and a high voltage. Typically, the low voltage is about 0.3V and the high voltage is about 3.3V or 5V.

In the conventional inversion driving method, the common electrode voltage output from the common voltage generator is in the form of a square wave as shown in FIG. 2, but the common electrode voltage measured in the panel is applied to the data line for the following reasons. The waveform changes in accordance with the magnitude of the gradation voltage. In general, the dielectric constant of the liquid crystal material changes according to the intensity of the electric field applied to the liquid crystal material, that is, the difference between the gradation voltage and the common voltage, and thus the liquid crystal capacitance also changes according to the strength of the electric field. More specifically, in the liquid crystal of the normal twisted nematic (TN) mode, the larger the intensity of the electric field applied to the liquid crystal material, the more the liquid crystal capacity, that is, the load increases, and as a result, the actual common electrode measured in the panel of the liquid crystal display The voltage changes according to the size of the liquid crystal capacitance.

For example, in a normally white mode liquid crystal displaying white when no pixel voltage is applied, a difference between the common electrode voltage and the gray voltage is small when a gray voltage displaying white is applied to the entire screen. As a result, since the liquid crystal capacitance becomes smaller, the common electrode voltage actually applied to the panel of the liquid crystal display device exhibits the characteristic of overshooting as shown by the dotted waveform of FIG. 3. On the other hand, when a gray voltage displaying black is applied to the entire screen, the difference between the common electrode voltage and the gray voltage is increased, and thus the liquid crystal capacitance is increased. Therefore, the common electrode voltage applied to the liquid crystal display panel is a solid waveform in FIG. It shows severely distorted characteristics. As a result, there is a significant difference in the waveform of the common electrode voltage when displaying white and when displaying black.

This difference is a serious problem if the same color is not displayed on the entire screen. That is, for example, when a black square is displayed on a white background, the waveform of the common electrode voltage actually applied to the panel of the liquid crystal display device is significantly different between the pixels adjacent to the white and the black in the horizontal line. crosstalk). This causes a problem that the display quality of the LCD deteriorates.

The present invention has been made to solve such a problem, and is to prevent display quality deterioration due to deformation of the common electrode voltage.

1 is an equivalent circuit of a unit pixel of a general TFT-LCD,

2 shows a common electrode voltage in the conventional line inversion driving method,

FIG. 3 shows the waveform of the common electrode voltage of FIG. 2 when displaying white and black on the screen, FIG.

4 is an overall configuration diagram of a liquid crystal display according to an embodiment of the present invention;

5 is a detailed view of a common electrode voltage generator according to an embodiment of the present invention;

6 illustrates a waveform of a common electrode voltage having one step voltage according to an embodiment of the present invention.

7 shows waveforms of a common electrode voltage according to an embodiment of the present invention when displaying white and black;

8 illustrates waveforms of a common electrode voltage having a plurality of step voltages according to an embodiment of the present invention.

The liquid crystal display of the present invention includes a panel, a gate driver, a data driver, and a common electrode voltage generator, and the panel includes a plurality of gate lines, a plurality of data lines insulated from and intersecting the plurality of gate lines, and connected to the gate lines. A plurality of thin film transistors having a gate electrode and a source electrode connected to the data line, a pixel electrode connected to a drain electrode of the thin film transistor, and a gate driver for turning the thin film transistor on and off on the gate line. The gate driver applies a gate signal, and the data driver applies a data voltage representing an image signal to the data line, and the common electrode voltage generator is configured to a voltage of 0.6 to about the high voltage of the common electrode voltage when switching from a low voltage to a high voltage to the common electrode of the panel. Staircase with one or more steps in the 0.8 size range Tube voltage is applied to the electrode.

In addition, the common electrode voltage may apply a stepped waveform having one or more steps in a size range of about 0.2 to 0.4 of the high voltage when switching from the high voltage to the low voltage.

The common electrode voltage generator is connected in series between the power supply voltage, the power supply voltage, and the grounding point to divide the power supply voltage, and has a plurality of resistance lines having a plurality of terminals for outputting voltage to the outside between the resistors, It is preferable to include a switch for switching between terminals, a control unit for controlling the selection of the terminal between the switching time and the resistance row of the switch,

The common electrode voltage generation unit preferably outputs a stepped common electrode voltage having a step voltage smaller than the power supply voltage by switching of a switch.

The driving method of the liquid crystal display device of the present invention is a method of driving a liquid crystal display device that displays a desired image by applying an electric field corresponding to a difference between a common electrode voltage and a data voltage representing an image signal to the liquid crystal material and adjusting the intensity of the electric field. In the common electrode voltage, when switching from a low voltage to a high voltage, a stepped waveform having one or more step voltages in the range of 0.6 to 0.8 of the high voltage magnitude is applied.

In addition, the common electrode voltage may apply a stepped waveform having one or more step voltages in the range of 0.2 to 0.4 of the high voltage size when switching from the high voltage to the low voltage.

The step voltage of the common electrode voltage may be applied for 0.05 to 0.15 of the horizontal synchronizing signal period which is a holding time of the high voltage and the low voltage of the common electrode voltage.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

4 is an overall configuration diagram of a liquid crystal display according to an exemplary embodiment of the present invention, and FIG. 5 is a detailed view of a common electrode voltage generator according to an exemplary embodiment of the present invention.

The liquid crystal display according to the present exemplary embodiment includes a TFT-LCD panel 100, a gate driver 200, a data driver 300, and a common electrode voltage generator 400.

The data driver 300 applies a data voltage representing an image signal to each data line D of the TFT-LCD panel 100, and the gate driver 200 applies a data voltage applied to each data line D to the pixel. A gate voltage for turning on the TFT of each pixel is output so that it can be applied to the electrode.

In the TFT-LCD panel 100, a data line D transferring a data voltage from the data driver 300 and a gate line G transferring a gate voltage from the gate driver 200 cross each other. The source electrode and the gate electrode of the TFT of each pixel are connected to this data line and the gate line. In FIG. 4, for convenience of description, only the equivalent circuit for one pixel is illustrated in the TFT-LCD panel 100, where the voltage charged in the pixel electrode is represented by Vp and the voltage charged in the storage capacitor electrode is represented by Vst.

Hereinafter, the operation of the common electrode voltage generator of the exemplary embodiment of the present invention will be described with reference to the drawings.

FIG. 6 is a waveform of a common electrode voltage output from the common voltage generator 400 according to the present embodiment, and FIG. 7 shows white and black colors on a TFT-LCD in a normally white mode liquid crystal display. The waveform of the common electrode voltage measured in the TFT-LCD panel is shown.

In the present invention, the degradation of the display quality caused by the deformation of the waveform of the conventional common electrode voltage is solved by applying the common electrode voltage in the form of a step as described below.

Since the conventional square wave type common electrode voltage Vcom is rapidly changed when a low voltage is input and then switches to a high voltage, there is a distortion of the common electrode voltage.

In this embodiment, before switching the common electrode voltage from the high voltage to the low voltage and from the low voltage to the high voltage, as shown in Fig. 6, a step voltage having a magnitude between the high voltage and the low voltage is set for a predetermined period. Rough while. At this time, the period for applying the step voltage is appropriately about 0.05 to 0.15 of the horizontal synchronization signal period, which is a period in which the high voltage and the low voltage are maintained in the waveform of the common electrode voltage. As such, when the step voltage is applied for a predetermined period, the voltage difference between the gray level voltages of the liquid crystals of the pixel and the common electrode voltage is reduced at the instant of switching, thereby reducing the load on the liquid crystal. The distortion of the waveform is also reduced. In this case, the step voltage should be a voltage between the high voltage and the low voltage in order to alleviate the load burden of the liquid crystal as described above. The magnitude of the step voltage is about 0.6 to 0.8 of the high voltage when switching from the low voltage to the high voltage. When switching from high voltage to low voltage, about 0.2 ~ 0.4 of high voltage is appropriate. This is to reduce the load burden when rising or falling.

In addition, as shown in FIG. 8, a plurality of step voltages may be applied to the common electrode in order to prevent degradation of display quality of the liquid crystal display. That is, a stepped waveform having two or more step voltages sequentially applied from a low voltage may be applied as the common electrode voltage. In this case, the load burden on the liquid crystal is further reduced than when there is one step voltage.

For example, in a normally white mode in which a liquid crystal displays white in a state in which a pixel voltage is not applied, a pixel is displayed due to an intermediate step voltage when white is displayed in the entire screen. By making the difference between the voltage and the common electrode voltage slightly, it is possible to prevent the load of the capacitor of the pixel from being drastically reduced at the moment of switching, thereby avoiding the overshooting phenomenon appearing in the waveform of the conventional common electrode voltage. On the other hand, when black is displayed on the entire screen, the difference between the pixel voltage and the common electrode voltage is reduced at the instant of switching due to the intermediate step voltage, and as a result, a sudden increase in the load of the capacitor of the pixel at the moment of switching is alleviated. Eliminate the distortion of the waveform of the electrode voltage. The waveform of the common electrode voltage when the white is displayed on the entire screen is the same as the dotted line in FIG. 7, and the waveform of the common electrode voltage when the black is displayed on the entire screen is the same as the solid line in FIG. Here, only the case where the load change of the pixel is the maximum and minimum of white and black has been described as an example, but the same applies when displaying other colors.

Accordingly, by applying the common electrode voltage having the waveform as shown in FIG. 6, there is almost no difference between the waveform of the common electrode voltage when displaying white and when displaying black on the screen as shown in FIG. As a result, even if black and white are displayed on the same line, there is no difference in the waveform of the common electrode voltage, thereby reducing the crosstalk phenomenon, thereby improving display quality.

Hereinafter, a process of generating a common electrode voltage having a waveform as shown in FIGS. 6 and 8 will be described.

The common electrode voltage generator 400 switches a power supply voltage Vcc, a plurality of resistors R1, R2, R3,..., Rn connected in series with the power supply voltage, and a terminal between the plurality of resistors. The control unit 410 is connected to the switch SW and controls the connection terminal and the connection time of the switching SW.

A plurality of resistors (R1, R2, R3, ..., Rn) divide the power supply voltage (Vcc), the switch is connected to a plurality of terminals (V1, V2, V3, ..., Vn) between the resistors The common electrode voltage Vcom is output to the outside. At this time, if a resistor having the same resistance value is used, a desired voltage can be easily output.

That is, the switch SW properly connects the electrode voltage terminal Vcom to one of the plurality of terminals in order to generate the waveform shown in FIGS. 6 and 8. The switching of such switching is performed by the control of the control unit 10. Under the control of the controller, the switch can output a waveform of the common electrode voltage as shown in FIG. 6 by switching.

The present invention is not limited to the above-described embodiment, and various modifications are possible, and an equivalent device capable of displaying the waveform of the common electrode voltage of the present embodiment can also be implemented.

According to the liquid crystal display of the present invention, even if various colors are displayed, distortion of the waveform of the common electrode voltage can be prevented and display quality can be improved.

Claims (8)

A plurality of thin film transistors having a plurality of gate lines, a plurality of data lines insulated from and intersecting the plurality of gate lines, a gate electrode connected to the gate line and a source electrode connected to the data line, and a drain of the thin film transistor A liquid crystal display panel having pixel electrodes connected to the electrodes; A gate driver for applying a gate signal for turning on / off the thin film transistor to the gate line; A data driver for applying a data voltage representing an image signal to the data line, And a common electrode voltage generator configured to apply a stepped common electrode voltage having at least one step in a size range of about 0.6 to 0.8 of the high voltage of the common electrode voltage when switching from a low voltage to a high voltage to the common electrode of the panel. Liquid crystal display. In claim 1, The common electrode voltage is, A liquid crystal display having a step shape having one or more steps in a size range of about 0.2 to 0.4 of a high voltage when switching from a high voltage to a low voltage. The method of claim 1 or 2, The step voltage of the common electrode voltage is, And 0.05 to 0.15 of a horizontal synchronization signal cycle which is a holding time of the high voltage and the low voltage of the common electrode voltage. In claim 1, The common electrode voltage generator, Power supply voltage, A plurality of resistor rows having a plurality of terminals connected in series between the power supply voltage and a ground point to divide the power supply voltage and output a voltage divided externally between resistors; A switch for switching between a plurality of terminals between the plurality of resistor rows; And a controller for controlling selection of a terminal between the switching time of the switch and the resistor row. In claim 4, The common electrode voltage generator, And a stepped common electrode voltage having a step voltage smaller than the power supply voltage by switching of the switch. In the driving method of the liquid crystal display device for displaying a desired image by applying an electric field according to the difference between the common electrode voltage and the data voltage representing the image signal, and adjusting the intensity of the electric field, And the common electrode voltage is a stepped waveform having one or more step voltages in a range of 0.6 to 0.8 of a high voltage size when switching from a low voltage to a high voltage. In claim 6, And wherein the common electrode voltage is a stepped waveform having one or more step voltages in a range of 0.2 to 0.4 of a high voltage size when switching from a high voltage to a low voltage. In claim 6 or 7, The step voltage of the common electrode voltage is, And a method of applying the liquid crystal display device during a period of 0.05 to 0.15 of a horizontal synchronizing signal period which is a holding time of the high voltage and the low voltage of the common electrode voltage.