KR19990059996A - Liquid crystal display device and driving method thereof - Google Patents

Abstract

The present invention includes a plurality of gate lines, a plurality of data lines insulated from and intersecting the gate lines, and a plurality of thin film transistors each having a gate line, a data line, and a pixel electrode connected to the gate electrode, the source electrode, and the drain electrode. A liquid crystal display including a driver, a step voltage generator, and a source driver, and a driving method thereof. The gate driver applies a gate on / off voltage for turning on or off the thin film transistor to the gate line, and the step voltage generator generates a step voltage including a first voltage and a second voltage for applying to the data line. This step voltage is input to the source driver, and the source driver applies this step voltage to the data line. In this case, the first voltage is first applied to the pixel electrode to charge the pixel electrode in advance, and then the second voltage is finally applied to the pixel electrode. In this manner, the time for charging the pixel can be reduced.

Description

Liquid crystal display device and driving method thereof

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a liquid crystal display (LCD) device and a driving method thereof, and in particular, reduces gate-on time by applying a stepped data voltage, thereby flickering the LCD panel. The present invention relates to a liquid crystal display and a driving method thereof for preventing the phenomenon.

An 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.

A plurality of gate lines parallel to each other and a plurality of data lines insulated from and intersecting the gate lines are formed on the substrate of the LCD, and an area surrounded by the gate lines and the data lines forms one pixel. A thin film transistor (hereinafter referred to as TFT) is formed at a portion where the gate line and the data line cross each other, and a gate electrode, a source electrode, and a drain electrode are connected to the gate, source, and drain of the TFT, respectively. The pixel electrode is connected to the drain electrode, and a liquid crystal material is injected between the pixel electrode and the opposite substrate on which the common electrode is formed.

Referring to the operation of the LCD panel as follows.

First, the TFT is turned on by applying a gate-on voltage to a gate electrode connected to the gate line to be displayed, and then a data voltage representing an image signal is applied to the source electrode to apply the data voltage to the drain electrode. Then, the data voltage is transmitted to the pixel electrode, and an electric field is formed by the potential difference between the pixel electrode and the common electrode. The intensity of this electric field is controlled by the magnitude of the data voltage, and the amount of light transmitted to the substrate is controlled by the intensity of this electric field.

1 shows an equivalent circuit of one pixel of an LCD panel.

As shown in Fig. 1, a source electrode and a gate electrode of the TFT are connected to the data line Dn and the gate line Gn, respectively, and a pixel electrode (not shown) is connected to the drain electrode of the TFT.

The liquid crystal capacitor Ccl is formed between the pixel electrode and the common electrode COM, and the storage capacitor Cst is formed between the pixel electrode and the front gate line Gn-1. In addition, a parasitic capacitor Cgd is generated due to misalignment of the gate electrode and the drain electrode. The liquid crystal capacitor Ccl and the sustain capacitor Cst serve as loads that the TFT LCD should drive.

In Fig. 1, when the TFT is turned on by the gate on voltage, the data voltage applied to the source of the TFT is applied to the liquid crystal capacitor Ccl and the sustain capacitor Cst through the drain. At this time, the applied voltage must be maintained even after the TFT is turned off, but the parasitic capacitor between the gate and the drain causes the data voltage to be distorted by dVp. This dVp is obtained by the following equation.

.....(One)

Here, dVg means the amount of change in the gate voltage.

Since the voltage distortion is a value that always acts downward with respect to the voltage of the pixel electrode regardless of the polarity of the data voltage, the voltage actually applied to the pixel is lower by dVp than the data voltage.

This data voltage distortion dVp is called a kickback voltage, which is one cause of flickering.

In driving TFT LCDs, various methods for reducing kickback voltage have been devised and used. One of these methods is to turn down the voltage from the on gate to the off gate. A method of minimizing the effect on the pixel by using the step down rather than down is being used.

In other words, if the gate-on voltage is lowered to the gate-off voltage at once, the dVg of Equation (1) becomes large and the kickback voltage becomes large, but if the voltage is lowered over several steps, the dVg in each phase becomes smaller and the kickback voltage Minimize the impact by This dVg can be made smaller as more steps are taken from the gate-on voltage to the gate-off voltage, thereby further reducing the influence of the kickback voltage.

However, as the TFT LCD becomes increasingly finer, the time from the gate on voltage to the gate off voltage is gradually shortened, so there is a problem in that it is limited to perform many steps from the gate on voltage to the gate off voltage.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and shortens the time for which the data voltage is charged to the pixel electrode, thereby reducing the gate-on period, thereby increasing the time from the gate-on voltage to the gate-off voltage. This is to minimize the effect of the kickback voltage by going through the step of switching to the gate-off voltage.

1 is a diagram showing an equivalent circuit of one pixel of a liquid crystal display panel.

2 is a diagram illustrating waveforms of voltages applied to pixels.

3 is a diagram showing a stepped data voltage used in the embodiment of the present invention.

FIG. 4 is a block diagram of a liquid crystal display for applying the data voltage of FIG. 3.

FIG. 5 is a diagram schematically illustrating the step voltage generator of FIG. 4. FIG.

FIG. 6 is a diagram illustrating in detail a step voltage generator of FIG. 4.

According to the liquid crystal display device of the present invention for achieving the above object,

A plurality of gate lines, a plurality of data lines insulated from and intersecting the gate lines, a plurality of thin film transistors each having a gate electrode, a source electrode, and a drain electrode connected to the gate line, the data line, and the pixel electrode; And a step voltage generator and a source driver.

The gate driver applies a gate on / off voltage for turning on or off the thin film transistor to the gate line, and the step voltage generator generates a step voltage for applying the data line. This step voltage is input to the source driver, and the source driver applies this step voltage to the data line.

Here, the step voltage is composed of a first voltage and a second voltage. The first voltage is applied to the pixel electrode first to charge the pixel electrode in advance, and the second voltage is the voltage applied finally to the pixel electrode.

In this case, the step voltage generator may include a charging voltage generator for generating the first voltage, a gray voltage generator for generating the second voltage, and a selection unit for selecting the first voltage and the second voltage.

On the other hand, according to the driving method of the liquid crystal display device of this invention,

Applying a gate on / off voltage to the gate line for turning the thin film transistor on or off, generating a step voltage, and applying a step voltage to the data line, respectively.

The generating of the staircase voltage may include generating a first voltage applied to the pixel electrode first to charge the pixel electrode in advance, and generating a second voltage finally applied to the pixel electrode.

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

When a constant data voltage is applied to the gate line to which the gate-on voltage is applied in the TFT-LCD, the voltage charged to the pixel by the RC delay of the data line is expressed in the form of an exponential function over time.

The waveform of the voltage applied to such a pixel is shown in FIG.

As shown in Fig. 2, when data voltages V1 and V2 (V1 > V2) having different magnitudes are applied to the gate line to which the gate-on voltage is applied, the voltages Vp1 and Vp2 charged to the pixels increase exponentially. do.

At this time, the voltages Vp1 and Vp2 charged in the pixel are respectively obtained by the following equations when the initial voltage is assumed to be zero.

Vp1 = V1 (1-e ^{-t / τ} )

Vp2 = V2 (1-e ^{-t / τ} )

Τ is a time constant and is obtained by multiplying the resistance of the data line and the capacitor.

In FIG. 2, when the voltages Vp1 and Vp2 applied to the pixels after the ta time have elapsed, it can be seen that the case where the data voltage is V1 is more charged than when the case is V2. That is, the larger the data voltage applied, the larger the voltage charged to the pixel electrode.

In consideration of this, as shown in FIG. 3, when the data voltage is applied in a stepped manner, the time for charging the gray voltage to the pixel electrode can be reduced.

In FIG. 3, a voltage Vt larger than the gray voltage Va to be applied to the pixel is first applied to the data line for t1 time. Then, the pixel voltage is charged faster than when the gray voltage Va is applied from the beginning. Then, when the appropriate voltage is charged in the pixel, the gradation voltage Va is applied for t2 time.

In this way, if the data voltage is initially applied at a high voltage Vt and then the gray voltage Va is applied to the pixel electrode, the pixel voltage can be charged faster than when Va is applied from the beginning. Even if it is small, the pixel voltage can be charged with the gray scale voltage. Therefore, the gate off time can be extended by that much.

FIG. 4 is a diagram illustrating an embodiment of a liquid crystal display device for applying such a stepped data voltage (hereinafter, referred to as a "stepped voltage").

In FIG. 4, the liquid crystal display according to the exemplary embodiment of the present invention includes an LCD panel 100, a gate on / off voltage generator 200, a gate driver 300, a source driver 400, and a step voltage generator 500. Is done.

The LCD panel 100 includes a plurality of data lines and a plurality of gate lines insulated from and intersecting the data lines. The plurality of gate lines and the region surrounded by the plurality of gate lines form one pixel region, and a thin film transistor is formed in the pixel region. Gate lines are respectively connected to gate electrodes of the thin film transistor, and data lines are respectively connected to source electrodes.

The gate on / off voltage generator 200 generates a gate / off voltage for turning on and off the thin film transistor, respectively, and the gate on / off voltage is applied to the gate line through the gate driver 300, respectively. The step voltage generator 500 generates the step voltage shown in FIG. 3, and the source driver 400 receives the step voltage and applies it to each data line.

5 is a diagram schematically illustrating the step voltage generator 500.

As shown in FIG. 5, the step voltage generator 500 includes a charge voltage generator 510, a gray voltage generator 520, and a selector 530.

The charging voltage generator 520 is for generating the gray voltage Va shown in FIG. 3. That is, it is applied to the pixel electrode in advance to charge the pixel voltage quickly. The gray voltage generator 520 is for generating the Va voltage shown in FIG. The gray voltage generator 520 generates voltages having a plurality of gray levels, and selects one of the voltages and applies Va to the pixel electrode.

The voltage Vt output from the charging voltage generator 510 and the voltage Va output from the gray voltage generator 520 are input to the selector 530. The selector 530 selects one of these two voltages Va and Vt and transfers it to the source driver. That is, the selector 530 selects the voltage Vt during the first t1 time, selects the voltage Va during the t2 time, and transfers the selected voltages to the source driver.

6 is a diagram illustrating in detail the step voltage generator 500.

In FIG. 6, the step voltage generator 500 includes a plurality of resistors Ra, Rb, R1, R2, R3, ..., Rn-1, Rn 540 connected in series between the reference voltage Vref and the ground point. ), A plurality of multiplexers (MUX) 550 respectively connected to the contacts between the resistors, a plurality of buffers 560 respectively connected to one end of the multiplexer, and a voltage selection signal Sv for selecting a voltage.

In Fig. 6, the resistors 540 connected in series are distribution resistors for distributing between the reference voltage Vref and the ground voltage. For example, the potential of the contact point of the resistor Ra and the resistor Rb is obtained by the following equation.

In a similar manner, the potential of the contact between each resistor can be found using the reference voltage and the distribution resistors.

In FIG. 6, the potential of the contact between the resistors Ra and Rn-1 produces the charging voltage Vt shown in FIG. 3, and the potential of the contact between Rb and Rn produces the gray scale voltage. One of these gray voltages produces the voltage Va of FIG.

The voltage selection signal Sv is connected to the multiplexer, respectively, and selects one of a charging voltage and a gray voltage according to the operation of the multiplexer. At this time, the charging voltage Vt is first output, and then the gradation voltage Va is output.

The charging voltages for the respective gradation voltages V1 and V2 are different from each other, and the charging voltage Vt may be used by connecting a higher or lower voltage as necessary.

As described above, according to the present invention, the gate-on period can be reduced by applying the step voltage to the pixel to shorten the time for which the desired gray-scale voltage is charged to the pixel electrode, thereby reducing the gate-on voltage to the gate-off voltage. By prolonging the time, it is possible to sufficiently convert the gate on voltage to the gate off voltage, thereby minimizing the influence of the kickback voltage.

Claims (6)

Multiple gate lines, A plurality of data lines insulated from and intersecting the gate lines; A plurality of thin film transistors having a gate electrode connected to the gate line, a source electrode connected to the data line, and a drain electrode connected to the pixel electrode; A gate driver configured to apply a gate on / off voltage to the gate line to turn the thin film transistor on or off; A step voltage generator for generating step voltages to be applied to the data lines; And a source driver for applying the step voltage to the data line. In claim 1, And the staircase voltage is first applied to the pixel electrode to previously charge the pixel electrode, and a second voltage finally applied to the pixel electrode. In claim 2, The step voltage generation unit A charging voltage generator for generating the first voltage; A gradation voltage generator for generating the second voltage; And a selector for selecting the first voltage and the second voltage. In claim 2, The step voltage generation unit First voltage generating means including a first resistor connected to a reference voltage and a second resistor connected in series with the first resistor; Second voltage generating means including a plurality of resistors connected in series between the second resistor and the ground point; A plurality of buffers each connected to a contact between the first and second resistors, and a plurality of contacts between the resistors; A multiplexer for selecting one of the potentials of the plurality of contacts according to a voltage selection signal, The first voltage is a potential of a contact between the first resistor and the second resistor, And the second voltage is one of contact voltages between the plurality of resistors. A driving method of a liquid crystal display device comprising a thin film transistor having a source electrode connected to a gate line, a gate electrode connected to a data line, and a drain electrode connected to a pixel electrode. Applying a gate on / off voltage to the gate line for turning on or off the thin film transistor; Generating a step voltage for applying to the data line; And applying the step voltage to the data line. In claim 5, The step of generating the step voltage Generating a first voltage applied to the pixel electrode first to charge the pixel electrode in advance; And generating a second voltage finally applied to the pixel electrode.