KR20050033279A - Common voltage generator and liquid crystal display using the same - Google Patents

Abstract

A common voltage generator and an LCD(Liquid Crystal Display) using the common voltage generator are provided to optimize a common voltage even when a temperature is varied and minimize deterioration of display quality due to a temperature variation. A common voltage generator includes a common voltage generating unit(40) for generating a common voltage, and a common voltage compensation unit for controlling the common voltage output from the common voltage generating unit in response to a temperature. The common voltage compensation unit includes a thermistor whose resistance is varied in inverse proportion to the temperature. The common voltage compensation unit includes a temperature sensor for sensing the temperature, a voltage dividing circuit for dividing the output voltage of the common voltage generating unit to generate a plurality of different common voltages, a voltage selector for selecting one of the common voltages, and a controller for controlling the voltage selector in response to the temperature sensed by the temperature sensor.

Description

Common Voltage Generator and Liquid Crystal Display Using The Same

The present invention relates to a voltage generator, and more particularly, to a common voltage generator for generating a common voltage supplied to pixels of a liquid crystal display. In addition, the present invention relates to a liquid crystal display device using the common voltage generator to prevent deterioration of display quality generated at the time of temperature change.

Conventional liquid crystal display devices display an image by adjusting the light transmittance of the liquid crystal using an electric field. To this end, the liquid crystal display includes a liquid crystal panel in which liquid crystal cells are arranged in a matrix and a driving circuit for driving the liquid crystal panel.

In the liquid crystal panel, as shown in FIG. 1, the thin film transistor Thin for driving the liquid crystal cell Clc at the intersection of the gate line GL and the data line DL and at the intersection of the gate line GL and the data line GL. Film Transistor (hereinafter referred to as "TFT") is formed. In addition, a storage capacitor Cst is formed in the liquid crystal panel to maintain the voltage of the liquid crystal cell Clc. When the data voltage is applied to the pixel electrode 11 and the common voltage Vcom is applied to the pixel electrode 11, the liquid crystal cell Clc changes the arrangement of the liquid crystal molecules by the electric field applied to the liquid crystal layer. It controls the amount of light or blocks the light. The data voltage is supplied at a gamma voltage preset according to the driving voltage characteristic of the liquid crystal cell.

2 illustrates a common voltage Vcom and gamma voltages Gamma1 to Gamma10 when driving the liquid crystal panel in dot inversion in the normally white mode.

Referring to FIG. 2, in order to drive the liquid crystal panel in dot inversion, the polarities of the gamma voltages Gamma1 to Gamma10 are inverted approximately every one horizontal period, and the common voltage Vcom is supplied with a constant DC voltage. In the normally white mode, the smaller the potential difference between the gamma voltages Gamma1 to Gamm10 and the common voltage Vcom increases the transmittance of light passing through the liquid crystal layer, while the gamma voltages Gamma1 to Gamma10 and the common voltage Vcom are increased. The larger the potential difference is, the smaller the transmittance of light passing through the liquid crystal layer.

3 illustrates the scan pulse SCP supplied to the gate line GL and the voltage Vlc charged in the liquid crystal cell Clc.

Referring to FIG. 3, the scan pulse SCP is set to a gate high voltage Vgh, which is set to a voltage for turning on the TFT, and a voltage for turning off the TFT. Swing between the gate low voltage (Vgl). During the scanning period in which the scan pulse SCP maintains the gate voltage, the liquid crystal cell Vlc charges the data voltage Vdata supplied as the gamma voltage and maintains the charged voltage for a predetermined time. However, if the common voltage Vcom is not set to the optimum voltage or is not maintained at the optimum voltage and is shifted by the offset voltage Voffset in the drawing toward the positive or negative polarity, the offset voltage Voffset is provided at both ends of the liquid crystal cell Clc. Residual DC voltage is applied. As a result, the liquid crystal cell Clc periodically charges the voltage of which the voltage is increased by the offset voltage Voffset and the voltage of which the voltage is lowered by the offset voltage without charging the data voltage Vdata corresponding to the video data. If the common voltage Vcom is not optimized in this manner, flicker may occur on the screen in units of frame periods, or afterimages may remain on the screen. For example, when the LCD is driven at 60 Hz, the screen flickers at a 30 Hz period. The feed through voltage (ΔVp) causing such flicker and afterimage may be described through Equation 1 below. The feed through voltage ΔVp is defined as a difference voltage between the data voltage supplied to the data line DL and the pixel voltage charged in the liquid crystal cell Clc. The larger the feed-through voltage DELTA Vp, the more flicker and afterimages become.

Here, 'Cgd' is a parasitic capacitance formed between the gate terminal and the drain terminal of the TFT as shown in FIG. 1, and ΔVg is the gate high voltage Vgh and the gate low voltage Vgl of the scan pulse SCP. Is the difference voltage.

Bar flicker or afterimage as shown in Equation 1 may be generated due to the parasitic capacitance Cgd of the TFT or the gate voltage of the scan pulse, but may also be generated due to the capacitance of the liquid crystal cell Clc. The capacitance of the liquid crystal cell Clc is a function related to the common voltage Vcom. Therefore, if the common voltage Vcom is not optimized, the feed through voltage DELTA Vp becomes large and the display quality is degraded.

The common voltage Vcom is generally optimized based on driving the liquid crystal display at room temperature. However, when the liquid crystal display device is used in a high temperature environment or a low temperature environment, the response characteristic of the liquid crystal cell Clc is changed to change the common voltage Vcom felt by the liquid crystal cell Clc. For example, even if the common voltage Vcom optimized at room temperature is supplied to the liquid crystal cell Clc in the low temperature environment and the liquid crystal cell Clc in the high temperature environment, the common voltage Vcom is the liquid crystal cell in the low temperature environment. It is felt lower in Clc) while higher in liquid crystal cell Clc in a high temperature environment.

Accordingly, it is an object of the present invention to provide a common voltage generator capable of optimizing a common voltage even when the use temperature is changed.

An object of the present invention is to provide a liquid crystal display device using the common voltage generator to prevent deterioration of display quality generated when a temperature is changed.

In order to achieve the above object, a common voltage generator according to an embodiment of the present invention includes a common voltage generator for generating a common voltage; And a common voltage compensator for adjusting the common voltage output from the common voltage generator according to temperature.

The common voltage compensator includes a thermistor whose resistance is changed in inverse proportion to the temperature.

The common voltage compensation unit includes a temperature sensor for sensing the temperature; A voltage divider circuit for dividing an output voltage of the common voltage generator to generate a plurality of common voltages having different voltages; A voltage selector for selecting any one of the common voltages; And a controller for controlling the voltage selector according to the temperature sensed by the temperature sensor.

According to an exemplary embodiment of the present invention, a liquid crystal display includes: a liquid crystal panel having a pixel electrode supplied with a data voltage and a common electrode supplied with a common voltage; A common voltage generator for generating the common voltage; And a common voltage compensator for adjusting the common voltage output from the common voltage generator according to temperature.

The common voltage compensator includes a thermistor whose resistance is changed in inverse proportion to the temperature.

The common voltage compensation unit includes a temperature sensor for sensing a temperature of the liquid crystal panel; A voltage divider circuit for dividing an output voltage of the common voltage generator to generate a plurality of common voltages having different voltages; A voltage selector for selecting any one of the common voltages and supplying the selected common voltage to the common electrode; And a controller for controlling the voltage selector according to the temperature sensed by the temperature sensor.

Other objects and features of the present invention in addition to the above objects will become apparent from the description of the embodiments with reference to the accompanying drawings.

Hereinafter, exemplary embodiments of the present invention will be described with reference to FIGS. 4 to 7.

Referring to FIG. 4, the common voltage generator according to the first exemplary embodiment of the present invention includes a common voltage generator 40 generating a common voltage Vcom and a thermistor connected to an output terminal of the common voltage generator 40. (Thermistor) (TH).

The common voltage generator 40 is an operational amplifier in which a first resistor R1, a variable resistor VR1, a second resistor R2, and a divided VDD voltage are connected between a VDD voltage source and a base voltage source GND. (Operational Amplifier: OP amp), the fifth resistor (R5) and the output node (n1) connected between the output terminal of the operational amplifier (OP amp) and the output node (n1) connected to the output terminal of the common voltage generator 40 And a capacitor C connected between the base voltage source GND.

The VDD voltage is divided by the voltage divider circuit including the first resistor, the variable resistor VR, and the second resistor R2 and then supplied to the non-inverting input terminal of the operational amplifier OP amp. When the resistance value of the variable resistor VR is changed, the common voltage Vcom is adjusted.

The operational amplifier OP amp amplifies the input voltage according to the amplification ratio determined by the third resistor R3 and the fourth resistor R4. DC or AC voltage Vin is supplied to the inverting terminal of the operational amplifier OP amp. The voltage level or polarity of the common voltage Vcom may change depending on the voltage Vin.

The fifth resistor R5 and the capacitor C form an integrator to smooth the output voltage of the operational amplifier OP amp to suppress fluctuation of the common voltage Vcom.

The thermistor TH is connected between the output terminal of the common voltage generator 40 and the ground voltage source GND to adjust the common voltage Vcom according to the temperature. This thermistor TH has a characteristic of a negative resistance temperature coefficient in which the resistance value decreases as temperature increases. Therefore, when the temperature decreases, the resistance value of the thermistor TH increases, so that the common voltage Vcom increases. On the other hand, when the temperature increases, the resistance value of the thermistor TH is lowered, and thus the common voltage Vcom is lowered.

As a result, the common voltage Vcom output from the common voltage generator 40 is converted into the corrected common voltage CVcom by the change of the resistance of the thermistor TH according to the temperature change.

The thermistor TH may be directly mounted on the glass substrate of the liquid crystal display device or may be installed adjacent to the glass substrate of the liquid crystal display device together with the common voltage generator 40. It can be mounted on the).

6 shows a common voltage generator according to a second embodiment of the present invention. In the common voltage generator of FIG. 6, the same components as those of the common voltage generator of FIG. 4 are denoted by the same reference numerals, and detailed description thereof will be omitted.

Referring to FIG. 6, a common voltage generator according to a second exemplary embodiment of the present invention includes a common voltage generator 40 generating a common voltage Vcom and a divided voltage connected to an output terminal of the common voltage generator 40. A circuit 42, a voltage selector 44 connected to the output terminals of the voltage divider circuit 42, a temperature sensor 46 for sensing the ambient temperature, an output terminal and the voltage selector 44 of the temperature sensor 46; Controller 48 is connected between the control terminals.

The voltage dividing circuit 42 includes a plurality of resistors R6 to R9 connected to the output terminal of the common voltage generator 40 and outputs common voltages Vcom1 to Vcom5 having different voltages from a node between the resistors. do. These common voltages Vcom1 to Vcom5 are supplied to the voltage selector 44.

The temperature sensor 46 senses the ambient temperature and supplies a temperature sensing signal St indicating the ambient temperature to the controller 48. The temperature sensor 46 may be mounted directly on the glass substrate of the liquid crystal display device or on the PCB together with the common voltage generator 40, the voltage divider circuit 42, the voltage selector 44 and the controller 48. have.

The controller 48 controls the voltage selector 44 in accordance with the temperature sensing signal St to cause the voltage selector 44 to select an optimum common voltage according to the current temperature.

The voltage selector 44 selects a high common voltage when the temperature is lowered among the common voltages Vcom1 to Vcom5 input through its input terminals under the control of the controller 48, while the temperature is increased to a high temperature. Selects a low common voltage and outputs the corrected common voltage (CVcom).

7 schematically illustrates a liquid crystal display according to an embodiment of the present invention.

Referring to FIG. 7, in the liquid crystal display according to the present invention, m × n liquid crystal cells Clc are arranged in a matrix type, m data lines D1 to Dm, and n gate lines G1 to Gn. ) Are intersected and TFTs are formed at the intersections thereof, the temperature sensor 19 installed adjacent to the liquid crystal panel 15, and the data lines D1 to Dm of the liquid crystal panel 15. A data driver circuit 13 for supplying data, a gate driver circuit 14 for supplying a scan signal to the gate lines G1 to Gn, and a direct current for generating drive voltages of the liquid crystal panel 15. The DC converter (hereinafter referred to as a "DC-DC converter") 16, the common voltage generator 17 connected to the DC-DC converter 16, and the common voltage according to the temperature change of the liquid crystal panel 15. Control the common voltage compensator 18, the data driver circuit 13, the gate driver circuit 14, and the common voltage compensator 18 to adjust Vcom. And a timing controller 20.

In the liquid crystal panel 15, liquid crystal is injected between two glass substrates. The data lines D1 to Dm and the gate lines G1 to Gn formed on the lower glass substrate of the liquid crystal panel 15 cross each other. The TFTs formed at the intersections of the data lines D1 to Dm and the gate lines G1 to Gn display liquid crystal data on the data lines D1 to Dn in response to scan signals from the gate lines G1 to Gn. It is supplied to the cell Clc. For this purpose, the gate electrodes of the TFTs are connected to the corresponding gate lines G1 to Gn, and the source electrodes are connected to the corresponding data lines D1 to Dm. The drain electrode of the TFT is connected to the pixel electrode of the liquid crystal cell Clc. A black matrix, a color filter, and a common electrode (not shown) are formed on the upper glass substrate of the liquid crystal panel 15. On the upper glass substrate and the lower glass substrate of the liquid crystal panel 15, a polarizing plate having an optical axis orthogonal to each other is attached, and an alignment layer for setting the pretilt angle of the liquid crystal is formed on the inner side of the liquid crystal panel 15 in contact with the liquid crystal. The storage capacitor Cst formed in each of the liquid crystal cells Clc is formed between the pixel electrode of the liquid crystal cell Clc and the front gate line or between the pixel electrode of the liquid crystal cell Clc and the common electrode line (not shown). It serves to keep the voltage of the liquid crystal cell Clc constant.

The temperature sensor 19 is substantially the same as the temperature sensor 46 shown in FIG. The temperature sensor 19 is mounted on a glass substrate of the liquid crystal panel 15 or a PCB adjacent thereto to sense the temperature of the liquid crystal panel 15 and supply a temperature sensing signal St to the timing controller 20.

The data driving circuit 13 converts the digital video data RGB into an analog gamma voltage corresponding to the gray scale value in response to the data control signal DDC from the timing controller 20 and converts the analog gamma voltage into the data lines D1 to Dm).

The gate driving circuit 14 sequentially supplies scan pulses to the gate lines G1 to Gn in response to the gate control signal GDC from the timing controller 20 so that the data of the liquid crystal panel 15 is horizontally supplied. Select a line.

The DC-DC converter 16 boosts or decompresses a VCC voltage from a system (not shown) to generate a VDD voltage of 6 V or more, a gamma voltage (Gamma1 to Gamma10), a gate high voltage (Vgh), and a gate low generated by a partial pressure of the VDD voltage. Generates voltage Vgl and the like. The VDD voltage and gamma voltages Gamma1 to Gamma10 are supplied to the data driving circuit 13 as analog gamma voltages. The gate high voltage Vgh is supplied to the gate driving circuit 14 as the high logic voltage of the scan pulse set above the threshold voltage of the TFT, and the gate low voltage Vgl is the low logic voltage of the scan pulse set to the off voltage of the TFT. The gate driving circuit 14 is supplied.

As described above, the common voltage generator 17 uses the VDD voltage as an input and amplifies the VDD voltage with an operational amplifier to generate a common voltage Vcom of 2.5 to 3.3V.

The common voltage compensator 18 adjusts the common voltage Vcom according to the temperature of the liquid crystal panel 15 to generate the corrected common voltage CVcom, so that the common voltage Vcom felt by the liquid crystal cell Clc when the temperature changes. To compensate for the change.

The timing controller 20 uses a vertical / horizontal synchronization signal and a clock signal to control the gate control signal GDC for controlling the gate driving circuit 14 and the data control signal DDC for controlling the data driving circuit 13. Occurs. The gate control signal GDC includes a gate start pulse GSP, a gate shift clock GSC, a gate output enable GOE, and the like. The data control signal (DDC) includes a source start pulse (GSP), a source shift clock (SSC), a source output signal (SOC), a polarity signal (POL), and the like. do. The timing controller 20 rearranges the digital video data RGB to supply the data driving circuit 13. In addition, the timing controller 20 controls the common voltage compensator 18 according to the temperature detection signal St. This timing controller 20 includes a controller 48 shown in FIG.

The thermistor TH of FIG. 4 also functions as the temperature sensor 19 and the common voltage compensator 18 in FIG. 7.

The voltage dividing circuit 42 and the voltage selector 44 of FIG. 6 are included in the common voltage compensator 18 of FIG. 7.

As described above, the common voltage generator according to the present invention can adjust the common voltage at the time of temperature change. The liquid crystal display according to the present invention uses the common voltage generator to compensate for the change of the common voltage felt by the liquid crystal cell at the time of temperature change, thereby optimizing the common voltage in any temperature environment, thereby reducing the picture quality according to the temperature change, that is, flicker or Afterimage can be minimized.

Those skilled in the art will appreciate that various changes and modifications can be made without departing from the technical spirit of the present invention. For example, although the common voltage generator according to the present invention has been described as a circuit for generating a common voltage of a liquid crystal display device, in a display device other than the liquid crystal display device or other power supply device, the necessary voltage can be compensated according to temperature without difficulty of configuration. Can be. Therefore, the technical scope of the present invention should not be limited to the contents described in the detailed description of the specification but should be defined by the claims.

1 is an equivalent circuit diagram of a pixel of a general liquid crystal display device.

2 is a waveform diagram illustrating a common voltage and a gamma voltage.

3 is a waveform diagram illustrating a voltage charged in a scan pulse and a liquid crystal cell.

4 is a circuit diagram illustrating a common voltage generator according to a first embodiment of the present invention.

FIG. 5 is a graph showing the characteristics of the thermistor shown in FIG. 4.

6 is a circuit diagram illustrating a common voltage generator according to a second embodiment of the present invention.

7 is a block diagram illustrating a liquid crystal display according to an exemplary embodiment of the present invention.

<Explanation of symbols for main parts of drawing>

11 pixel electrode 12 common electrode

13 data driving circuit 14 gate driving circuit

15 liquid crystal panel 16 DC-DC converter

17, 40: common voltage generator 18: common voltage compensator

42: voltage divider circuit 44: voltage selector

46: temperature sensor 48: controller

Claims (6)

A common voltage generator for generating a common voltage; And a common voltage compensator for adjusting a common voltage output from the common voltage generator according to a temperature. The method of claim 1, The common voltage compensator, And a thermistor whose resistance changes in inverse proportion to the temperature. The method of claim 2, The common voltage compensator, A temperature sensor for sensing the temperature; A voltage divider circuit for dividing an output voltage of the common voltage generator to generate a plurality of common voltages having different voltages; A voltage selector for selecting any one of the common voltages; And a controller for controlling the voltage selector according to the temperature sensed by the temperature sensor. A liquid crystal panel having a pixel electrode supplied with a data voltage and a common electrode supplied with a common voltage; A common voltage generator for generating the common voltage; And a common voltage compensator for adjusting a common voltage output from the common voltage generator according to a temperature. The method of claim 4, wherein The common voltage compensator, And a thermistor whose resistance changes in inverse proportion to the temperature. The method of claim 4, wherein The common voltage compensator, A temperature sensor for sensing a temperature of the liquid crystal panel; A voltage divider circuit for dividing an output voltage of the common voltage generator to generate a plurality of common voltages having different voltages; A voltage selector for selecting any one of the common voltages and supplying the selected common voltage to the common electrode; And a controller for controlling the voltage selector according to the temperature sensed by the temperature sensor.