JP2006126292A - Liquid crystal display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent reverse transition from bend alignment to splay alignment without impairing the brightness. <P>SOLUTION: The liquid crystal display is equipped with a display panel DP containing an OCB liquid crystal, which is subjected to transition from the splay alignment to the bend alignment, in advance, for display motion between a pair of electrodes PE, CE, and of which the reverse transition from the bend alignment to the splay alignment is interrupted with a voltage for black display periodically applied from the pair of electrodes PE, CE. Furthermore, the liquid crystal display device is equipped with an LED backlight BL to illuminate the display panel DP and a light source drive portion LD for driving the LED backlight BL so as to compensate for the temperature-dependent luminance fluctuations of the display panel DP. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a liquid crystal display device including an OCB (Optically Compensated Birefringence) mode liquid crystal display panel.

  A flat display device typified by a liquid crystal display device is widely used as a display device such as a computer, a car navigation system, or a television receiver.

A liquid crystal display device generally includes a liquid crystal display panel including a matrix array of a plurality of liquid crystal pixels, and a display panel control circuit that controls the display panel. The liquid crystal display panel has a structure in which a liquid crystal layer is sandwiched between an array substrate on which a plurality of pixel electrodes are formed and a counter substrate on which a common electrode facing the plurality of pixel electrodes is formed. A pair of pixel electrodes and a common electrode constitute a liquid crystal pixel in cooperation with a portion of the liquid crystal layer disposed between these electrodes.
In the case where the liquid crystal display device is mainly used for a television receiver that displays a moving image, an OCB mode liquid crystal display panel in which liquid crystal molecules exhibit good responsiveness is generally used (see Patent Document 1). In this liquid crystal display panel, the OCB liquid crystal has a spray orientation almost twisted as shown in FIG. 10A before power-on by an alignment film rubbed in parallel with each other on the pixel electrode and the common electrode. The liquid crystal display panel performs a display operation after the OCB liquid crystal is changed from the spray alignment to the bend alignment shown in FIGS. 10B and 10C by a relatively strong electric field applied in the initialization process when the power is turned on. .

FIG. 11 shows the energy of spray alignment and bend alignment with respect to the liquid crystal driving voltage. The reason why the OCB liquid crystal is in the spray alignment before the power is turned on is that the spray alignment is energetically more stable than the bend alignment in a state where no liquid crystal driving voltage is applied. Vc shown in FIG. 11 represents a transition boundary value of the liquid crystal driving voltage at which the spray alignment energy and the bend alignment energy antagonize, and is about Vc = 1.6V. Such an OCB liquid crystal has the property that even if it transitions once to bend alignment, it reversely transitions back to spray alignment if no voltage application state or voltage application state below the Vc level continues for a long period of time. In the spray orientation, the display angle characteristics are greatly different from those in the bend orientation, resulting in abnormal display.
JP 2002-202491 A

  Conventionally, in order to prevent reverse transition from bend alignment to spray alignment, for example, a driving method in which a large voltage is applied to the OCB liquid crystal in a part of each frame period is employed. In a normally white liquid crystal display panel, since this voltage corresponds to a black display voltage, it is called black insertion driving.

  However, since the response speed of the OCB liquid crystal decreases as the temperature of the display panel decreases, even if the white display voltage is applied for a certain period immediately after the black display voltage is applied, It becomes impossible to shift the transmittance of the display panel, that is, the luminance to the maximum value. In addition, a fluorescent tube such as a cold cathode tube is generally used as a backlight of a display panel. However, there is a problem that the luminance of the fluorescent tube itself decreases with a decrease in temperature. For this reason, for example, at a temperature of −30 ° C., the effective luminance is reduced to about half of the luminance obtained at 40 ° C.

  An object of the present invention is to provide a liquid crystal display device capable of preventing a decrease in luminance of a display panel accompanying a decrease in temperature.

  According to the present invention, for display operation between a pair of electrodes, a black display voltage is applied in which a reverse transition from a spray alignment to a bend alignment and a reverse transition from the bend alignment to the spray alignment is periodically applied from the pair of electrodes. Provided is a liquid crystal display device comprising a display panel including OCB liquid crystal to be blocked, an LED light source that illuminates the display panel, and a light source driving unit that drives the LED light source to compensate for temperature-dependent luminance variations of the display panel Is done.

  In this liquid crystal display device, the light source driving unit drives the LED light source so as to compensate for the luminance variation of the display panel depending on the temperature. For this reason, even when the response speed of the OCB liquid crystal is slowed down at a low temperature and the effective transmittance of the display panel is reduced, this decrease in transmittance, that is, the decrease in luminance is compensated by increasing the luminance of the LED light source. be able to. Therefore, a decrease in luminance of the display panel due to a decrease in temperature is prevented.

  Further, since the brightness of the LED light source does not decrease with a decrease in temperature unlike fluorescent tubes such as cold cathode fluorescent lamps, the brightness of the LED light source is substantially excluded from the luminance fluctuation factors of the display panel. Brightness compensation of the display panel can be performed. In addition, the use of LED light sources reduces power consumption at room temperature compared to driving a fluorescent tube that has a light-emitting ability that can compensate for the decrease in brightness of the display panel and compensate for the decrease in brightness that occurs at low temperatures. it can.

  Hereinafter, a liquid crystal display device according to an embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 1 schematically shows a circuit configuration of the liquid crystal display device. The liquid crystal display device compensates for variations in luminance of the liquid crystal display panel DP, a display panel control circuit CNT connected to the display panel DP, an LED backlight BL that is an LED light source that illuminates the display panel DP, and a temperature dependent display panel DP. The light source drive part LD which drives LED backlight BL is provided. The liquid crystal display panel DP has a structure in which a liquid crystal layer 3 is sandwiched between an array substrate 1 and a counter substrate 2 which are a pair of electrode substrates. The liquid crystal layer 3 is an OCB liquid crystal in which, for example, a normally white display operation is performed, an OCB liquid crystal that is previously transitioned from spray alignment to bend alignment and reverse transition from bend alignment to spray alignment is periodically blocked by a black display voltage. Including as a liquid crystal material. The display panel control circuit CNT controls the transmittance of the liquid crystal display panel DP by the liquid crystal driving voltage applied from the array substrate 1 and the counter substrate 2 to the liquid crystal layer 3. The transition from the spray orientation to the bend orientation is obtained by applying a relatively large electric field to the OCB liquid crystal by a predetermined initialization process performed by the display panel control circuit CNT when the power is turned on. The LED backlight BL includes, for example, a plurality of light emitting diodes (LEDs) arranged side by side at the end of the display panel DP and a light guide plate that guides light from these light emitting diodes to the entire display panel DP. The light source driver LD includes a temperature sensor 8 that is a detector that detects the temperature of the display panel DP (or its surroundings) and a luminance adjuster 9 that adjusts the luminance of the LED backlight BL based on the detection result of the temperature sensor 8. . The luminance adjusting unit 9 is configured to increase the luminance of the LED backlight BL by an amount corresponding to an effective luminance decrease of the display panel DP corresponding to the response speed of the OCB liquid crystal that is delayed as the temperature of the display panel DP decreases.

  The array substrate 1 includes a plurality of pixel electrodes PE arranged in a substantially matrix form on a transparent insulating substrate such as glass, and a plurality of gate lines Y (Y0 to Ym) arranged along a row of the plurality of pixel electrodes PE. , A plurality of source lines X (X1 to Xn) arranged along a column of the plurality of pixel electrodes PE, and the gate lines Y and the source lines X arranged in the vicinity of the intersection positions and driven through the corresponding gate lines Y, respectively. In this case, the corresponding source line X and the corresponding pixel electrode PE are conducted to have a plurality of pixel switching elements W. Each pixel switching element W is made of, for example, a thin film transistor, the gate of the thin film transistor is connected to the gate line Y, and the source-drain path is connected between the source line X and the pixel electrode PE.

  The counter substrate 2 includes, for example, a color filter disposed on a transparent insulating substrate such as glass, and a common electrode CE disposed on the color filter so as to face the plurality of pixel electrodes PE. Each pixel electrode PE and common electrode CE are made of a transparent electrode material such as ITO, for example, and are covered with alignment films that are rubbed in parallel to each other, and have a liquid crystal molecular arrangement corresponding to the electric field from the pixel electrode PE and common electrode CE. A pixel PX is formed together with the pixel region of the liquid crystal layer 3 to be controlled.

  Each of the plurality of pixels PX has a liquid crystal capacitor CLC between the pixel electrode PE and the common electrode CE, and is further connected to one end of the plurality of auxiliary capacitors Cs. Each auxiliary capacitor Cs is formed by capacitive coupling between the pixel electrode PE of the pixel PX and the gate line Y adjacent to the pixel PX on one side and controlling the pixel switching element W of the pixel PX. It has a sufficiently large capacitance value with respect to the parasitic capacitance of W. In FIG. 1, a plurality of dummy pixels arranged around the matrix array of the plurality of pixels PX constituting the display screen are omitted. These dummy pixels are wired in the same manner as the pixels PX in the display screen, and are provided to make all the pixels PX in the display screen have the same conditions with respect to parasitic capacitance and the like. The gate line Y0 is a gate line for such a dummy pixel.

  The display panel control circuit CNT includes a gate driver YD that sequentially drives the plurality of gate lines Y0 to Ym so that the plurality of switching elements W are conducted in units of rows, and the switching elements W in each row are conducted by driving the corresponding gate lines Y. A source driver XD that outputs the pixel voltage Vs to the plurality of source lines X1 to Xn in each period, and a plurality of pixel data input from the external signal source SS for each frame period (vertical scanning period) to the plurality of pixels PX. The image data conversion circuit 4 that converts the resolution, gradation, and the like of the image data composed of the image data, and the operation timings of the gate driver YD and the source driver XD for the image data obtained as the conversion result of the image data conversion circuit Including a controller 5 for controlling the control and the like. The pixel voltage Vs is a voltage applied to the pixel electrode PE on the basis of the common voltage Vcom of the common electrode CE, and the polarity is inverted with respect to the common voltage Vcom so as to perform, for example, frame inversion driving and line inversion driving.

  The gate driver YD and the source driver XD are, for example, integrated circuit (IC) chips mounted on a flexible wiring sheet disposed along the outer edge of the array substrate 1. The temperature sensor 8 is mounted on a flexible wiring sheet together with the integrated circuit chip. On the other hand, the image data conversion circuit 4 and the controller 5 are arranged on an external printed wiring board PCB. As described above, the controller 5 controls the control signal CTY for sequentially driving the plurality of gate lines Y, and the pixel data obtained in units of pixels PX for one row as the conversion result of the image data conversion circuit 4 and output in series. DATA is assigned to each of the plurality of source lines X, and a control signal CTX for designating output polarity is generated. The control signal CTY is supplied from the controller 5 to the gate driver YD, and the control signal CTX is supplied from the controller 5 to the source driver XD together with the pixel data DATA obtained as a conversion result from the image data conversion circuit 4.

  The display panel control circuit CNT further passes through the gate driver YD to the adjacent adjacent gate line Y on one side adjacent to the gate line Y connected to the switching elements W when the switching elements W for one row become non-conductive. The compensation voltage generation circuit 6 for generating the compensation voltage Ve for compensating the fluctuation of the pixel voltage Vs generated in the pixels PX for one row by the parasitic capacitance of the switching elements W, and the image data DATA are converted into the pixel voltage Vs. A gradation reference voltage generation circuit 7 for generating a predetermined number of gradation reference voltages VREF used for the purpose is included.

  The gate driver YD sequentially selects a plurality of gate lines Y1 to Ym in one frame period under the control of the control signal CTY, and supplies an ON voltage that makes the pixel switching elements W in each row conductive for one horizontal scanning period to the selected gate line Y. . The image data conversion circuit 4 outputs a conversion result composed of pixel data DATA for the pixels PX for one row every horizontal scanning period, and the source driver XD outputs a predetermined number of signals supplied from the gradation reference voltage generation circuit 7 described above. The pixel data DATA is converted into the pixel voltage Vs with reference to the gradation reference voltage VREF, and output in parallel to the plurality of source lines X1 to Xn.

  When the gate driver YD drives, for example, the gate line Y1 with the on-voltage to make all the pixel switching elements W connected to the gate line Y1 conductive, the pixel voltage Vs on the source lines X1 to Xn is changed to these pixel switching elements W. To the corresponding pixel electrode PE and one end of the auxiliary capacitor Cs. Further, the gate driver YD outputs the compensation voltage Ve from the compensation voltage generation circuit 6 to the previous gate line Y0 adjacent to the gate line Y1, and performs one horizontal scanning on all the pixel switching elements W connected to the gate line Y1. Immediately after being turned on for a period, an off voltage for turning off the pixel switching element W is output to the gate line Y1. The compensation voltage Ve reduces the electric charge drawn from the pixel electrode PE by these parasitic capacitances when these pixel switching elements W become non-conductive, and substantially cancels the fluctuation of the pixel voltage Vs, that is, the punch-through voltage ΔVp.

  2 to 6 show liquid crystal displays obtained at various temperatures of 60 ° C., 40 ° C., 20 ° C., 0 ° C. and −20 ° C. when each of the two types of OCB liquid crystal is applied to the liquid crystal layer 3 shown in FIG. The response characteristic of panel DP is shown. Referring to FIGS. 2 to 6, the rise of the transmittance when the white display voltage is applied for a certain period immediately after the black insertion due to the application of the black display voltage is accompanied by a decrease in the temperature of the display panel DP. It becomes slow and the transmittance that can be reached within this certain period is limited. Further, at a low temperature of −20 ° C. shown in FIG. 6, the transmittance does not change to 0% even when black insertion is performed. Since black insertion also plays a role of improving the visibility by clearing the retinal afterimage generated in the viewer's vision in moving image display, the brightness of the display panel DP is not sufficiently lowered, so that the visibility is also lowered.

  FIG. 7 shows the temperature dependence of the transmittance ratio of the display panel DP normalized with respect to the value obtained at 40 ° C. in white display by each of the two types of OCB liquid crystals shown in FIGS. For such a display panel DP, the temperature sensor 8 detects the temperature of the display panel DP, and the brightness adjusting unit 9 adjusts the brightness of the LED backlight BL based on the detection result of the temperature sensor 8. Here, the luminance adjusting unit 9 is configured to increase the luminance of the LED light source by an amount corresponding to the effective luminance decrease of the display panel DP corresponding to the response speed of the OCB liquid crystal that is delayed as the temperature of the display panel DP decreases. . The light source controller LD operates to reflect the brightness adjustment by uniformly increasing or decreasing the amount of current flowing through each light emitting diode in the LED backlight BL in driving the LED backlight BL.

  FIG. 8 shows the temperature dependence of the luminance ratio of the LED backlight BL standardized on the basis of the value obtained at 40 ° C. for each of the two types of OCB liquid crystals shown in FIGS. The temperature dependence of the luminance ratio of the LED backlight BL shown in FIG. 8 is adapted to the temperature dependence of the transmittance ratio of the display panel DP shown in FIG. FIG. 9 shows the temperature dependence of the luminance ratio of the display panel DP normalized based on the value obtained at 40 ° C. when such a display panel DP and the LED backlight BL are combined. As can be seen from FIG. 9, the luminance ratio is fixed to “1” at least in the range of −20 ° C. to 60 ° C. That is, the brightness of the display panel DP is maintained at a value obtained at a temperature of 40 ° C.

  In the liquid crystal display device of the present embodiment, the light source driver LD drives the LED backlight BL so as to compensate for the luminance variation of the display panel DP depending on the temperature. For this reason, even when the response speed of the OCB liquid crystal is slowed down at a low temperature and the effective transmittance of the display panel DP is reduced, the reduction in the transmittance, that is, the decrease in the luminance is caused to increase the luminance of the LED backlight BL. Can compensate. Therefore, a decrease in luminance of the display panel DP due to a decrease in temperature is prevented.

  Further, since the luminance of the LED backlight BL does not decrease with a decrease in temperature unlike a fluorescent tube such as a cold cathode tube, the luminance of the LED backlight BL light source is substantially reduced from the luminance variation factor of the display panel DP. Therefore, appropriate brightness compensation of the display panel DP can be performed. Furthermore, the use of the LED backlight BL consumes at room temperature as compared with driving a fluorescent tube having a light emitting capability that can compensate for the decrease in brightness of the display panel DP that occurs at a low temperature. Electric power can be reduced.

  In addition, this invention is not limited to the above-mentioned embodiment, It can deform | transform variously in the range which does not deviate from the summary.

  In the above-described embodiment, the light source driving unit LD is configured to uniformly increase / decrease the amount of current flowing through the plurality of light-emitting diodes provided in the LED backlight BL. The number of light emitting diodes that should emit light may be increased or decreased so as not to occur.

It is a figure which shows schematically the circuit structure of the liquid crystal display device which concerns on one Embodiment of this invention. 2 is a graph showing response characteristics of a liquid crystal display panel obtained at a temperature of 60 ° C. when each of two types of OCB liquid crystals is applied to the liquid crystal layer shown in FIG. 1. 2 is a graph showing response characteristics of a liquid crystal display panel obtained at a temperature of 40 ° C. when each of two types of OCB liquid crystals is applied to the liquid crystal layer shown in FIG. 1. 2 is a graph showing response characteristics of a liquid crystal display panel obtained at a temperature of 20 ° C. when each of two types of OCB liquid crystals is applied to the liquid crystal layer shown in FIG. 1. 2 is a graph showing response characteristics of a liquid crystal display panel obtained at a temperature of 0 ° C. when each of two types of OCB liquid crystals is applied to the liquid crystal layer shown in FIG. 1. 2 is a graph showing response characteristics of a liquid crystal display panel obtained at a temperature of −20 ° C. when each of two types of OCB liquid crystals is applied to the liquid crystal layer shown in FIG. 1. It is a graph which shows the temperature dependence of the transmittance | permeability ratio of the display panel normalized on the basis of the value obtained at 40 degreeC in the white display by each of two types of OCB liquid crystal shown in FIGS. It is a graph which shows the temperature dependence of the luminance ratio of the LED backlight normalized on the basis of the value obtained at 40 degreeC about each of two types of OCB liquid crystal shown in FIGS. It is a graph which shows the temperature dependence of the luminance ratio of the display panel normalized on the basis of the value obtained at 40 degreeC when the display panel shown in FIG. 1 and an LED backlight are combined. It is a figure which shows the OCB liquid crystal which is changed from a spray orientation to a bend orientation for display operation. 11 is a graph showing the energy of spray alignment and bend alignment shown in FIG. 10 with respect to the liquid crystal driving voltage.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Array substrate, 2 ... Opposite substrate, 3 ... Liquid crystal layer, 4 ... Image data conversion circuit, 5 ... Controller, 6 ... Compensation voltage generation circuit, 7 ... Tone reference voltage generation circuit, 8 ... Temperature sensor, 9 ... Luminance Adjustment unit, BL ... LED backlight, DP ... liquid crystal display panel, PE ... pixel electrode, CE ... common electrode, CLC ... liquid crystal capacitor, Cs ... auxiliary capacitor, PX ... liquid crystal pixel, W ... switching element, Y ... gate line, X ... source line, CNT ... display panel control circuit, LD ... light source drive unit, YD ... gate driver, XD ... source driver.

Claims (3)

In order to perform a display operation between a pair of electrodes, a transition from a spray orientation to a bend orientation is performed in advance, and a reverse transition from the bend orientation to the spray orientation is prevented by a black display voltage periodically applied from the pair of electrodes. A display panel including an OCB liquid crystal, an LED light source that illuminates the display panel, and a light source driving unit that drives the LED light source so as to compensate for a luminance variation of the display panel depending on temperature. Liquid crystal display device. The light source driving unit includes a detection unit that detects a temperature of the display panel or its surroundings, and a luminance adjustment unit that adjusts the luminance of the LED light source based on a detection result of the detection unit. The liquid crystal display device described. The luminance adjustment unit is configured to increase the luminance of the LED light source by an amount corresponding to an effective luminance decrease of the display panel corresponding to the response speed of the OCB liquid crystal delayed as the temperature decreases. The liquid crystal display device according to claim 2.

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