Liquid crystal display apparatus

A liquid crystal display apparatus comprises a liquid crystal display panel, a temperature sensor which detects temperature of the liquid crystal display panel, and a controller which controls a voltage applied to the liquid crystal display panel. The controller sets a black-insertion ratio to 0% and changes the voltage applied in white-display mode to a voltage equal to or higher than the critical voltage, or sets a black-insertion ratio to a finite value and changes the voltage applied in the white-display mode to a voltage lower than the critical voltage, in accordance with the temperature detected by the temperature sensor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2005-333046, filed Nov. 17, 2005, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display apparatus for use in liquid crystal television sets, monitors for car-navigation systems, OA apparatuses and mobile apparatuses.

2. Description of the Related Art

Liquid crystal displays are widely used as planar display apparatuses for computers, car-navigation systems and television receivers.

It is proposed that a liquid crystal display panel of OCB-mode be used in liquid crystal displays for television receivers that display mainly moving pictures. This is because the liquid crystal molecules of this panel exhibit good response. See, for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-202491.

The liquid crystal display panel of OCB-mode comprises two substrates, a liquid crystal layer, and transparent electrodes. The liquid crystal layer is held between the substrates. Transparent electrodes are formed on the substrates, and are used as means for applying a voltage Before the power switch of the liquid crystal display having the panel is turned on, the liquid crystal molecules of the liquid crystal layer are aligned in a specific state called splay alignment. When the power switch is turned on, a relatively high voltage is applied between the transparent electrodes for a short time, changing the alignment of the liquid crystal molecules to so-called bend alignment. The use of the bend alignment characterizes the liquid crystal display panel of OCB-mode.

Most liquid crystal display panels of OCB-mode have an active-matrix substrate having a plurality of TFTs. Therefore, the panel can fast respond to input data, reducing the one-frame period to half the conventional one-frame period. For example, Jpn. Pat. Appln. KOKAI Publication No. 2000-214827 and Jpn. Pat. Appln. KOKAI Publication No. 2002-107695 disclose that a signal-display period and a black-display period are set in each one-frame period and the panel is driven, by utilizing the fast response of the panel.

In the liquid crystal display panel of OCB-mode, the liquid crystal molecules are prevented from undergoing inverse transition from bend alignment to splay alignment. That is, a high voltage is applied to the liquid crystal layer for a part of the one-frame period, thus driving the liquid crystal display panel of OCB-mode In the normally-white mode, the high voltage corresponds to a voltage that achieves black display. Therefore, the panel is driven in so-called black-insertion driving, thereby preventing the inverse transition to the splay alignment. Hence, the panel can acquire high transmittance.

However, the transmittance falls when the panel is driven in the black-insertion driving at low temperatures (0° C. or less).

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a liquid crystal display apparatus that has high transmittance, can be driven without inverse transition, and can maintain the high transmittance even at low temperatures.

To achieve the object, according to an aspect of the present invention, there is provided a liquid crystal display apparatus comprising:

a liquid crystal display panel;

a temperature sensor which detects temperature of the liquid crystal display panel; and

a controller which controls a voltage applied to the liquid crystal display panel,

the controller being configured to set a black-insertion ratio to 0% and change the voltage applied in white-display mode to a voltage equal to or higher than the critical voltage, in accordance with the temperature detected by the temperature sensor, or to set a black-insertion ratio to a finite value and change the voltage applied in the white-display mode to a voltage lower than the critical voltage, in accordance with the temperature detected by the temperature sensor.

DETAILED DESCRIPTION OF THE INVENTION

Embodiment of the present invention will be described in detail, with reference to the accompanying drawings.

FIG. 1schematically shows the circuit configuration of a liquid crystal display apparatus according to an embodiment of the present invention. The liquid crystal display apparatus has a liquid crystal display panel DP and a display panel control circuit CNT connected to the liquid crystal display panel DP. The panel DP has an array substrate1, a counter substrate2, and a liquid crystal layer3. The substrates1and2form a pair of electrodes substrates. The liquid crystal layer3is held between these substrates1and2.

The liquid crystal display panel DP is driven in a normally-white and contains an OCB liquid crystal as liquid crystal material. In the normally-white display mode, the liquid crystal molecules of the layer3have been transferred from the splay alignment to the bend alignment. A black-display voltage is cyclically applied, thus preventing the inverse transition from bend alignment to splay alignment.

The display panel control circuit CNT controls the transmittance of the liquid crystal display panel DP, by applying a liquid-crystal driving voltage to the liquid crystal layer3from the array substrate1and counter substrate2. The transition from splay alignment to bend alignment is achieved by applying a relatively intense electric field to the OCB liquid crystal in the initialization process that the control circuit CNT performs when the power switch of the liquid crystal display is turned on.

The array substrate1has a transparent insulating substrate, a plurality of pixel electrodes PE, a plurality of gate lines Y (Y0to Ym), a plurality of source lines X (X1to Xn), and a plurality of pixel-switching elements W. The transparent insulating substrate is made of glass or the like. The pixel electrodes PE are arranged in rows and columns on the transparent insulating substrate, forming a matrix. The gate lines Y are arranged, extending along the rows of pixel electrodes PE. The source lines X are arranged, extending along the columns of pixel electrodes PE. The pixel-switching elements W are arranged near the intersections of the gate lines Y and source lines X.

Each pixel switching element W electrically connects one source line X to one pixel electrode PE when it is driven by the gate line Y. The pixel switching elements W are, for example, thin film transistors. Each thin film transistor has its gate electrode connected to one gate electrode Y, its source electrode connected to one source line X and its drain electrode connected to one pixel electrode PE.

The counter substrate2includes a transparent substrate, a color filter, and a common electrode CE. The transparent substrate is made of, for example, glass or the like. The color filter is arranged on the transparent substrate. The common electrode CE is arranged on the color filter. The common electrode CE is opposed to the pixel electrodes PE. The pixel electrodes PE and the common electrode CE are made of transparent electrode materials, such as ITO (Indium Tin Oxide) and are covered with alignment films, respectively. The alignment films have been rubbed in parallel directions. The pixel electrodes PE, parts of the common electrode CE and the pixel regions of the liquid crystal layer3form pixels PX. In each pixel region of the liquid crystal layer3, the liquid crystal molecules are aligned in accordance with the electric field applied between the corresponding pixel electrode PE and the common electrode CE.

Each pixel PX has a liquid crystal capacitance CLC between the pixel electrode PE and the common electrode CE and is connected to one end of an auxiliary capacitance Cs. Each of the auxiliary capacitance Cs is a coupling capacitance between the pixel electrode of the pixel PX and the gate line Y that controls the pixel switching element W of the pixel PX provided on one side of this pixel PX. The auxiliary capacitance Cs is much larger than the parasitic capacitance of the pixel switching element W.

The liquid crystal display ofFIG. 1has dummy pixels, which are not shown inFIG. 1. The dummy pixels are arranged around the pixels PX matrix, or the display screen. The dummy pixels have the same configuration as the pixels PX forming the display screen. The dummy pixels are used, imparting the same parasitic capacitance to all pixels PX forming the display screen. The gate line Y0is a gate line that is opposed to the dummy pixels.

The display panel control circuit CNT includes a gate driver YD, a source driver XD, an image processing circuit4, and a controller5. The gate driver YD drives the gate lines Y, one after another, to turn on the switching elements W in units of rows. The source driver XD applies a pixel voltage Vs to the source lines X while the switching elements W of each row remain on.

The image processing circuit4processes video data that is cyclically updated, during every one-frame period (i.e., vertical scan period). The video data is gradation data that represents different gradation levels to be presented by the pixels. The controller5controls the operation timing of the gate driver YD and that of the source driver XD, in accordance with the video data processed by the image processing circuit4. The video data is supplied to the image processing circuit4from an external signal source SS. At the same time, a sync signal is supplied to the controller5from the external signal source SS.

The gate driver YD and the source driver XD are, for example, integrated circuit (IC) chips mounted on a flexible wiring sheet. The flexible wiring sheet is arranged, surrounding the array substrate1. The image processing circuit4and the controller5are arranged on an external printed circuit board PCB. The gate driver YD and the source driver XD have shift registers so that they may perform vertical scanning to at lest one of the select gate lines Y, and horizontal scanning to select at least one of the source lines X.

The controller5includes a vertical-timing control circuit11and a horizontal-timing control circuit12. The vertical-timing control circuit11generates a control signal CTY for the gate driver YD, from the synchronizing signal supplied from external signal source SS. The horizontal-timing control circuit12generates a control signal CTX for the source driver XD, from the synchronizing signal supplied from external signal source SS. The vertical-timing control circuit11includes a black-insertion-timing controlling unit that adds, to the control signal CTY, data representing the black-inserting timing.

The image processing circuit4includes a gamma correction unit14and a black-insertion-data conversion unit15. The gamma correction unit14performs gamma correction on pixel data items contained in the image data supplied from external signal source SS and representing different gradation levels. The black-insertion-data conversion unit15performs black-insertion-data conversion on the pixel data items that have been gamma-corrected by the gamma correction unit14.

The display panel control circuit CNT further includes a compensation-voltage generating circuit6and a gradation-reference voltage generating circuit7. The compensation-voltage generating circuit6generates compensation voltage Ve. The compensation voltage Ve is applied through the gate driver YD to a gate line Y immediately preceding any gate line Y that is connected to the switching elements W of one low when these elements W are off. The compensation voltage Ve compensates for a change of pixel voltage Vs, which occurs in the pixels PX of one row duce to the parasitic capacitance of the switching elements W. The gradation-reference voltage generating circuit7generates a prescribed number of gradation-reference voltages VREF. These gradation reference voltages VREF will be used to change video data DATA to a pixel voltage Vs.

As will be described later in detail, a temperature sensor20is connected to the black-insertion-timing control unit13. The sensor20can detect the temperature of the liquid crystal display panel DP.

The OCB liquid crystal used in the liquid crystal display panel DP will be described with reference toFIG. 2.

FIG. 2illustrates the two alignment states that liquid crystal molecules of the OCB liquid crystal can assume, namely splay alignment and bend alignment. Generally, the splay alignment is more stable than the bend alignment, as long as no voltage is applied to the liquid crystal layer. When a sufficiently high voltage is applied to the liquid crystal layer, however, the bend alignment is more stable than the splay alignment. In most cases, the OCB mode is used while assuming the bend alignment. Hence, a high voltage is applied for some time after the power switch of the display is turned on, thus changing the alignment state from splay alignment to bend alignment. Note that the state transition from splay alignment to bend alignment is called “transition,” and the state transition from bend alignment to splay alignment is called “inverse transition.”

The stability of liquid-crystal alignment will be explained in greater detail.

FIG. 3Ashows how the free energy changes with the voltage applied while the OCB liquid crystal molecules remain in splay alignment, and how it changes with the voltage while the OCB liquid crystal molecules remain in bend alignment. Both curves shown inFIG. 3Across a line indicating a certain voltage value Vc (hereinafter called critical voltage). In the low-voltage region on the left-hand side of the line, the energy is smaller in the splay alignment than in the bend alignment. In the high-voltage region on the right-hand side of the line, the energy is smaller in the bend alignment than in the splay alignment.

FIG. 3Bshows how the transmittance of the OCB liquid crystal layer changes with the voltage applied to the layer, while the OCB liquid crystal molecules remain in bend alignment. The voltage Vb at which the transmittance is minimal is called black voltage. In order to increase the transmittance in the white display mode, the dynamic range of the voltage applied to the liquid crystal layer should be as broad as possible. The voltage should range, for example, from V1to Vb, as indicated by curve [1] inFIG. 3B.

However, the voltage V1applied in the white-display mode is lower than critical voltage Vc. Therefore, the alignment state undergoes inverse transition, changing from the bend alignment to the stable splay alignment. Consequently, the image displayed will have defects. To prevent the inverse transition, a voltage ranging from V2to Vb must be applied to the liquid crystal layer, as indicated by curve [2] inFIG. 3B, at some expense of the transmittance, thereby setting voltage V2for the white-display mode, to a value greater than the critical voltage Vc.

Black-insertion driving has been devised as a drive scheme that imparts high transmittance to the liquid crystal layer as indicated by curve [1] inFIG. 3Band that causes no inverse transition of alignment state. In an ordinary black insertion driving, the liquid crystal layer is driven in signal-display mode (i.e., white display) for 80% of the one-frame period, and in black display mode (i.e., black insertion) for the remaining 20% of the one-frame period.

FIG. 4andFIG. 5show the timing of the black-insertion driving that is performed in the liquid crystal display panel DP, in the white-display mode and the black-display mode, respectively. InFIG. 5, the period τf is equivalent to the one-frame period. The period τf consists of periods τs and τb. The period τs is a signal-display period, and period τb is a black-insertion period. Voltage Vb and critical voltage Vc shown inFIGS. 4 and 5correspond to the black voltage and critical voltage that are shown inFIG. 3B, respectively.

The white display shown inFIG. 4will be described. In the black-insertion period, a signal at voltage ±Vb is applied to the liquid crystal layer. As a result, the transmittance of the liquid crystal layer becomes to almost 0. In a signal-display period, the voltage corresponding to white display (i.e., ±V1is lower than critical voltage Vc) is applied to the liquid crystal layer. As result, the transmittance (T1shown inFIG. 4) will correspond to the voltage applied. The liquid crystal molecules respond, with some delay, to the stepwise change in voltage. Therefore, the wave representing the change of transmittance is somewhat blunt as is illustrated inFIG. 4.

To perform black display, voltage ±Vb is applied not only in the black-display period, but also in the signal-display period. In this case, the transmittance becomes almost 0 in both the black-display period and the signal-display period.

In this driving, a signal of voltage ±Vb is intermittently supplied even if the voltage applied to the liquid crystal layer falls below the critical voltage Vc as the white display proceeds. Thus, the alignment state is changed back to the bend alignment. The liquid crystal display panel DP can therefore reliably operate, without causing inverse transition.

The black-non-insertion driving of the display panel DP will be explained, in comparison with the black-insertion driving.

FIG. 6andFIG. 7show the timing of the black-non-insertion driving that is performed in the liquid crystal display panel DP, in the white-display mode and the black-display mode, respectively.

In this driving, the entire one-frame period is a signal indication period. In the white-display mode (FIG. 6), for example, the transmittance is T2that corresponds to the voltage V2. In the black-display mode (FIG. 7), the transmittance corresponds to voltage Vb (almost 0).

In the black-insertion driving, the transmittance remains 0 for the black-insertion period. As a result, the transmittance averaged in terms of time is somewhat low. Nonetheless, the transmittance is greatly improved during the display period, thanks to the low voltage for the white display. In total, the transmittance can be higher in the black-insertion driving than in the black-non-insertion driving. The black-insertion driving can achieve an additional advantage, namely improved visibility of moving pictures.

As mentioned above, the black-insertion driving is advantageous in that a high transmittance is obtained. However, it has the problem that the transmittance falls at low temperatures (0° C., more or less).

FIG. 8shows how the transmittance of the liquid crystal display panel changes with the temperature of the panel. That is, the transmittance relatively fast follows the change of the voltage at temperatures near room temperature. At low temperatures, however, its response is slow due to the increase in the viscosity of liquid crystal. As shown inFIG. 8, the response waveform of transmittance becomes blunt, and the transmittance decreases.

The present invention solves the problem that the transmittance falls at such low temperatures.

FIG. 9illustrates how the liquid crystal display panel DP according to the example of the embodiment of this invention is driven and controlled.

When the temperature is higher than a certain value (e.g., 0° C.), the panel PD is driven in black-insertion driving as shown at [1] inFIG. 9. When the temperature is lower than the above-mentioned value, the panel PD is driven in black-non-insertion driving as shown at [2] inFIG. 9. That is, the voltage applied during white display falls below the critical voltage Vc at any temperature higher than 0° C. The voltage is equal to or higher than the critical voltage Vc at any temperature equal to or lower than 0° C. In practice, the voltage applied during white display falls below Vc, or is equal to or higher than −Vc, if the temperature is higher than 0° C. Alternatively, the voltage applied during white display falls below −Vc, or is equal to or higher than Vc, if the temperature is lower than 0° C.

Thus, at temperatures higher than 0° C., the liquid crystal display panel DP can obtain high transmittance when drive and controlled as described above. At temperatures lower than 0° C., too, the panel DP can have high transmittance, because the transmittance is not influenced by such a slow response as shown inFIG. 8.

To perform the control described above, the black-insertion-timing control unit13of the controller5, shown inFIG. 1, changes the control conditions in accordance with the temperature of the liquid crystal display panel DP, which the temperature sensor20has detected.

FIG. 10AandFIG. 10Bshow how a liquid crystal display panel DP according to another example o the embodiment of this invention is driven and controlled.

The control is fundamentally identical to the control shown inFIG. 9. It differs in that the drive conditions are changed continuously.

The drive mode is not switched at a specific temperature, from the black-insertion driving to the black-non-insertion driving, or vice versa, as shown inFIG. 9. Instead, as shown inFIG. 10B, the black-insertion ratio is continuously changed. In the white-display mode, too, the voltage is continuously changed as illustrated inFIG. 10A.

When the liquid crystal display panel DP is so controlled as described above, the displaying condition (brightness) on the screen continuously changes, not abruptly, even if the temperature of the panel DP changes. Hence, the user of the liquid crystal display panel DP feels nothing wrong with the images displayed. The panel DP can be driven as shown inFIG. 10AorFIG. 10Bby means of a combination of a temperature sensor and a controller.

More specifically, in the configuration ofFIG. 1, the black-insertion-timing control unit13of the controller5only needs to change the drive conditions continuously, in accordance with the temperature of the liquid crystal display panel DP, which the temperature sensor20has detected.

The panel DP can be driven as shown inFIG. 10Aor10B, too, in order to accomplish, for example, field-sequence driving.

As has been described above, the liquid crystal display apparatus according to the embodiment of this invention needs to comprise only the liquid crystal display panel DP, the temperature sensor20that detects the temperature of the panel DP, and the controller5that controls the voltage applied to the liquid crystal display panel DP. The controller5needs only to set the black-insertion ratio is 0% and change the voltage applied in the white-display mode to a voltage higher than the critical voltage Vc, in accordance with the temperature detected by the sensor20. Alternatively, the controller needs only to set the black-insertion ratio to a finite value and change the voltage applied in the white-display mode to a voltage lower than the critical voltage Vc, in accordance with the temperature detected by the sensor20.