Abstract:
Disclosed is a liquid crystal display (LCD) wherein driving stability is secured. The LCD includes Optically Compensated Birefringence (OCB) mode liquid crystal cells formed where a data line and a scan line cross over within a display region of an LCD panel, a panel driver for driving the data and the scan lines, and OCB mode dummy liquid crystal cells formed within a non-display region of the LCD panel to surround the liquid crystal cells. Liquid crystal in each of the dummy liquid crystal cells sustains a bend state when the liquid crystal cells in the display region are driven corresponding to a data signal applied from the data line.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to and the benefit of Korean Patent Application No. 10-2005-0112575, filed on Nov. 23, 2005, in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference.  
       BACKGROUND  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a liquid crystal display (LCD) and a driving method thereof, and more particularly, to an LCD wherein driving stability can be secured and a driving method for such an LCD.  
         [0004]     2. Discussion of Related Art  
         [0005]     In recent years, various types of flat panel displays that have a lower weight and volume than cathode ray tubes (CRTs) have been developed. Liquid crystal displays (LCDs), field emission displays (FEDs), plasma display panels (PDPs), organic light emitting displays (OLEDs) and the like are used as flat panel displays.  
         [0006]     Since LCDs have advantages in terms of miniaturization, lightweight, low power consumption and the like, they have been gradually highlighted for replacing the existing CRTs. At present, LCDs have been used not only in portable devices such as cellular phones and Personal Digital Assistants (PDAs) but also in large and medium sized devices such as monitors and TVs.  
         [0007]     In an LCD, liquid crystal is injected between an upper substrate having a common electrode and the like formed thereon and a lower substrate having a thin film transistor, a pixel electrode and the like formed thereon. Different electric potentials are applied to the pixel electrode and the common electrode so that an electric field is formed therebetween. The arrangement of the liquid crystal between the upper and lower substrates is changed due to the electric field between the pixel and the common electrodes, and thus an image is displayed while transmittance of light is being controlled.  
         [0008]     Among the LCDs, an Optically Compensated Birefringence (OCB) mode LCD has the advantages of a wide viewing angle and a high response speed. Therefore, the OCB mode LCD is being actively studied.  
         [0009]     As shown in  FIGS. 1A and 1B , in an OCB mode LCD, liquid crystal injected between an upper substrate  10  and a lower substrate  12  is initially set to be in a splay state where a voltage V smaller than a transition voltage Vcr is applied to the liquid crystal (V&lt;Vcr). In such a splay state, light is irregularly transmitted as the voltage V increases. Accordingly, in the splay state, unevenness or flicker is produced on an image.  
         [0010]     If a voltage V larger than a transition voltage Vcr is applied to the liquid crystal in the splay state, the liquid crystal is converted into a bend state (V&gt;Vcr). In such a bend state, the transmittance of the liquid crystal is linearly decreased as the voltage V increases. Thus, in a conventional OCB mode LCD, a voltage larger than the transition voltage Vcr is applied to liquid crystal cells to convert the liquid crystal into the bend state, and a predetermined image is then displayed.  
         [0011]     However, in the conventional OCB mode LCD, the liquid crystal of some of the liquid crystal cells forming the LCD panel returns from a bend state to a splay state. In practice, the liquid crystal returning from the bend state to the splay state mainly occurs in the liquid crystal cells located at the outermost edges of the LCD panel. It can be speculated that these liquid crystal cells do not sustain their bend states because the liquid crystal cells located at the outermost edges of the LCD panel are not affected by the surrounding electric field.  
       SUMMARY OF THE INVENTION  
       [0012]     Accordingly, it is an aspect of the present invention to provide an LCD wherein driving stability can be secured and a driving method for such an LCD.  
         [0013]     A first aspect of the present invention provides an LCD including OCB mode liquid crystal cells formed where data lines and scan lines cross over within an effective display region of an LCD panel, a panel driver for driving the data and the scan lines, and OCB mode dummy liquid crystal cells formed within a non-display region of the LCD panel to surround the liquid crystal cells within the effective display region. The effective display region may also be called a display region. The liquid crystal contained in each of the dummy liquid crystal cells sustains a bend state when the liquid crystal cells are driven corresponding to a data signal applied from the data line.  
         [0014]     The panel driver may include a scan driver for sequentially applying scan signals to scan lines coupled to the dummy liquid crystal cells and the liquid crystal cells, and a data driver for applying a data signal to data lines coupled to the dummy liquid crystal cells and the liquid crystal cells. The scan driver and the data driver may be both included in a panel driver. The data driver controls a voltage value of a data signal applied to the dummy liquid crystal cells such that a voltage larger than a transition voltage can be applied to liquid crystal in each of the dummy liquid crystal cells.  
         [0015]     A second aspect of the present invention provides a method of driving an LCD, the method including displaying an image from OCB mode liquid crystal cells formed within an effective display region, and applying a voltage larger than a transition voltage to liquid crystal in each dummy liquid crystal cell formed within a non-display region when an image is displayed from the liquid crystal cells. The non-display region may surround the display region allowing the dummy liquid crystal cells to impose an electric field upon the liquid crystal cells within the display region.  
         [0016]     The transition voltage may be a voltage with which the liquid crystal in each of the dummy liquid crystal cells can change from a splay state to a bend state. The application of a voltage larger than the transition voltage to the dummy liquid crystal cells, includes turning on a TFT included in each of the dummy liquid crystal cells, and applying a data signal to a pixel electrode included in each of the dummy liquid crystal cells when the TFTs are turned on.  
         [0017]     In one embodiment, an OCB mode LCD includes an LCD panel that has scan lines, data lines, a display region having liquid crystal cells, and a non-display region having dummy liquid crystal cells. The scan lines cross over the data lines at least within the display region. The non-display region is formed surrounding the display region. The LCD also includes a scan driver for applying scan signals to the scan lines and a data driver for applying data signals to the data lines. The liquid crystal in the dummy liquid crystal cells is maintained in a bend state to impose an electric field upon the liquid crystal cells in the display region. The liquid crystal cells and the dummy liquid crystal cells may be both formed where the scan lines and the data lines cross over. Then the liquid crystal cells and the dummy liquid crystal cells are both coupled to the scan lines and the data lines that are respectively applying the scan signals and the data signals to these cells. In this embodiment, the data signals applied to the dummy liquid crystal cells have a voltage greater than a transition voltage of the liquid crystal. Alternatively, the dummy liquid crystal cells may be coupled to a dummy line applying a dummy voltage to the dummy liquid crystal cells. Then, the dummy voltage applied to the dummy liquid crystal cells is greater than the transition voltage of the liquid crystal.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     These and/or other aspects and features of the invention will become apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings of which:  
         [0019]      FIGS. 1A and 1B  are views illustrating movements of an OCB mode liquid crystal depending on a voltage applied thereto;  
         [0020]      FIG. 2  is a block diagram of an OCB mode LCD according to a first embodiment of the present invention;  
         [0021]      FIG. 3  is a circuit diagram of a liquid crystal cell and dummy liquid crystal cell of the LCD of  FIG. 2 ;  
         [0022]      FIG. 4  is a block diagram of an OCB mode LCD according to a second embodiment of the present invention; and  
         [0023]      FIG. 5  is a circuit diagram of a dummy liquid crystal cell of the LCD of  FIG. 4 . 
     
    
     DETAILED DESCRIPTION  
       [0024]     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying  FIGS. 2, 3 ,  4 , and  5 .  
         [0025]      FIG. 2  is a block diagram of an OCB mode LCD according to a first embodiment of the present invention.  
         [0026]     Referring to  FIG. 2 , the OCB mode LCD according to the first embodiment of the present invention includes an LCD panel  100 , a data driver  104  for applying data signals to data lines D 1  to Dm of the LCD panel  100  and a scan driver  102  for applying scan signals to scan lines S 1  to Sn of the LCD panel  100 .  
         [0027]     In the embodiment of  FIG. 2 , the scan driver  102  and the data driver  104  are shown to be separated from each other. However, the two drivers  102  and  104  may form a single integrated circuit (IC) as a Chip On Glass (COG). A circuit including both the scan driver  102  and the data driver  104  may be referred to as a panel driver.  
         [0028]     The scan driver  102  applies the scan signals to the scan lines S 1  to Sn. Accordingly, dummy liquid crystal cells  112  and liquid crystal cells  110  are selected by the horizontal lines.  
         [0029]     The data driver  104  converts data input from the outside into the data signals and applies the data signals for one horizontal line to the data lines D 1  to Dm in every one horizontal period.  
         [0030]     The LCD panel  100  is divided into an effective display region  106  and a non-display region  108 . The effective display region  106  may also be called a display region. In the effective display region  106 , the liquid crystal cells  110  are arranged in a matrix. Each of the liquid crystal cells  110  displays a portion of the image corresponding to a data signal applied thereto. That is, each of the liquid crystal cells  110  located within the effective display region  106  is used to display a portion of the image corresponding to the applied data signal.  
         [0031]     In the non-display region  108 , the dummy liquid crystal cells  112  are arranged to surround the liquid crystal cells  110  formed within the effective display region  106 . A voltage larger than a transition voltage Vcr is applied to liquid crystal in each of the dummy liquid crystal cells  112  to sustain a bend state. That is, the dummy liquid crystal cells  112  located within the non-display region  108  provide an electric field to the liquid crystal cells  110  formed at the outer edges of the effective display region  106  while sustaining their bend states, so that the liquid crystal cells  110  formed within the effective display region  106  are prevented from returning to their splay states. Meanwhile, the dummy liquid crystal cells  112  located within the non-display region  108  have no influence on the image to be displayed. For example, a black matrix may be formed on the non-display region  108  of the LCD panel  100  to cover the dummy liquid crystal cells  112 , thereby preventing light from the dummy liquid crystal cells  112  from being radiated to the outside.  
         [0032]      FIG. 3  is a circuit diagram of the liquid crystal cell and the dummy liquid crystal cell of the LCD shown in  FIG. 2 .  
         [0033]     Referring to  FIG. 3 , each of the liquid crystal cells  110  and the dummy liquid crystal cells  112  includes a thin film transistor (TFT) located between a scan line S and a data line D, and a liquid crystal capacitor Clc is coupled to the TFT.  
         [0034]     The liquid crystal capacitor Clc is used to express the equivalent capacitance formed by liquid crystal interposed between a pixel electrode Pe coupled to a drain electrode of the TFT and a common electrode Ce formed on an upper substrate of the LCD. Further, each of the liquid crystal cells  110  and the dummy liquid crystal cells  112  also includes a storage capacitor Cst.  
         [0035]     During operation, the TFT is first turned on in response to a scan signal applied to the scan line S. If the TFT is turned on, a data signal applied to the data line D is applied to the pixel electrode Pe via the TFT. Then, the light transmittance of the liquid crystal is controlled by a voltage applied between the pixel electrode Pe and the common electrode Ce. A voltage corresponding to the data signal may be charged in the storage capacitor Cst to be applied to the pixel electrode Pe for one frame period.  
         [0036]     In the described embodiments of the present invention, a voltage larger than the transition voltage Vcr is applied to liquid crystal in each of the dummy liquid crystal cells  112  when the liquid crystal cells  110  are driven. Then, the dummy liquid crystal cells  112  sustain their bend states. There may be various methods to apply a voltage larger than the transition voltage Vcr to the liquid crystal in the dummy liquid crystal cells  112 . For example, in a case where the common electrode Ce is fixed to a ground voltage, the voltage of the data signal applied to the pixel electrode Pe is such that a voltage larger than the transition voltage Vcr can be applied to the liquid crystal. Further, in a case where the common electrode Ce is inverted into a positive or negative voltage, a data signal corresponding to the inversion voltage is applied to the dummy liquid crystal cells  112  so that a voltage larger than the transition voltage Vcr can be applied to the liquid crystal. As such, if the liquid crystal in the dummy liquid crystal cells  112  sustains a bend state while a voltage larger than the transition voltage Vcr is applied to the liquid crystal, the liquid crystal cells  110  formed within the effective display region  106  can stably sustain their bend states so that a desired image can be displayed.  
         [0037]      FIG. 4  is a block diagram of an OCB mode LCD according to a second embodiment of the present invention. When describing  FIG. 4 , like elements to the elements shown in  FIG. 2  are designated by like reference numerals and their detailed description is omitted.  
         [0038]     Referring to  FIG. 4 , the OCB mode LCD of the second embodiment includes an LCD panel  100 ′, a data driver  104  for applying the data signals to data lines Dl to Dm of the LCD panel  100 ′ and a scan driver  102  for applying scan signals to scan lines S 1  to Sn of the LCD panel  100 ′.  
         [0039]     In an effective display region  106 ′, liquid crystal cells  110 ′ are formed. Each of the liquid crystal cells  110 ′ displays a portion of the image corresponding to a data signal applied thereto. The liquid crystal cells  110 ′ are similar to the liquid crystal cells  110  of the first embodiment.  
         [0040]     In a non-display region  108 ′ of the LCD panel  100 ′, dummy liquid crystal cells  120  are arranged to surround the liquid crystal cells  110 ′. A voltage larger than the transition voltage Vcr is applied to liquid crystal in each of the dummy liquid crystal cells  120  to sustain a bend state. That is, the dummy liquid crystal cells  120  provide an electric field to the liquid crystal cells  110 ′ to sustain the bend state in the liquid crystal cells  110 ′. As a result, the liquid crystal cells  110 ′ are prevented from returning to their splay states.  
         [0041]     In the second embodiment, the dummy liquid crystal cells  120  are commonly coupled to a dummy line DL. The dummy line DL is coupled to a dummy voltage Vd. A voltage value of the dummy voltage Vd is set and controlled such that the liquid crystal contained or included in the dummy liquid crystal cells  120  sustains a bend state.  
         [0042]      FIG. 5  is a circuit diagram of the dummy liquid crystal cell of the LCD shown in  FIG. 4 .  
         [0043]     Referring to  FIG. 5 , each of the dummy liquid crystal cells  120  includes liquid crystal interposed between a pixel electrode Pe and a common electrode Ce. The pixel electrode Pe, the liquid crystal and the common electrode Ce are equivalently expressed as a liquid crystal capacitor Clc. Further, each of the dummy liquid crystal cells  120  also includes a storage capacitor Cst. Some embodiments may not include the storage capacitor Cst.  
         [0044]     All the pixel electrodes Pe included in the dummy liquid crystal cells  120  are electrically connected to a dummy line DL. That is, in the second embodiment, the pixel electrode Pe is not electrically connected to the dummy line DL via a TFT but is electrically connected to the dummy line DL such that a dummy voltage Vd is applied to the pixel electrode Pe. Here, a voltage value of the dummy voltage Vd is set or controlled such that a voltage larger than the transition voltage Vcr is applied to the liquid crystal contained or included in each of the dummy liquid crystal cells  120 . For example, in a case where the common electrode Ce is fixed to a ground voltage, the dummy voltage Vd is set to a voltage larger that the transition voltage Vcr such that the liquid crystal sustains a bend state. Further, in a case where the common electrode Ce is inverted into a positive or negative voltage, the voltage value of the dummy voltage Vd is set to have a relationship with the inversion voltage such that the liquid crystal sustains a bend state. As such, if the liquid crystal in the dummy liquid crystal cells  112  sustains a bend state while a voltage larger than the transition voltage Vcr is applied to the liquid crystal, the liquid crystal cells  110  formed within the effective display region  106  can sustain their stable bend states so that a desired image can be displayed.  
         [0045]     As described above, in an LCD and a method for driving the LCD according to the embodiments of the present invention, dummy pixels are formed within a non-display region, and liquid crystal of the dummy pixels sustains a band state, thereby preventing liquid crystal cells included within an effective display region from being returned to a splay state. Accordingly, in the LCD and the method for driving the LCD according to the embodiments of the present invention, an image can be stably displayed within an effective display region.  
         [0046]     Although certain exemplary embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes might be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.