Patent Publication Number: US-7911429-B2

Title: Programmable liquid crystal display device for controlling viewing angle and driving method thereof

Description:
This application claims the benefit of the Korean Patent Application No. 10-2006-137375 filed on Dec. 29, 2006, which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device and a driving method thereof for controlling an angle at which an image is viewed. 
     2. Discussion of the Related Art 
     In general, an LCD device controls light transmitted through liquid crystals in accordance with video data to display an image corresponding to the video data. The LCD device makes possible a large sized screen beyond a limit, a slim profile, and lightweight. Accordingly, the LCD device is commonly used as a display device for computers and television receivers as a substitute for cathode ray tube (CRT) display devices. 
     Recently, users of portable devices such as portable phones, personal digital assistants (PDAs), and portable computers have a desire to prevent the data being displayed on the portable device from being viewed by other people. Therefore, it is desirable for LCD display devices in portable devices to include a narrow viewing angle mode as well as a normal viewing angle mode so that privacy and security can be achieved. 
     To achieve viewing angle control, a liquid crystal panel with viewing angle control mode includes a separate interference subpixel added to a color pixel. An example of a liquid crystal panel with viewing angle control mode includes a double structure liquid crystal panel. As shown in  FIG. 1 , a double structure liquid crystal panel includes a normal panel  10  in which color pixels are formed, and an interference panel  12  located on the normal panel  10 . The interference panel  12  includes interference subpixels. The normal panel  10  is used for displaying an image while the interference panel  12  blocks light propagating in a lateral direction of the normal panel  10 . In other words, the double structure liquid crystal panel switches between viewing angle modes of an image by blocking light propagating in a lateral direction of the normal panel  10  using the interference panel  12 . 
     An LCD device with the double structure liquid crystal panel controls a viewing angle by selectively driving the interference panel  12  to select between a wide viewing angle mode and a narrow viewing angle mode. In other words, the LCD device turns on or off the interference panel  12  depending on the desired viewing angle mode. However, because external light needs to pass through the double liquid crystal layer in the above-described double structure liquid crystal panel, brightness of an image is significantly decreased. Additionally, the double structure liquid crystal panel has increased thickness and weight. 
     An alternative viewing angle control mode liquid crystal panel includes a region division-type liquid crystal panel in which color pixels and interference subpixels are arranged on the same plane. Color pixels on the region division-type liquid crystal panel include red, green, and blue subpixels as well as interference subpixels. Since the interference subpixels and the color subpixels are arranged on the same plane, the region division-type liquid crystal panel does not increase the thickness and weight of the panel. Also, since the region division-type liquid crystal panel controls the viewing angle using only one liquid crystal layer, the amount of light, brightness, and color purity are not diminished. 
     An LCD device including a viewing angle control mode liquid crystal panel selectively drives interference subpixels on the viewing angle control mode liquid crystal panel to display an image at a normal viewing angle and at a narrow viewing angle. In other words, only the color subpixels are driven when the LCD device displays an image at a normal viewing angle. On the other hand, the color subpixels and the interference subpixels are driven when the LCD device displays an image at a narrow viewing angle. A related art LCD device that controls the viewing angle using interference subpixels increases the brightness using interference subpixels according to a predetermined pattern during a narrow viewing angle mode by driving interference subpixels and color subpixels simultaneously. 
     In the related art LCD device that controls the viewing angle using interference subpixels, the increased brightness of interference subpixels according to the predetermined pattern effectively prevents viewing of images with outlines having a large degree of brightness change, such as a text including letters, from a lateral side. However, it is difficult to block an image with a small degree of brightness change, such as a picture, from being viewed from the lateral side. In addition, since the pattern is fixed, text images with small outline changes may be viewed, though dimly, from the lateral side. Accordingly, maintaining confidentiality and security of displays regardless of the kind of an image have been difficult in the related art LCD device with viewing angle controls. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a liquid crystal display device with a viewing angle control mode that substantially obviates one or more problems due to limitations and disadvantages of the related art, and a driving method thereof. 
     An object of the present invention is to provide a liquid crystal display device and a driving method thereof for controlling a viewing angle of the liquid crystal display device to control confidentiality and security of a display regardless of the kind of an image. 
     Another object of the present invention is to provide a liquid crystal display device and a driving method thereof for controlling a viewing angle to control levels of confidentiality and security of the display. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a liquid crystal display device includes a liquid crystal panel having a plurality of color subpixels and a plurality of interference subpixels, an input unit for inputting color subpixel data to be applied to the color subpixels, a programmable interference data generating unit for storing viewing mode data to be applied to the interference subpixels, the viewing mode data including interference subpixel data and offset subpixel data, the programmable interference data generating unit selectively outputting one of the stored interference subpixel data and offset subpixel data, and a panel driving unit for driving the color subpixels and the interference subpixels in response to the color subpixel data and selected one of the interference subpixel data and the offset subpixel data. 
     In another aspect, a method for driving a liquid crystal display device to control a viewing angle, the method includes the steps of storing interference subpixel data to be used for interference subpixels on a liquid crystal panel in a memory, inputting color subpixel data to be used for color subpixels on the liquid crystal panel, selecting between the interference subpixel data stored in the memory and offset subpixel data, and driving the color subpixels and the interference subpixels in response to the color subpixel data and the selected one of the interference subpixel data and the offset subpixel data. 
     In yet another aspect, a liquid crystal display device includes a liquid crystal panel including a plurality of color subpixels and at least one interference subpixel, a programmable interference data generating unit to generate viewing mode data to be applied to the interference subpixel, the viewing mode data being selected to switch between a narrow viewing angle mode and a wide viewing angle mode, a video data combining unit to combine color data to be applied to the color subpixels with the viewing mode data, and a panel driving unit to apply the color data and the viewing mode data to corresponding color subpixels and the interference subpixel. 
     The programmable interference data generating unit for a liquid crystal display device with a plurality of color subpixels and at least one interference subpixel includes a first storage unit to store interference subpixel data to be applied to the interference subpixel, a second storage unit to store offset subpixel data to be applied to the interference subpixel, and a selector to select between the interference subpixel data and the offset subpixel data, wherein the interference subpixel data is selected for a narrow viewing angle mode and the offset subpixel data is selected for a wide viewing angle mode. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings: 
         FIG. 1  is a schematic cross-sectional view illustrating a liquid crystal panel TO control a viewing angle according to a related art; 
         FIG. 2  is a schematic block diagram illustrating a programmable liquid crystal display device TO control a viewing angle according to an exemplary embodiment of the present invention; 
         FIGS. 3A to 3C  are views explaining a polarization characteristic of an liquid crystal panel according to an exemplary embodiment of the present invention; 
         FIGS. 4A and 4B  are lateral side views illustrating an image state on a liquid crystal panel of the liquid crystal display device of  FIG. 2  according to a wide viewing angle mode and a narrow viewing angle mode, respectively; and 
         FIG. 5  is a block diagram of an exemplary embodiment of a programmable interference data generating unit of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
       FIG. 2  is a schematic block diagram illustrating a programmable liquid crystal display device in accordance with an exemplary embodiment of the present invention to control a viewing angle of the liquid crystal display device. 
     The programmable liquid crystal display device of  FIG. 2  includes a programmable interference data generating unit  40  for generating interference subpixel data to be supplied to interference subpixels ESP 11  to ESP mn  on a liquid crystal panel  30  and a video data combining unit  42  for adding interference data IFD from the programmable interference data generating unit  40  to video data VD received from an external video source. 
     The liquid crystal panel  30  includes red subpixels RSP 11 -RSP mn , green subpixels GSP 11 -GSP mn , blue subpixels BSP 11 -BSP mn , and interference subpixels ESP 11 -ESP mn  formed in regions defined by a plurality of data lines DL 1 -DL 2m  arranged in a horizontal direction and a plurality of gate lines GL 1 -GL 2n  arranged in a vertical direction. Each of the subpixels RSP 11 -RSP mn , GSP 11 -GSP mn , BSP 11 -BSP mn , and ESP 11 -ESP mn  includes a liquid crystal cell CLC connected to a common electrode Vcom and a thin film transistor (TFT) MN to selectively apply a subpixel drive signal to the liquid crystal cell CLC from the data line DL in response to a scan signal on the gate line GL. Red subpixels RSP 11 -RSP mn  are connected to odd-numbered gate lines (e.g., GL 1 , GL 3 , GL 5  . . . GL 2n-1 ) and odd-numbered data lines (e.g., DL 1 , DL 3 , DL 5  . . . DL 2m-1 ). Green subpixels GSP 11 -GSP mn  are connected to odd-numbered gate lines (e.g., GL 1 , GL 3 , GL 5  . . . GL 2n-1 ) and even-numbered data lines (e.g., DL 2 , DL 4 , DL 6  . . . DL 2m ). Blue subpixels BSP 11 -BSP mn  are connected to even-numbered gate lines (e.g., GL 2 , GL 4 , GL 6  . . . GL 2n ) and even-numbered data lines (e.g., DL 2 , DL 4 , DL 6  . . . DL 2n ). Interference subpixels ESP 11 -ESP mn  are connected to even-numbered gate lines (e.g., GL 2 , GL 4 , GL 6  . . . GL 2n ) and odd-numbered data lines (e.g., DL 1 , DL 3 , DL 5  . . . DL 2m-1 ). 
     Each of the interference subpixels ESP 11 -ESP mn  form one group together with each red, green, and blue subpixels RSP 11 -RSP mn , GSP 11 -GSP mn , and BSP 11 -BSP mn  adjacent in an upper direction and a right direction to constitute color pixels PXC 11 -PXC mn  to control a viewing angle. Accordingly, a first color pixel PXC 11  on a first line includes red and green subpixels RSP 11  and GSP 11  connected to a first gate line GL 1  in common and connected to a first data line DL 1  and a second data line DL 2 , respectively. Interference and blue subpixels ESP 11  and BSP 11  are connected to a second gate line GL 2  in common and connected to a first data line DL 1  and a second data line DL 2 , respectively. In this manner, the last color pixel PXC mn  on the last line includes red and green subpixels RSP mn  and GSP mn  connected to a (2n−1)th gate line GL 2n-1  in common and red and green subpixels RSP mn  and GSP mn  connected to a (2m−1)th data line DL 2m-1  and a (2m)th data line DL 2m , respectively. Interference and blue subpixels ESP mn  and BSP mn  are connected to a (2n)th gate line GL 2n  in common and interference and blue subpixels ESP mn  and BSP mn  connected to a (2m−1)th data line DL 2m-1  and a (2m)th data line DL 2m , respectively. 
     As shown in  FIG. 3A , the color subpixels (RSP, GSP, BSP) are driven by a horizontal electric field to polarize transmitted light so that an amount of light decreases as a viewing angle increases from the front of the liquid crystal panel  30  to the side in a lateral direction. Accordingly, an image displayed on the liquid crystal panel  30  can be viewed from a lateral side that is at a wide angle with respect to the front of the liquid crystal panel  30 . In other words, the liquid crystal panel  30  is operating at a wide viewing angle mode. On the other hand, as shown in  FIG. 3B , the interference subpixel ESP is driven using a vertical electric field to polarize the light so that an amount of light perceived at about 40° from either side of the front of the liquid crystal panel  30  is maximized. 
     As shown in  FIG. 3C , light propagating in the side directions due to the interference subpixels ESP merges with light propagating in both side directions from the color subpixels RSP, GSP, and BSP to have a polarization strength characteristic that maximizes an amount of light propagating to a lateral direction at about 40° from the front of the liquid crystal panel  30  and decreases an amount of light at the front of the liquid crystal panel  30 . Therefore, the amount of light polarized on both sides at about 40° from the front of the liquid crystal panel  30  due to the interference subpixels ESP is added to the amount of light from the color subpixels RSP, GSP, and BSP. As a result, an image on the liquid crystal panel  30  viewed at an angle of about 40° from both sides of the liquid crystal panel  30  appears different than the original image data. 
     As shown in  FIG. 2 , the programmable interference data generating unit  40  supplies interference data IFD to a video data combining unit  42  in response to a wide/narrow (W/N) mode control signal from an external video source (e.g., a graphic card of a computer). The interference data IFD is used to switch a viewing angle of the liquid crystal panel  30  between a wide and narrow viewing angle. The interference data IFD includes interference subpixel data (E d ) of an image having an interference pattern. The interference subpixel data (E d ) allows interference light to be added to both side directions from the front of the liquid crystal panel  30  when the W/N mode control signal has a predetermined logic (e.g., a high or low logic) indicating a narrow viewing angle mode. 
     An interference pattern includes a fixed interference pattern and an erasable interference pattern set in advance by a manufacturer. The fixed interference pattern includes logo or letters of a predetermined company, for example, and is stored in memory so that it cannot be deleted. Alternatively, the erasable interference pattern may be updated at any time by the user. To update the erasable interference pattern, the programmable interference data generating unit  40  is connected to an external video source (e.g., a graphic module of a computer system) via a user pattern detecting unit  50 . The programmable interference data generating unit  40  inputs a user interference pattern (i.e., an interference pattern loaded by the user) from the user pattern detecting unit  50  to update an existing erasable interference pattern. The user pattern detecting unit  50  detects a user interference pattern contained (or transmitted) during a vertical blanking interval of a vertical synchronization signal of data streams from the external video source. The user pattern detecting unit  50  then supplies the detected user interference pattern to the programmable interference data generating unit  40 . In the case where the W/N mode control signal has an initialization logic designating a wide viewing angle mode, the interference data IFD includes offset subpixel data (E off ) having an offset value that prevents interference light from propagating to both side directions from the front side of the liquid crystal panel  30 . 
     The video data combining unit  42  receives video data VD including color subpixel data of red, green, and blue subpixels (RSP, GSP, BSP) from the external video source (not shown). The video combining unit  42  adds interference data IFD from the programmable interference data generating unit  40  to the video data VD. Also, the video data combining unit  42  rearranges color subpixel data and interference subpixel data (E d ) (or offset subpixel data E off ) so that these data coincide with an arrangement state of the subpixels on the liquid crystal panel  30  to generate combined video data CVD. The combined video data CVD prepared by the video data combining unit  42  includes a subpixel data stream where red and green subpixel data (R d  and G d ) are alternated when red and green subpixels (RSP and GSP) connected to odd-numbered gate lines GL 1 -GL 2n-1  of the liquid crystal panel  30  are scanned. The combined video data CVD also includes a subpixel data stream where interference (or offset) and blue subpixel data E d  (or E off ) and B d  are alternated when interference and blue subpixels ESP and BSP connected to even-numbered gate lines GL 2 -GL 2n  of the liquid crystal panel  30  are scanned. 
     The programmable LCD device further includes a gate driver  44  for sequentially driving gate lines GL 1 -GL 2n  on the liquid crystal panel  30 , a data driver  46  for driving data lines DL 1 -DL 2m  on the liquid crystal panel  46 , and a timing controller  48  for controlling operation timings of the data and gate drivers  44  and  46 . The gate driver  44  generates 2n scan signals sequentially enabling the gate lines GL 1 -GL 2n  in response to gate timing signals GTS from the timing controller  48 . The gate lines GL 1 -GL 2n  on the liquid crystal panel  30  are enabled sequentially and exclusively by a period corresponding to half of the period of a horizontal synchronization signal by the 2n scan signals. 
     The data driver  46  supplies subpixel drive signals to 2m data lines DL 1 -DL 2m  whenever one of the gate lines GL 1 -GL 2n  is enabled in response to data timing signals DTS from the timing controller  48 . For this purpose, the data driver  46  receives combined video data CVD transmitted in series from the video data combining unit  42 . When columns of red and green subpixels (RSP and GSP) connected to one of odd-numbered gate lines GL 1 -GL 2n-1  are scanned, the data driver  46  inputs a subpixel data stream where red and green subpixel data (R d  and G d ) are alternated to allow red subpixel drive signals to be supplied to odd-numbered data lines DL 1 -DL 2m-1 , respectively, and to allow green subpixel drive signals to be supplied to even-numbered data lines DL 2 -DL 2m , respectively. On the other hand, when columns of interference and blue subpixels (ESP and BSP) connected to one of even-numbered gate lines GL 2 -GL 2n  are scanned, the data driver  46  inputs a subpixel data stream where interference (or offset) and blue subpixel data E d  (or E off ) and B d  are alternated to allow interference subpixel drive signals to be supplied to odd-numbered data lines DL 1 -DL 2m-1 , respectively, and to allow blue subpixel drive signals to be supplied to even-numbered data lines DL 2 -DL 2m , respectively. 
     When the W/N mode control signal generates a predetermined logic for a narrow viewing angle, an interference subpixel drive signal has a voltage allowing an interference subpixel ESP to transmit interference light to both side directions from the front side of the liquid crystal panel  30 . An amount of light transmitted to both side directions by the interference subpixel ESP is controlled by a voltage level of the interference subpixel drive signal. The amount of light transmitted to both side directions due to the interference subpixels (ESP) is added to the amount of light transmitted to both side directions by the color subpixels (RSP, GSP, BSP), so that a brightness interference component at the side directions is perceived. Accordingly, as shown in  FIG. 4B , an image that cannot be recognized from the side direction is displayed on the liquid crystal panel  30 . Also, the interference subpixel drive signals have different voltage levels for each of the interference subpixels ESP 11 -ESP mn  corresponding to interference patterns, so that a difference in an amount of brightness interference is generated between color pixels PXC. Accordingly, an image displayed on the liquid crystal panel  30  cannot be recognized from the side directions. Consequently, confidentiality and security of the display are maintained during a narrow viewing angle mode. 
     Moreover, the degree of recognition of an image from the side directions can be controlled depending on the density of letters and characters contained in the erasable interference pattern. For this purpose, a user loads an interference pattern containing letters and characters of an appropriate density depending on the desired degree of security and confidentiality to the programmable interference data generating unit  40 . Therefore, the programmable LCD device that controls the viewing angle according to the present invention not only enhances maintaining confidentiality and security of the display but also controls the degree of confidentiality and security. 
     When the W/N mode control signal has an initialization logic indicating a wide viewing angle, the interference subpixel ESP responds to an offset pixel drive signal having an offset voltage that prevents interference light from being transmitted to the side directions as well as to the front side of the liquid crystal panel  30 . An amount of light passing through the interference subpixels ESP disappear due to the offset subpixel drive signal so that only red, green, and blue subpixels (RSP, GSP, BSP) allow light to pass through the entirety of the liquid crystal panel  30 . Accordingly, as shown in  FIG. 4A , an image displayed on the liquid crystal panel  30  can be viewed from the side directions as well as the front side. 
     The timing controller  48  receives synchronization signals SYNC (i.e., horizontal and vertical synchronization signals and a data clock) from an external video source. The timing controller  48  generates gate timing signals GTS to be supplied to the gate driver  44  and data timing signals DTS to be supplied to the data driver  46  using the synchronization signals SYNC. Also, the timing controller  48  generates interference control signals ECS required for a data generating operation and an erasable interference pattern updating operation of the programmable interference data generating unit  40 , and combining control signals CCS required for data combining operation of the video data combining unit  42 . 
       FIG. 5  is a detailed block diagram of an exemplary embodiment of the programmable interference data generating unit of  FIG. 2 . As shown in  FIG. 5 , the programmable interference data generating unit  40  includes a first memory  60  and a second memory  62  in which interference patterns are stored, respectively, and a register  64  in which offset subpixel data is stored. The register  64  may be replaced by a plurality of switches, for example, that can generate offset subpixel data E off . 
     The interference pattern stored in the first memory  60  may be updated with a new user pattern from the user pattern detecting unit  50  of  FIG. 2 . In other words, the first memory  60  updates the already existing interference pattern into a new user pattern whenever a new user pattern is input from the user pattern detecting unit  50 . Confidentiality and security of data displayed on the liquid crystal panel  30  can be enhanced or alleviated by an updating operation of the first memory  60 . Therefore, a degree of recognition of an image from the side directions may be controlled. The first memory  60  may be a non-volatile memory (e.g., static random access memory (SRAM) or electrically erasable programmable random access memory (EEPROM)) which maintains a stored erasable interference pattern even when power is not supplied. The first memory  60  can have the same storage capacity corresponding to or smaller than the number of interference subpixels on the liquid crystal panel  30 . This is because the first memory  60  can generate interference subpixel data corresponding to the number of the interference subpixels on the liquid panel  30  using a repeated reading operation of the first memory  60  even when the storage capacity of the first memory is smaller than the number of the interference subpixels on the liquid crystal panel  30 . 
     The fixed interference pattern stored in the second memory  62  is a pattern programmed by a manufacturer, for example, and is not erasable, so that an interference pattern to be used for performing a viewing angle control is available even when the interference pattern is not stored in the first memory  60 . Reading operations of the first and second memories  60  and  62  are selectively performed under control of a memory controller  68 . The memory controller  68  controls a reading operation of the second memory  62  to be performed when there is no interference pattern stored in the first memory  60 . The memory controller  68  controls a selective reading operation of the first and second memories  60  and  62 , and an interference pattern updating operation of the first memory  60  in response to an interference control signal ESC from the timing controller  48  of  FIG. 2 . 
     A selector  66  transmits either an offset subpixel data (E off ) from the register  64  or an interference subpixel data (E d ) from one of the first and second memories  60  and  62  as interference data IFD to the video data combining unit  42  of  FIG. 2  in response to a logic value of the W/N mode control signal. When the W/N mode control signal has a predetermined logic (i.e., a high or low logic) indicating a narrow viewing angle mode, the selector  66  allows interference subpixel data (E d ) from one of the first and second memories  60  and  62  to be supplied as interference data IFD to the video data combining unit  42  of  FIG. 2 . On the other hand, if the W/N mode control signal has an initialization logic (i.e., a high or low logic) designating a wide viewing angle mode, the selector  66  allows offset subpixel data (E off ) from the register  64  to be supplied as interference data IFD to the video data combining unit  42  of  FIG. 2 . 
     As described above, a programmable LCD device, and a driving method thereof, for controlling a viewing angle according to the present invention includes an interference pattern to be inserted into the displayed image using interference subpixels on a liquid crystal panel. The interference pattern is loaded to a programmable interference data generating unit such that the interference pattern includes letters and characters having density depending on the level of confidentiality and security desired. Therefore, the programmable LCD device and the driving method thereof in accordance with the present invention not only provide confidentiality and security of the displayed image but also allow control of the degree of the confidentiality and security. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the programmable LCD device of the present invention and a driving method thereof without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.