Abstract:
A liquid crystal display comprising: a receiver for receiving power and differential signal; a backlight power supply which supplies the power to a backlight unit; a power-off sensor which senses power-off of the backlight power supply and distorts one of the differential signals; and a controller which senses the distortion of the differential signal and generates an after-image removing gray-scale signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from Korean Patent Application No. 2007-0049638, filed on May 22, 2007 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
       BACKGROUND OF INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to a liquid crystal display that is capable of removing an after-image during a power-off operation, and a driving method therefor. 
         [0004]    2. Description of Related Art 
         [0005]    In general, a liquid crystal display includes a thin film transistor (TFT) substrate and a color filter substrate on which an electric field generating electrode is respectively formed and which face each other, and a liquid crystal layer interposed between the two substrates. The liquid crystal is moved by an electric field generated by applying voltage to the electrodes, thereby varying light transmittance to form an image. 
         [0006]    The liquid crystal display includes a liquid crystal panel including a plurality of liquid crystal cells which are connected to gate lines and data lines, a data driver which applies a gray-scale display voltage to the data lines, a gate driver which applies a gate driving signal to the gate lines, a controller which controls the data driver and the gate driver, and a power supply which supplies a driving voltage. 
         [0007]    The liquid crystal cell includes a liquid crystal capacitor for charging a gray-scale display voltage, and a TFT for applying the gray-scale display voltage to the liquid crystal capacitor in response to a gate-on voltage. The driving voltage which is supplied by the power supply includes a power voltage, a ground voltage, a gate-on voltage, a gate-off voltage, a common voltage and an analog power voltage. 
         [0008]    When power supplied to the power supply is cut off, the power supply outputs an overall output voltage including the power voltage, the gate-on voltage, the gate-off voltage, the common voltage and the analog power voltage into 0V of a ground voltage level. When the gate-on voltage of the ground voltage level is applied to a gate of the TFT, the gray-scale display voltage applied to the liquid crystal capacitor is discharged as a leakage current through a channel of the TFT. 
         [0009]    In the conventional liquid crystal display, when the power supply is cut off and the backlight unit is powered off, a pattern which is displayed on the liquid crystal panel does not disappear with the cut off of power but remains for a certain time as an after-image. 
       SUMMARY OF INVENTION 
       [0010]    Accordingly, it is an aspect of the present invention to provide a liquid crystal display which is capable of removing an after-image, 
         [0011]    The foregoing aspect of the present invention can be achieved by providing a liquid crystal display comprising: a receiver for receiving power and differential signals from outside; a backlight power supply which supplies the power to a backlight unit; a power-off sensor which senses power-off of the backlight power supply and distorts one of the differential signals; and a controller which senses the distortion of a differential signal and generates an after-image removing gray-scale signal. 
         [0012]    The controller may include a gray-scale signal generator which generates the after-image removing gray-scale signal. 
         [0013]    The gray-scale signal generator may include a differential signal receiver, and a signal comparator which compares the differential signals. 
         [0014]    The gray-scale signal generator may include a memory and outputs the after-image removing gray-scale signal which is previously stored therein when the power-off of the backlight unit is sensed. 
         [0015]    The gray-scale signal generator outputs a white gray-scale signal when a liquid crystal mode is a normally white mode, and outputs a black gray-scale signal when the liquid crystal mode is a normally black mode. 
         [0016]    The power-off sensor may include a transistor which functions as a switch and at least one resistor. 
         [0017]    The power-off sensor grounds the differential signal. 
         [0018]    The power-off sensor may include a bipolar transistor or a metal oxide silicon (MOS) transistor. 
         [0019]    The transistor may include a first terminal connected to a differential signal line which connects the receiver and the controller; a second terminal connected to the resistor which is connected to the backlight power supply; and a third terminal connected to a ground terminal. 
         [0020]    The differential signals may include a first clock signal and a clock second signal which has an inverse phase to the first signal. 
         [0021]    The foregoing aspect of the present invention can also achieved by providing a method of driving a liquid crystal display, comprising: detecting power-off of a backlight unit by using backlight power and differential signals; generating an after-image removing gray-scale signal when the power-off of the backlight unit is detected; and applying the after-image removing gray-scale signal to a data driver. 
         [0022]    One of the differential signals is grounded by using a transistor which functions as a switch and detects the power-off of the backlight unit. 
         [0023]    A white or a black gray-scale signal is generated according to a liquid crystal mode, in the generating the after-image removing gray-scale signal. 
         [0024]    The white gray-scale signal is generated when the liquid crystal mode is a normally white mode, and the black gray-scale signal is generated when the liquid crystal mode is a normally black mode, in the generating the after-image removing gray-scale signal. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0025]    The above and/or other aspects of the present invention will become apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings, in which: 
           [0026]      FIG. 1  is a block diagram illustrating a liquid crystal display according to an exemplary embodiment of the present invention. 
           [0027]      FIG. 2  is a block diagram illustrating a signal order during a power-off operation in the liquid crystal display according to the exemplary embodiment of the present invention. 
           [0028]      FIG. 3  is a diagram illustrating a power-off sensor shown in  FIG. 1 . 
           [0029]      FIG. 4  is a block diagram illustrating a controller in  FIG. 1 . 
           [0030]      FIG. 5  is a flowchart illustrating a driving method of the liquid crystal display according to the exemplary embodiment of the present invention. 
           [0031]      FIG. 6  and  FIG. 7  are views for illustrating a power-off operating order in the driving method of the liquid crystal display according to the exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0032]      FIG. 1  is a block diagram illustrating a liquid crystal display according to an exemplary embodiment of the present invention. 
         [0033]    As shown in  FIG. 1 , the liquid crystal display includes a main system  10 , a receiver  20 , a backlight power supply  40 , a controller  60 , a driving power supply  30 , a power-off sensor  50 , a liquid crystal panel  70 , a data driver  90 , a gate driver  100  and a gamma voltage generator  80 . 
         [0034]    The main system  10  supplies pixel data (RGB), a control signal (CONT), a driving voltage (VDD), a backlight driving voltage (BLV) and the like to the receiver  20 . The main system  10  digitalizes and compresses the pixel data (RGB) and the control signal (CONT), etc., and lowers a voltage by a differential signal and supplies it to the receiver  20 . The differential signal includes a first signal and a second signal which has an inversed phase to the first signal. The differential signal may use a low voltage differential signaling (LVDS). 
         [0035]    The receiver  20  receives the pixel data (RGB), the control signal (CONT), etc., and then supplies them to the controller  60 . The receiver  20  supplies the driving voltage (VDD) and the backlight driving voltage (BLV) which are supplied from the main system  10  to the driving power supply  30  and the backlight power supply  40 , respectively. 
         [0036]    When the driving voltage (VDD) is supplied from outside to the driving power supply  30 , the driving power supply  30  supplies the driving voltage (VDD) to the controller  60  including a digital circuit, the data driver  90  and the gate driver  100 . The driving power supply  30  generates a gate-on voltage (VON) and a gate-off voltage (VOFF) as the driving voltage (VDD) and supplies the gate-on voltage (VON) and the gate-off voltage (VOFF) to the gate driver  100 . The driving power supply  30  outputs an overall output voltage including the gate-on voltage (VON), the gate-off voltage (VOFF), a common voltage (VCOM) and an analog driving voltage (AVDD) into a ground voltage level when a supply of the driving voltage (VDD) is cut off from outside. The ground voltage level may be 0V. 
         [0037]    The backlight driving voltage (BLV) is supplied to the backlight power supply  40  through the receiver  20 . The backlight power supply  40  supplies a backlight on voltage and a backlight off voltage to a backlight unit (not shown). The backlight unit may include a light emitting diode (LED). 
         [0038]    The backlight power supply  40  generates a SET signal when the backlight on/off voltage is supplied. For example, the SET signal includes a high/low level. The SET signal becomes a high level when the backlight on voltage is output, and becomes a low level when the backlight off voltage is output. 
         [0039]    The power-off sensor  50  is connected to the power supply  40 , the receiver  20 , and the controller  60 . The power-off sensor  50  distorts a clock signal (CLK) which is inputted from the receiver  20  through the SET signal of the backlight driving voltage (BLV). The power-off sensor  50  will be more specifically described later. 
         [0040]    The controller  60  restores the received pixel data (RGB) and the control signal (CONT) which are converted into differential signals by the receiver  20  so as to compare the pair of signals and detect an error thereof. For example, the controller  60  restores and outputs the pixel data (RGB) and the control signal (CONT) by using a voltage difference between the differential signals which are converted into low voltage differential signals. The controller  60  generates and outputs a data control signal (DCS) to control the data driver  90 , and a gate control signal (GCS) to control the gate driver  100  by using the restored control signal (CONT). The data control signal (DCS) which is generated from the controller  60  includes a source start pulse (SSP), a source shift clock (SSC), etc., and the gate control signal (GCS) includes a gate start pulse (GSP), a gate shift clock (GSC), etc. The controller  60  re-arrays the pixel data (RGB) supplied from the receiver  20  in accordance with a driving order of the data driver  90  and supplies them to the data driver  90 . 
         [0041]    The controller  60  receives the clock signal (CLK) which is distorted by the power-off sensor  50  during the outside power-off and determines a signal error. When the controller  60  senses the signal error, the controller  60  operates in a built-in self test pattern (BIST) mode. The BIST mode is an operating mode in which predetermined after-image removing gray-scale signals (WS and BS) are output to the data driver  90  together with the pixel data (RGB). Accordingly, when the controller  60  operates in the BIST mode, the data driver  90  can apply a gray-scale display voltage of the same gray-scale to a liquid crystal cell according to a minimum applying voltage. A detailed description thereof will be described later together with the power-off sensor  50 . 
         [0042]    The liquid crystal panel  70  includes an upper substrate on which a color filter and a common electrode are formed, a lower substrate on which a TFT is formed, a liquid crystal layer which is interposed between the upper substrate and the lower substrate. The lower substrate includes a liquid crystal capacitor (Clc), and the TFT which is connected to a plurality of gate lines (GL 1 , . . . , GLn) and a plurality of data lines (DL 1 , . . . , DLm) and applies the gray-scale display voltage to the liquid crystal capacitor (Clc) in response to the gage on voltage (VON). The TFT includes a gate which is connected to the gate line (GL 1 ), a source which is connected to the data line (DL 1 ) and a drain which is connected to a pixel electrode of the liquid crystal capacitor (Clc). 
         [0043]    The gamma voltage generator  80  divides the analog driving voltage (AVDD) supplied from the driving power supply  30  and generates a gamma voltage (VGMA), and then supplies the gamma voltage (VGMA) to the data driver  90 . 
         [0044]    The data driver  90  generates the gray-scale display voltage corresponding to the pixel data (RGB) and the after-image removing gray-scale signals (WS and BS) using the gamma voltage, and applies the gray-scale display voltage to the TFT which is driven by the gate-on voltage (VON), thereby displaying the gray-scale display voltage in a unit of the gate line (GL 1 , . . . , GLn). To this end, the data driver  90  receives the data control signal (DCS) and the data signal (RGB) from the controller  60  and also receives the gamma voltage (VGMA) from the gamma voltage generator  80 . 
         [0045]    The data driver  90  is prepared with a data driving integrated circuit (IC) and adhered to the liquid crystal display panel  70  in a tape carrier package (TCP) type. Alternatively, the data driver  90  may be directly installed on a non-display area of the liquid crystal panel  70  in a chip on glass (COG) type. 
         [0046]    The gate driver  100  sequentially applies the gate-on voltage (VON) to the plurality of gate lines (GL 1 , . . . , GLn) and applies the gate-off voltage (VOFF) to a gate line to which the gate-on voltage (VON) is not applied. That is, the gate driver  100  turns on the plurality of the TFTs which are respectively connected to the sequentially selected gate lines (GL 1 , . . . , GLn) at the same time. To this end, the gate driver  100  receives the control signal (GCS) from the controller  60 , and receives the gate-on voltage (VON) and the gate-off voltage (VOFF) from the driving power supply  30 . 
         [0047]    The gate driver  100  may be prepared with the gate driving IC and adhered to the liquid crystal panel  70 . Alternatively, the gate driver  100  may be integrated on the non-display area of the liquid crystal panel as an amorphous silicon gate (ASG) when the TFT is formed. 
         [0048]    The driving time point of an after-image removing gray-scale signal in the liquid crystal display according to the exemplary embodiment of the present invention will be described with reference to  FIG. 2 . 
         [0049]      FIG. 2  is a drawing illustrating a signal order during a power-off in the liquid crystal display according to the exemplary embodiment of the present invention. 
         [0050]    As shown in  FIG. 2 , the backlight driving voltage (BLV) is powered off the moment when the outside power is cut off. Then, the differential signals (LVDS) including the pixel data (RGB) and the control signal (CONT) are powered off. Thereafter, the driving voltage (VDD) which is supplied through the driving power supply  30  is powered off. 
         [0051]    The after-image removing gray-scale signals (WS and BS) are applied to the data driver  90  during a delay time (T 1 ) between an off time point of the backlight driving voltage and an off time point of the differential signal in order to get rid of the after-image displayed in the liquid crystal panel. Then, the data driver  90  applies the gray-scale voltage corresponding to the after-image removing gray-scale signals (WS and BS) to the liquid crystal panel and displays a white or a black color on the liquid crystal panel. 
         [0052]    The power-off sensor  50  and the controller  60  for generating the after-image removing gray-scale signals will be described in detail by referring to  FIG. 3  and  FIG. 4 . 
         [0053]      FIG. 3  is a block diagram illustrating the power-off sensor  50  in  FIG. 1 . 
         [0054]    Referring to  FIG. 3 , the power-off sensor  50  includes first to third resistors  55 ,  56  and  57  and a transistor  51 . 
         [0055]    The first to third resistors  55 ,  56  and  57  are connected in series and in parallel between the backlight power supply  40  and the transistor  51 . 
         [0056]    The transistor  51  is connected between the receiver  20  and the controller  60  and switches the clock (CLK) supplied to the controller  60  from the receiver  20  to ground. The transistor  51  may employ a P-type bipolar transistor. Alternatively, the transistor  51  may be a metal oxide silicon (MOS) transistor performing the same function as the P-type bipolar transistor. 
         [0057]    The SET signal indicated by the backlight on/off voltage is applied from the power supply  40  to the base of the transistor  51  through the first to third resistors  55 ,  56  and  57 . The SET signal includes a high level and a low level. The SET signal becomes high when the backlight-on voltage is output, and becomes low when the backlight-off voltage is output. Then, a second signal (CLK 2 ) among the clock signals (CLK 1  and CLK 2 ) applied to the controller  60  from the receiver  20  is applied to an emitter of the transistor  51 . Further, a collector of the transistor  51  is connected to a ground (GND). 
         [0058]    When the backlight unit is powered on and the high SET signal is applied, the transistor  51  is turned off because a voltage between the base and the emitter is lower than a threshold voltage of the transistor  51 . On the other hand, when the backlight unit is powered off and the low SET signal is applied, the transistor  51  is turned on because the voltage between the base and the emitter is higher than the threshold voltage of the transistor  51 . Accordingly, the transistor  51  is turned on when the backlight unit is powered off, thereby grounding the second signal (CLK 2 ) applied to the controller  60  from the receiver  20 . 
         [0059]    The transistor  51  according to the exemplary embodiment of the present invention is not limited to the grounding of the second signal (CLK 2 ), but may ground the first signal (CLK 1 ). 
         [0060]      FIG. 4  is a block diagram illustrating the controller in  FIG. 1 . 
         [0061]    Referring to  FIG. 4 , the controller  60  includes a differential signal receiver  61 , a signal comparator  63  and a gray-scale signal generator  65 . 
         [0062]    The differential signal receiver  61  receives the pixel data (RGB), the control signal (CONT), the first signal (CLK 1 ), and the second signal (CLK 2 ). As shown in  FIG. 3 , the second signal CLK 2  can be grounded by the power-off sensor  50 . The differential signal receiver  61  applies the first and the second signals (CLK 1  and CLK 2 ) to the signal comparator  63 . 
         [0063]    The signal comparator  63  compares the phase difference of the first and the second signals (CLK 1  and CLK 2 ) and determines a signal error. The signal comparator  63  can detect a power-off state of the backlight unit by sensing the second signal (CLK 2 ) when it is distorted (grounded) by the power-off sensor  50 . The signal comparator  63  informs the gray-scale signal generator  65  of the signal error. 
         [0064]    The gray-scale signal generator  65  generates an after-image removing gray-scale signal according to the signal error information. More specifically, the gray-scale signal generator  65  applies the after-image removing gray-scale signals (WS and BS) to the data driver  90  in response to the signal error information. To this end, when the backlight unit is powered off, the gray-scale signal generator  65  is converted into the BIST mode from a normal mode and outputs a predetermined white or black after-image removing gray-scale signal (WS or BS). For example, the gray-scale signal generator  65  may include an EEPROM and stores the gray-scale signal for removing the after-image. The gray-scale signal generator  65  stores the gray-scale signal so as to apply the gray-scale display voltage of the same gray-scale by the minimum applying voltage to the liquid crystal cell. 
         [0065]    The gray-scale signal generator  65  may store a gray-scale signal for displaying a white or a black according to a liquid crystal mode. Accordingly, the gray-scale signal generator  65  applies the white after-image removing gray-scale signal (WS) to the data driver  90  when the liquid crystal mode is in a normally white mode (NW). When the liquid crystal mode is in a normally black mode (NB), the gray-scale signal generator  65  applies the black after-image removing gray-scale signal (BS) to the data driver  90 . 
         [0066]    The controller  60  applies the after-image removing gray-scale signals (WS and BS) to the data driver  90 , thereby displaying a white or a black image on the liquid crystal panel. Accordingly, the controller  60  can remove the after-image due to a discharge error of the liquid crystal panel when the backlight unit is powered off and the driving voltage is off. 
         [0067]    The liquid crystal display according to the exemplary embodiment of the present invention is not only applied to the case that the liquid crystal display is driven to remove the after-image only the time of the power-off of the controller  60 , but also to the case that the liquid crystal display is driven to get rid of the after-image in the BITS mode when the backlight unit is temporarily powered off, for example, in the case of a channel conversion. 
         [0068]    The driving method of the liquid crystal display according to an exemplary embodiment of the present invention will be described referring to  FIG. 5 . 
         [0069]      FIG. 5  is a flowchart illustrating the driving method of the liquid crystal display according to the exemplary embodiment of the present invention. 
         [0070]    As shown in  FIG. 5 , the driving method of the liquid crystal display according to the exemplary embodiment of the present invention includes the operations of power-off (step S 300 ), detecting power-off of the backlight unit (step S 320 ), generating an after-image removing gray-scale signal (step S 330 ), and applying the after-image removing signal (step S 340 ). 
         [0071]    At first, in the power-off operation (step S 300 ), a user cuts off power supply of the main system  10  in the liquid crystal display. For example, the user may cut off power through a power switch of a mobile phone or a lap top computer. 
         [0072]    Next, in the operation of detecting the power-off of the backlight unit (step S 320 ), the power on/off state of the backlight unit is detected by the power-off sensor  50  from the backlight power supply  40  and the differential signal applied to the controller  60  from the receiver  20  is distorted. 
         [0073]    The power-off sensor  50  is connected to the backlight power supply  40  and applies a high/low SET signal to a P-type transistor during power on/off of the backlight unit. When the backlight unit is powered off, the P-type transistor is turned on by the low SET signal and grounds the differential signal applied to the controller  60  from the receiver  20 . When the differential signal is applied from the receiver  20 , the controller  60  compares a phase difference between a signal grounded by the power-off sensor  50  and a normal signal to detect a signal error. 
         [0074]    Then, in the operation of generating the after-image removing gray-scale signal (step S 330 ), the controller  60  which has detected the signal error is converted into the BIST mode from a normal mode and generates the after-image removing gray-scale signal through the gray-scale signal generator  65 . The BIST mode displays a predetermined image so as to control an image display operation of the liquid crystal panel when it is determined that the pixel data (RGB) is abnormal by a noise or a short-circuit of the signal line, etc. 
         [0075]    Finally, in the operation of supplying the after-image removing gray-scale signal (step S 340 ), the controller  60  in the BIST mode applies the after-image removing gray-scale signal to the data driver  90  so as to control the image display operation of the liquid crystal panel. The gray-scale signal generator  65  applies a white gray-scale signal when the liquid crystal panel is in the normally white mode, and applies a black gray-scale signal when the liquid crystal panel is in the normally black mode. Accordingly, the liquid crystal panel displays an image corresponding to the after-image removing gray-scale signal between the backlight power-off time point and the differential signal off time point, and thus is driven while being advantageous for discharge. 
         [0076]    Hereinafter, a power-off operation according to the liquid crystal modes will be described with reference to  FIG. 6  and  FIG. 7 . 
         [0077]      FIG. 6  and  FIG. 7  are views for illustrating the power-off operating order in the driving method of the liquid crystal display according to the exemplary embodiment of the present invention. 
         [0078]      FIG. 6  illustrates images of the liquid crystal panel according to the power-off operation when the liquid crystal mode is in the normally white mode. When an image of an area A displayed in a power on state is off, the liquid crystal panel applies the white gray-scale signal to the data driver  90  between the power-off time point of the backlight voltage (BLV) and the power-off time point of the differential signal to display the white gray-scale. Then, the liquid crystal panel displays a black gray-scale by the power-off of the driving voltage after displaying the white gray-scale. 
         [0079]      FIG. 7  illustrates images of the liquid crystal panel according to the power-off operation when the liquid crystal mode is in the normally black mode. When the image of the area A displayed in the power on state is off, the liquid crystal panel applies the black gray-scale signal to the data driver  90  between the power-off time point of the backlight voltage (BLV) and the power-off time point of the differential signal to display the black gray-scale. Then, the liquid crystal panel displays the black gray-scale by the power-off of the driving voltage after displaying the black gray-scale. 
         [0080]    As described above, the liquid crystal display according to the present invention includes the power-off sensor to distort the differential signal during the power-off of the backlight unit. Accordingly, the liquid crystal display generates a certain gray-scale signal and displays the certain gray-scale on the liquid crystal after power-off of the backlight unit during power-off of the liquid crystal display, thereby removing the after-image of the liquid crystal panel and preventing discharge error. 
         [0081]    Although a few exemplary embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.