Abstract:
An apparatus and method for driving scanning lines of a liquid crystal panel for reducing generation of flickers appearing on the image displayed by the liquid panel due to misalignments in the liquid crystal panel. The apparatus includes a driving unit for selectively generating a first scan signal or a second scan signal based on control signals to drive the plurality of scanning lines, where one of the control signals indicates whether or not at least one of the TFTs of the liquid crystal panel is misaligned. The driving unit can includes a driving IC chip, or a driving IC chip and a switch.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an apparatus and method for driving a liquid crystal display (LCD) and, more particularly, to an apparatus and method for driving scanning lines formed in the LCD panel. 
     2. Description of Background Art 
     Generally, a conventional LCD apparatus controls the light transmissivity of liquid crystal in the liquid crystal panel based on the voltage level of video signals applied to the liquid crystal panel to display images. The liquid crystal panel includes liquid crystal cells arranged in a matrix pattern, and control switches for selectively applying the video signals to each liquid crystal cell. The control switches are formed with thin film transistors (TFTs) with their sources and drains formed in an alternating pattern. Often during the fabrication process of the liquid crystal panel, the TFTs are misaligned, causing them to have different parasitic capacitances. The differences in the parasitic capacitance between the TFTs distort tile video signals applied to tile liquid crystal cells, causing undesirable flickers in the picture being displayed by the liquid crystal panel. 
     FIG. 1A shows a schematic view of such a conventional liquid crystal panel having a delta structure. As shown therein, the conventional liquid crystal panel includes a plurality of odd-numbered scanning lines  4 ′, a plurality of even-numbered scanning lines  4 ″ formed between the odd-numbered scanning lines  4 ′, and a plurality of data lines  2  crossing the scanning lines  4 ′ and  4 ″. Each of the odd-numbered scanning lines  4 ′ is connected to a plurality of first TFTs (odd-numbered TFTs)  8 A arranged in a row, and each of the even-numbered scanning lines  4 ″ is connected to a plurality of second TFTs (even-numbered TFTs)  8 B arranged in another row. Each of the TFTs  8 A and  8 B is connected to a liquid crystal cell (or pixel electrode  6 ). Each of the first TFTs  8 A is turned on and off based on a scanning pulse applied through the corresponding scanning line  4 ′ connected to the first TFTs  8 A. Each of the second TFTs  8 B is turned on and off based on a scanning pulse applied through the corresponding scanning line  4 ″ connected to the second TFT  8 B. 
     By turning on the first TFT  8 A, a current path between the corresponding data line  2  connected to the TFT  8 A and the corresponding liquid crystal cell  6  is established, and by turning on the second TFT  8 B, a current path between the corresponding data line  2  connected to the TFT  8 B and the corresponding liquid crystal cell  6  is established. Through the established current paths, data signals from the data lines  2  are applied to the corresponding TFTs  8 A and  8 B. The sources and drains of the first or second TFTs  8 A or  8 B are simultaneously formed using a same fabrication process, and their formation positions can vary with respect to the corresponding gate. 
     FIG. 1B shows an enlarged view of portion  1 B shown in FIG.  1 A. As shown in FIG. 1B, an active region AR partially overlaps the source ST, the drain DT and the gate GT of each TFT  8 A or  8 B to establish first and second parasitic capacitors Cgs and Cgd. The first parasitic capacitor Cgs emerges between the gate GT and the source ST while the second parasitic capacitor Cgd emerges between the gate GT and the drain DT of the TFT  8 A or  8 B. The characteristic of the parasitic capacitors Cgs and Cgd can differ substantially from each other depending on the exact positions of the source ST and the drain DT moved. For example, the parasitic values of the capacitors Cgs and Cgd differ from each depending on the position of the TFTs and the exact position of the source ST and the drain DT. For instance, if all the sources ST and the drains DT of the TFTs  8 A and  8 B are misaligned to the right from the original or predetermined position when they are formed, the first parasitic capacitor Cgs of the odd-numbered TFTs  8 A has a larger capacitance value than the first parasitic capacitor Cgs of the even-numbered TFTs  8 B, while the second parasitic capacitor Cgd of the odd-numbered TFTs  8 A has a smaller capacitance value than the second parasitic capacitor Cgd of the even-numbered TFTs  8 B. On the contrary, if the sources ST and the drains DT are misaligned to the left from the desired position, the first parasitic capacitor Cgs of the odd-numbered TFTs  8 A has a smaller capacitance value than the first parasitic capacitor Cgs of the even-numbered TFTs  8 B, while the second parasitic capacitor Cgd of the odd-numbered TFTs  8 A has a larger capacitance value than the second parasitic capacitor Cgd of the even-numbered TFTs  8 B. 
     The second parasitic capacitors Cgd do not directly influence the liquid crystal cells  6 ; however, the first parasitic capacitors Cgs vary the voltage level of video signals applied to the liquid crystal cells  6 . This occurs because, as shown in FIG. 2A, the first parasitic capacitor Cgs is serially connected to a parallel circuit  6   a  of the corresponding liquid crystal cell  6  having a support capacitor Cst. The serial connection of the first parasitic capacitor Cgs lowers the voltage level of the corresponding liquid crystal cell  6  at the time the corresponding TFT is turned off. 
     Specifically, as shown in FIG. 2B, the TFT ( 8 A or  8 B) is turned on when a high level voltage (Vgh) of the scan signal SS is applied to the TFT through the corresponding scanning line  4 . As the TFT is turned on, a video signal VD on the corresponding data line  2  is applied to the corresponding liquid crystal cell  6  and the support capacitor Cst. At this time, the voltage Vd of the video signal VD is charged into the liquid crystal cell  6  and the support capacitor Cst, and the voltage difference between the high level voltage Vgh and the video signal voltage Vd is charged to the first parasitic capacitor Cgs. When the scan signal SS transits from the high level voltage Vgh to a low level voltage Vgl, the TFT  8  is turned off to disconnect the current path from the data line  2  to the liquid crystal cell  6  and the support capacitor Cst. Then the first parasitic capacitor Cgs suddenly discharges the charged voltage into the corresponding scanning line  4  and, simultaneously, is charged by the voltage from the liquid crystal cell  6  and the support capacitor Cst. As a result, the voltage charged in the liquid crystal cell  6  is instantaneously reduced. By such a charge and discharge operation of the parasitic capacitor Cgs, an output signal CLS of the liquid crystal cell  6  sharply decreased by a predetermined voltage ΔVp from the video signal voltage Vd as shown in FIG.  2 B. The drop voltage ΔVp in the output signal CLS of the liquid crystal cell  6  can be expressed as follows:                Δ                 Vp2     =         (   Cgs   )          (   Vg   )         Clc   +   Cst   +   Cgs               (   1   )                                
     wherein Vg represents a voltage difference between the high level voltage Vgh and the low level voltage Vgl of the scan signal SS, and Clc represents a capacitance value of the corresponding liquid crystal cell  6 . Since the drop voltage ΔVp changes in accordance with the capacitance value of the parasitic capacitor Cgs as seen from the above equation (1), the drop voltage ΔVp in the output of the liquid crystal cell  6  connected to odd-numbered TFT  8 A and the drop voltage ΔVp in the output of the liquid crystal cell  6  connected to even-numbered TFT  8 B differ from each other when misalignment occurs during the fabrication of the TFTs  8 A and  8 B. 
     For example, if the overlapped width of the source ST and the active region AR is set to be 3 μm, but the position of the source ST formed on the substrate is shifted to the left by 1 μm due to misalignment, the source ST and the active region AR of each odd-numbered TFT  8 A are overlapped by 2 μm while the source ST and the active region AR of each even-numbered TFT  8 B are overlapped by 4 μm. If the video signal VD has an intermediate gray level voltage, then the drop voltage ΔVp in the output signal of tile liquid crystal cell  6  connected to the odd-numbered TFT  8 A becomes 192 mV, while the drop voltage ΔVp for the liquid crystal cell  6  connected to the even-numbered TFT  8 B becomes 380 mV. Although the odd-numbered line liquid crystal cells and the even-numbered line liquid crystal cells respond to the same video signals, the outputs thereof differ from each other. 
     As described above, in the delta structure of a conventional liquid crystal panel, the parasitic capacitance of the odd-numbered TFTs can differ substantially from that of the even-numbered TFTs due to misalignment occurring during the fabrication process. This causes the voltage variation ratio of the odd-numbered line liquid crystal cells to be different from that of the even-numbered liquid crystal cells, which causes flickers to be appear in the pictures displayed by the same liquid crystal panel. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an apparatus and method for driving scanning lines of a liquid crystal panel, which prevent generation of flickers appearing due to misalignment of TFTs. 
     In order to achieve this and other objects of the invention, an apparatus for driving scanning lines of a liquid crystal panel having TFTs according to the present invention includes a driving unit for selectively generating a first scan signal or a second scan signal based on control signals to drive the plurality of scanning lines, one of the control signals indicating whether or not at least one of the TFTs is misaligned. 
     In a liquid crystal display device including a liquid crystal panel having thin film transistors (TFTs) and a plurality of scanning lines connected to the TFTs, a method for driving the plurality of scanning lines according to the present invention includes the steps of selectively generating a first scan signal or a second scan signal based on control signals, one of the control signals indicating whether or not at least one of the TFTs is misaligned; and applying the generated first or second scan signal to the liquid crystal panel to drive the plurality of scanning lines. 
     These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed descriptions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects of the invention will be apparent from the following detailed description of the embodiments of the present invention with reference to the accompanying drawings, in which: 
     FIG. 1A is a layout of a conventional liquid crystal panel; 
     FIG. 1B is a detailed view of portion  1 B in FIG. 1A; 
     FIG. 2A is an electrical equivalent circuit diagram of FIG. 1B; 
     FIG. 2B is an operational waveform diagram for the elements shown in FIG. 2A; 
     FIG. 3 is a schematic diagram showing a configuration of an apparatus for driving scanning lines of a liquid crystal panel according to an embodiment of the present invention; and 
     FIG. 4 is a schematic diagram showing a configuration of an apparatus for driving scanning lines of a liquid crystal panel according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 3, there is shown an apparatus for driving scanning lines of a liquid crystal panel according to an embodiment of the present invention. As shown therein, the apparatus of the present invention includes a switch  10  for receiving voltage signals from first to fourth input lines  11 ,  13 ,  15  and  17 , respectively; and a driving integrated circuit (IC) chip  12  connected between the switch  10  and a liquid crystal panel  14 . Scan signals SS (e.g., as shown in FIG. 2B) applied to the scanning lines of the liquid crystal panel  14  can be composed of a different combination of signals Vgh 1 , Vgh 2 , Vgl 1  and Vgl 2 . The high level of the scan signal SS can correspond to the high level voltage signal Vgh 1  or Vgh 2 , and the low level of the scan signal SS can correspond to the low level voltage signal Vgl 1  or Vgl 2 . The liquid crystal panel  14  can be the conventional liquid crystal panel as shown in FIG. 1A or other liquid crystal panels known in the art. 
     The first to fourth input lines  11  to  17  are connected to a power supply (not shown). The first high level voltage signal Vgh 1  is applied to the first input line  11 , where the voltage level of the second high signal Vgh 2  is higher than that of the first high signal Vgh 1 . The first low level voltage signal Vgl 1  is applied to the third input line  15 , where the voltage level of the second low signal Vgl 2  is lower than that of the first low signal Vgl 1 . 
     The switch  10  transfers one of the voltage signals Vgh 1 , Vgh 2 , Vgl 1  and Vgl 12  to the driving IC chip  12 . If the TFTs of the liquid crystal panel  14  are not misaligned during the fabrication process, the switch  10  stably delivers any one of the first and second high level voltage signals Vgh 1  and Vgh 2  to the driving IC chip  12  as the high level scan signal SS. However, if the TFTs of the liquid crystal panel  14  are formed as misaligned during the fabrication process, the switch  10  alternatingly applies the first and second high level voltage signals Vgh 1  and Vgh 2  to the driving IC chip  12  as the odd-numbered and even-numbered scanning lines of the panel  14  are driven sequentially. For example, the switch  10  applies the first high level voltage signal Vgh 1  to the driving IC chip  12  as the high level scan signal SS when the odd-numbered scanning lines of the liquid crystal panel  14  are driven, and applies the second high level voltage signal Vgh 2  to the driving IC chip  12  as the high level scan signal SS when the even-numbered scanning lines of the liquid crystal panel  14  are driven. 
     To generate a low level scan signal SS, the switch  10  transfers any one of the first and second low level voltage signals Vgl 1  and Vgl 2  to the driving IC chip  12 . If the TFTs of the liquid crystal panel  14  are not misaligned during the fabrication process, the switch  10  stably delivers any one of the first and second low level voltage signals Vgl 1  and Vgl 2  to the driving IC clip  12 . However, if the TFTs are misaligned, the switch  10  alternatingly applies tile first and second low level voltage signals Vgl 1  and Vgl 2  to the driving IC chip  12  as the even-numbered and odd-numbered scanning lines are sequentially driven. For example, the switch  10  applies the first low level voltage signal Vgl 1  to the driving IC chip  12  as the low level scan signal SS when the odd-numbered scanning lines of the liquid crystal panel  14  are driven, and applies the second low level voltage signal Vgl 2  to the driving IC chip  12  as the low level scan signal SS when the even-numbered scanning lines of the liquid crystal panel  14  are driven. 
     In order to perform the above-described switching operation by the switch  10 , a mode signal MCS and a line switching signal LSS are applied to the switch  10  from an external source. The mode signal MCS indicates whether or not the TFTs of the liquid crystal panel  14  are misalignment, and the line switching signal LSS indicates whether the odd-numbered or even-numbered scanning lines are driven. The switch  10  includes two control switches commonly responding to the mode signal MCS and the line switching signal LSS. 
     In another example of the present invention, the switch  10  receives only one low level voltage signal rather than both the first and second low level voltage signals Vgl 1  and Vgl 2 , and transfers the one low level voltage signal to the driving IC chip  12  at all times as the low level scanning signal SS. In this case, the switch  10  employs a single control switch for carrying out the switching just between the high level voltage signals Vgh 1  and Vgh 2 . 
     In still another example of the present invention, the switch  10  receives only one high level voltage signal rather than both the first and second high level voltage signals Vgh 1  and Vgh 2 , and transfers the one high level voltage signal to the driving IC chip  12  at all times as the high level scanning signal SS. In this case, the switch  10  employs a single control switch just for switching between the low level voltage signals Vgl 1  and Vgl 2 . 
     As described above, the driving IC chip  12  applies a pulse shaped scan signal SS to the scanning lines of the liquid crystal panel  14  to drive the even-numbered and odd-numbered scanning lines. The pulse shaped scan signal SS is composed of the high level voltage signal Vgh 1  or Vgh 2  and the low level voltage signal Vgl 1  or Vgl 2  output from the switch  10 . The scan signal SS has a constant swing width depending upon whether or not the TFTs of the liquid crystal panel  14  are misaligned, or has a different swing width depending on the scanning lines. In other words, when the TFTs of the liquid crystal panel  14  are not misaligned according to the mode signal MCS, the scan signal SS composed of the first or second low voltage Vgl 1  or Vgl 2 , and the first or second high voltage Vgh 1  or Vgh 2  is applied to the liquid crystal panel  4 . If the TFTs are misaligned according to the mode signal MCS, a swing width of the scan signal SS applied to the odd-numbered scanning lines differs from that of the scan signal SS applied to the even-numbered scanning lines of the liquid crystal panel  4 . 
     For example, if the pulsed-shaped scan signal SS for driving the odd-numbered scanning lines of the liquid crystal panel  14  is composed of the first low level voltage signal Vgl 1  and the first high level voltage signal Vgh 1 , then the scan signal SS for driving the even-numbered scanning lines is composed of the second low level voltage signal Vgl 2  and the second high level voltage signal Vgh 2 . On the other hand, if the scan signal SS for driving the odd-numbered scanning lines is composed of the second low level voltage signal Vgl 2  and the second high level voltage signal Vgh 2 , then the scan signal SS for driving the even-numbered scanning lines is composed of the first low level voltage signal Vgl 1  and the first high voltage signal Vgh 1 . 
     As described above, a swing width of the odd-numbered scan signal differs from that of the even-numbered scan signal when the TFTs of the liquid crystal panel  14  are misaligned, so that the liquid crystal cells corresponding to the odd-numbered scanning lines have the same voltage variation ratio as the liquid crystal cells corresponding to the even-numbered scanning lines. This prevents flickers from appearing in the images displayed by the liquid crystal panel  14 . 
     Referring now to FIG. 4, there is shown an apparatus for driving scanning lines of a liquid crystal panel according to another embodiment of the present invention. In this embodiment, the switch  10  is incorporated into the driving IC chip  22  such that the driving IC chip  22  selects the different voltage signals Vgh 1 -Vgl 2  to generate a scan signal SS for driving the even and odd-numbered scanning lines of the liquid crystal panel  14 . 
     As shown therein, the scanning line driving apparatus includes a driving integrated circuit (IC) chip  22  connected to the first to fourth input lines  11 ,  13 ,  15  and  17 , and the liquid crystal panel  14  coupled to the driving IC chip  22 . The first to fourth input lines  11  to  17  are connected to a lower supply (not shown), and receive the voltage signals Vgh 1 , Vgh 2 , Vgl 1  and Vgl 2 , respectively. The voltage level of the second high signal Vgh 2  is higher than that of the first high signal Vgh 1 , and the voltage level of the second low signal Vgl 2  is lower than that of the first low signal Vgl 1 . 
     The driving IC chip  22  switches between the high and low level voltages Vgh 1 , Vgh 2 , Vgl 1 , and Vgl 2  as the switch  10  of the first embodiment. The mode signal MCS and the line switching signal LSS are applied to the driving IC chip  22  as discussed above in connection with the first embodiment. 
     In another example of the present invention, the driving IC chip  22  receives only one low level voltage signal rather than both the first and second low level voltage signals Vgl 1  and Vgl 2 , and transfers the one low level voltage signal to the liquid crystal panel  14  at all times as the low level scanning signal SS. In this case, the chip  22  employs a single control switch for carrying out the switching just between the high level voltage signals Vgh 1  and Vgh 2 . 
     In still another example of the present invention, the driving IC chip  22  receives only one high level voltage signal rather than both the first and second high level voltage signals Vgh 1  and Vgh 2 , and transfers the one high level voltage signal to the liquid crystal panel  14  at all times as the high level scanning signal SS. In this case, the chip  14  employs a single control switch just for switching between the low level voltage signals Vgl 1  and Vgl 2 . 
     Accordingly, the apparatus and method for driving the scanning lines of a liquid crystal panel according to the present invention provides a scan signal with varying swing width in accordance with the scanning lines of the liquid crystal panel when the TFTs of the liquid crystal panel are misaligned. This provides a uniform voltage variation ratio for each of the liquid crystal cells, and as a result, flickers do not appear on images displayed by the liquid crystal panel and the quality of the liquid crystal panel is improved. 
     Although the present invention has been explained by the embodiments shown in the drawings described above, it should be understood to an ordinary skilled person in the art that the invention is not limited to the embodiments, but rather that various changes or modifications thereof are possible without departing from the spirit of the invention.