Abstract:
A scan driver circuit for driving scanning lines is proposed. The scan driver circuit includes a pull-down control module, a pull-down module, a recovering control module, a recovering module, a downlink module, a first bootstrap capacitor, a constant low voltage supply, and a constant high voltage supply. The present inventive scan driver circuit has advantages of simple structure and low power consumption.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to the field of display driving, and more particularly, to a scan driver circuit. 
         [0003]    2. Description of the Prior Art 
         [0004]    Gate driver on array (GOA) is a scan driver circuit fabricated on an array substrate of a conventional thin-film transistor (TFT) liquid crystal display (LCD). Scanning lines are scanned one by one adopting the GOA. The conventional scan driver circuit comprises a pull-down control module, a pull-down module, a downlink module, a recovering control module, a bootstrap capacitor, and a recovering control module. 
         [0005]    Operating on high temperature, the scan driver circuit tends to delay and leak electricity, which affects the reliability of the scan driver circuit. 
         [0006]    Therefore, it is necessary to provide a scan driver circuit for solving the problem occurring in the conventional technology. 
       SUMMARY OF THE INVENTION 
       [0007]    An object of the present invention is to provide a scan driver circuit of which the structure is simple with high reliability for substituting for a conventional scan driver circuit of which the structure is complicated with low reliability. 
         [0008]    According to the present invention, a scan driver circuit for driving scanning lines comprises: a pull-down control module, for receiving an previous stage scanning signal or a next stage scanning signal and generating a low-voltage-level scanning level signal at the corresponding scanning line according to the previous stage scanning signal or the next stage scanning signal; a pull-down module, for lowering a scanning signal at the corresponding scanning line according to the scanning level signal; a recovering control module, for receiving an previous stage clock signal or a next stage clock signal and generating a recovering signal at the corresponding scanning line according to the previous stage clock signal or the next stage clock signal; a recovering module, for elevating a scanning signal at the corresponding scanning line according to the recovering signal; a downlink module, for generating and sending a current stage clock signal and a pull-down control signal according to the scanning signal at the scanning line; a first bootstrap capacitor, for generating a scanning level signal with a low level or a high level at the scanning line; a constant low voltage supply, for supplying the low-voltage-level signal; and a constant high voltage supply, for supplying the high-voltage-level signal. The pull-down control module comprises a twelfth transistor, a controlling terminal of the twelfth transistor inputs a low-voltage-level scanning signal, an input terminal of the twelfth transistor inputs a next stage scanning signal, and an output terminal of the twelfth transistor is connected to the pull-down module. The recovering control module comprises a thirteenth transistor, a controlling terminal of the thirteenth transistor inputs a low-voltage-level scanning signal, an input terminal of the thirteenth transistor inputs an previous stage clock signal, and an output terminal of the thirteenth transistor outputs a recovering signal at the scanning line. 
         [0009]    In one aspect of the present invention, the pull-down control module comprises a first transistor, a controlling terminal of the first transistor inputs a low-voltage-level scanning signal, an input terminal of the first transistor inputs the previous stage scanning signal, and an output terminal of the first transistor is connected to the pull-down module. 
         [0010]    In another aspect of the present invention, the pull-down module comprises a second transistor, a controlling terminal of the second transistor inputs an previous stage pull-down control signal, an input terminal of the second transistor is connected to the output terminal of the first transistor, and an output terminal of the second transistor outputs a low-voltage-level scanning level signal at the scanning line. 
         [0011]    In another aspect of the present invention, the recovering control module comprises a third transistor, a controlling terminal of the third transistor inputs the low-voltage-level scanning signal, an input terminal of the third transistor inputs the next stage clock signal, and an output terminal of the third transistor outputs a recovering signal at the scanning line. In another aspect of the present invention, the recovering module comprises a fourth transistor, a fifth transistor, a sixth transistor, a seventh transistor, and an eighth transistor. A controlling terminal of the fourth transistor is connected to the output terminal of the third transistor, an input terminal of the fourth transistor is connected to the constant low voltage supply, and an output terminal of the fourth transistor is connected to a controlling terminal of the fifth transistor, a controlling terminal of the eighth transistor, an output terminal of the sixth transistor, and an output terminal of the seventh transistor. An input terminal of the fifth transistor is connected to the constant high voltage supply, and an output terminal of the fifth transistor is connected to the output terminal of the second transistor. A controlling terminal of the sixth transistor inputs the pull-down control signal at a current stage, and an input terminal of the sixth transistor is connected to the constant high voltage supply. A controlling terminal of the seventh transistor inputs the pull-down control signal at an previous stage, and an input terminal of the seventh transistor is connected to the constant high voltage supply. An input terminal of the eighth transistor is connected to the constant high voltage supply, and an output terminal of the eighth transistor outputs a current stage scanning signal at the scanning line. 
         [0012]    In another aspect of the present invention, the recovering module further comprises a fourteenth transistor, a controlling terminal of the fourteenth transistor is connected to the output terminal of the fourth transistor, an input terminal of the fourteenth transistor is connected to the constant high voltage supply, and an output terminal of the fourteenth transistor is connected to the controlling terminal of the sixth transistor. 
         [0013]    In another aspect of the present invention, the recovering module further comprises a fifteenth transistor, a controlling terminal of the fifteenth transistor inputs the pull-down control signal at an previous stage, an input terminal of the fifteenth transistor is connected to the constant high voltage supply, and an output terminal of the fifteenth transistor is connected to an output terminal of the fourth transistor. 
         [0014]    In another aspect of the present invention, the pull-down module further comprises a sixteenth transistor, a controlling terminal of the sixteenth transistor inputs the pull-down control signal at an previous stage, an input terminal of the sixteenth transistor is connected to the constant low voltage supply, and an output terminal of the sixteenth transistor is connected to an output terminal of the second transistor. 
         [0015]    In another aspect of the present invention, the downlink module comprises a ninth transistor and an eleventh transistor, a controlling terminal of the ninth transistor is connected to an output terminal of the second transistor, an input terminal of the ninth transistor is connected to an output terminal of the eighth transistor, and an output terminal of the ninth transistor outputs the current stage clock signal. A controlling terminal of the eleventh transistor is connected to an output terminal of the second transistor, an input terminal of the eleventh transistor is connected to an output terminal of the ninth transistor, and an output terminal of the eleventh transistor outputs a current stage pull-down control signal. 
         [0016]    In still another aspect of the present invention, the recovering module further comprises a fourteenth transistor and a fifteenth transistor. A controlling terminal of the fourteenth transistor is connected to an output terminal of the fourth transistor, an input terminal of the fourteenth transistor is connected to the constant high voltage supply, and an output terminal of the fourteenth transistor is connected to the controlling terminal of the sixth transistor. A controlling terminal of the fifteenth transistor inputs the pull-down control signal at an previous stage, an input terminal of the fifteenth transistor is connected to the constant high voltage supply, and an output terminal of the fifteenth transistor is connected to the output terminal of the fourth transistor. 
         [0017]    In yet another aspect of the present invention, the pull-down module further comprises a sixteenth transistor, a controlling terminal of the sixteenth transistor inputs the pull-down control signal at a previous stage, an input terminal of the sixteenth transistor is connected to the constant low voltage supply, and an output terminal of the sixteenth transistor is connected to the output terminal of the second transistor. 
         [0018]    According to the present invention, a scan driver circuit for driving scanning lines comprises: a pull-down control module, for receiving an previous stage scanning signal or a next stage scanning signal and generating a low-voltage-level scanning level signal at the corresponding scanning line according to the previous stage scanning signal or the next stage scanning signal; a pull-down module, for lowering a scanning signal at the corresponding scanning line according to the scanning level signal; a recovering control module, for receiving an previous stage clock signal or a next stage clock signal and generating a recovering signal at the corresponding scanning line according to the previous stage clock signal or the next stage clock signal; a recovering module, for elevating a scanning signal at the corresponding scanning line according to the recovering signal; a downlink module, for generating and sending a current stage clock signal and a pull-down control signal according to the scanning signal at the scanning line; a first bootstrap capacitor, for generating a scanning level signal with a low level or a high level at the scanning line; a constant low voltage supply, for supplying the low-voltage-level signal; and a constant high voltage supply, for supplying the high-voltage-level signal. 
         [0019]    In one aspect of the present invention, the pull-down control module comprises a first transistor, a controlling terminal of the first transistor inputs a low-voltage-level scanning signal, an input terminal of the first transistor inputs the previous stage scanning signal, and an output terminal of the first transistor is connected to the pull-down module. 
         [0020]    In another aspect of the present invention, the pull-down module comprises a second transistor, a controlling terminal of the second transistor inputs an previous stage pull-down control signal, an input terminal of the second transistor is connected to the output terminal of the first transistor, and an output terminal of the second transistor outputs a low-voltage-level scanning level signal at the scanning line. 
         [0021]    In another aspect of the present invention, the recovering control module comprises a third transistor, a controlling terminal of the third transistor inputs the low-voltage-level scanning signal, an input terminal of the third transistor inputs the next stage clock signal, and an output terminal of the third transistor outputs a recovering signal at the scanning line. 
         [0022]    In another aspect of the present invention, the recovering module comprises a fourth transistor, a fifth transistor, a sixth transistor, a seventh transistor, and an eighth transistor. A controlling terminal of the fourth transistor is connected to the output terminal of the third transistor, an input terminal of the fourth transistor is connected to the constant low voltage supply, and an output terminal of the fourth transistor is connected to a controlling terminal of the fifth transistor, a controlling terminal of the eighth transistor, an output terminal of the sixth transistor, and an output terminal of the seventh transistor. An input terminal of the fifth transistor is connected to the constant high voltage supply, and an output terminal of the fifth transistor is connected to the output terminal of the second transistor. A controlling terminal of the sixth transistor inputs the pull-down control signal at a current stage, and an input terminal of the sixth transistor is connected to the constant high voltage supply. A controlling terminal of the seventh transistor inputs the pull-down control signal at an previous stage, and an input terminal of the seventh transistor is connected to the constant high voltage supply. An input terminal of the eighth transistor is connected to the constant high voltage supply, and an output terminal of the eighth transistor outputs a current stage scanning signal at the scanning line. 
         [0023]    In another aspect of the present invention, the recovering module further comprises a fourteenth transistor, a controlling terminal of the fourteenth transistor is connected to the output terminal of the fourth, transistor, an input terminal of the fourteenth transistor is connected to the constant high voltage supply, and an output terminal of the fourteenth transistor is connected to the controlling terminal of the sixth transistor. 
         [0024]    In another aspect of the present invention, the recovering module further comprises a fifteenth transistor, a controlling terminal of the fifteenth transistor inputs the pull-down control signal at an previous stage, an input terminal of the fifteenth transistor is connected to the constant high voltage supply, and an output terminal of the fifteenth transistor is connected to an output terminal of the fourth transistor. 
         [0025]    In still another aspect of the present invention, the pull-down module further comprises a sixteenth transistor, a controlling terminal of the sixteenth transistor inputs the pull-down control signal at an previous stage, an input terminal of the sixteenth transistor is connected to the constant low voltage supply, and an output terminal of the sixteenth transistor is connected to an output terminal of the second transistor. 
         [0026]    In yet another aspect of the present invention, the downlink module comprises a ninth transistor and an eleventh transistor, a controlling terminal of the ninth transistor is connected to an output terminal of the second transistor, an input terminal of the ninth transistor is connected to an output terminal of the eighth transistor, and an output terminal of the ninth transistor outputs the current stage clock signal; 
         [0027]    a controlling terminal of the eleventh transistor is connected to an output terminal of the second transistor, an input terminal of the eleventh transistor is connected to an output terminal of the ninth transistor, and an output terminal of the eleventh transistor outputs a current stage pull-down control signal. 
         [0028]    Compared with the conventional technology, the feature of the present invention is the disposition of a pull-down control module and the disposition of a recovering control module, which not only improves the reliability of the scan driver circuit but simplifies the structure of the scan driver circuit. The scan driver circuit of the present invention can substitute for a conventional scan driver circuit of which the structure is complicated with low reliability. 
         [0029]    These and other features, aspects and advantages of the present disclosure will become understood with reference to the following description, appended claims and accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  shows a schematic diagram of the structure of the scan driver circuit according to a preferred embodiment of the present embodiment. 
           [0031]      FIG. 2A  is a circuit diagram of the scan driver circuit according to a first preferred embodiment of the present invention. 
           [0032]      FIG. 2B  shows waveforms of signals applied in the scan driver circuit according to the first preferred embodiment of the present invention. 
           [0033]      FIG. 3  is a circuit diagram of a scan driver circuit according to a second preferred embodiment of the present invention. 
           [0034]      FIG. 4A  is a circuit diagram of a scan driver circuit according to a third preferred embodiment of the present invention. 
           [0035]      FIG. 4B  shows waveforms of signals applied in the scan driver circuit according to the third preferred embodiment of the present invention. 
           [0036]      FIG. 5A  is a circuit diagram of a scan driver circuit  40  according to a fourth preferred embodiment of the present invention. 
           [0037]      FIG. 5B  shows waveforms of signals applied in the scan driver circuit  40  according to the fourth preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0038]    Spatially relative terms, such as “beneath”, “below”, “lower”, “above”, “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. 
         [0039]    The invention is described below in detail with reference to the accompanying drawings, wherein like reference numerals are used to identify like elements illustrated in one or more of the figures thereof, and in which exemplary embodiments of the invention are shown. 
         [0040]    Please refer to  FIG. 1  showing a schematic diagram of the structure of the scan driver circuit  10  according to a preferred embodiment of the present embodiment. The scan driver circuit  10  is used for driving scanning lines in cascade. The scan driver circuit  10  comprises a pull-down control module  11 , a pull-down module  12 , a recovering control module  13 , a recovering module  14 , a downlink module  15 , a leakage-proof module  16 , a first bootstrap capacitor C 1 , a constant low voltage supply VGL, and a constant high voltage supply VGH. 
         [0041]    The pull-down control module  11  is used for receiving an previous stage scanning signal G_N−1 and generating a low-voltage-level scanning level signal at the corresponding scanning line according to the previous stage scanning signal G_N−1. The pull-down module  12  is used for lowering a scanning signal G_N at the corresponding scanning line according to the scanning level signal. The recovering control module  13  is used for receiving a next stage clock signal CK_N+1 and generating a recovering signal at the corresponding scanning line according to the next stage clock signal CK_N+1. The recovering module  14  is used for elevating the scanning signal G_N at the corresponding scanning line according to the recovering signal. The downlink module  15  is used for generating and sending a current stage clock signal CK_N according to the scanning signal G_N at the scanning line. The first bootstrap capacitor C 1  is used for generating a scanning level signal with a low level or a high level at the scanning line. The constant low voltage supply VGL is used for supplying the low-voltage-level signal. The constant high voltage supply VGH is used for supplying the high-voltage-level signal. 
         [0042]    Please refer to  FIG. 2A  and  FIG. 2B .  FIG. 2A  is a circuit diagram of the scan driver circuit  10  according to a first preferred embodiment of the present invention.  FIG. 2B  shows waveforms of signals applied in the scan driver circuit  10  according to the first preferred embodiment of the present invention. The pull-down control module  11  comprises a first transistor PT 1 . The controlling terminal of the first transistor PT 1  inputs a low-voltage-level scanning signal D 2 U. The input terminal of the first transistor PT 1  inputs the previous stage scanning signal G_N−1. The output terminal of the first transistor PT 1  is connected to the pull-down module  12 . The pull-down module  12  outputs the previous stage scanning signal G_N−1. 
         [0043]    The pull-down module  12  comprises a second transistor PT 2 . The controlling terminal of the second transistor PT 2  is connected to the output terminal of the first transistor PT 1 . The input terminal of the second transistor PT 2  is connected to the output terminal of the first transistor PT 1 . The output terminal of the second transistor PT 2  outputs a low-voltage-level scanning signal G_N−1 at the previous stage scanning line. 
         [0044]    The recovering control module  13  comprises a third transistor PT 3 . The controlling terminal of the third transistor PT 3  inputs the low-voltage-level scanning signal D 2 U. The input terminal of the third transistor PT 3  inputs the next stage clock signal CK_N+1, The output terminal of the third transistor PT 3  outputs a recovering signal at a scanning line, i.e., the next stage clock signal CK_N+1. 
         [0045]    The recovering module  14  comprises a fourth transistor PT 4 , a fifth transistor PT 5 , a sixth transistor PT 6 , a seventh transistor PT 7 , and a second bootstrap capacitor C 2 . The controlling terminal of the fourth transistor PT 4  is connected to the output terminal of the third transistor PT 3 . The input terminal of the fourth transistor PT 4  is connected to the constant low voltage supply VGL. The output terminal of the fourth transistor PT 4  is connected to the controlling terminal of the fifth transistor PT 5 , the controlling terminal of the seventh transistor PT 7 , the output terminal of the sixth transistor PT 6 , and the controlling terminal of an eighth transistor PT 8 . 
         [0046]    The input terminal of the fifth transistor PT 5  is connected to the constant high voltage supply VGH. The output terminal of the fifth transistor PT 5  is connected to the output terminal of the second transistor PT 2 . 
         [0047]    The controlling terminal of the sixth transistor PT 6  inputs a current stage pull-down control signal ST_N. The input terminal of the sixth transistor PT 6  is connected to the constant high voltage supply VGH. 
         [0048]    The controlling terminal of the seventh transistor PT 7  inputs a previous stage pull-down control signal ST_N−1. The input terminal of the seventh transistor PT 7  is connected to the constant high voltage supply VGH. 
         [0049]    The input terminal of the eighth transistor PT 8  is connected to the constant high voltage supply VGH. The output terminal of the eighth transistor PT 8  outputs the current stage scanning signal G_N at the scanning line. 
         [0050]    One terminal of the second bootstrap capacitor C 2  is connected to the constant high voltage supply VGH. The other terminal of the second bootstrap capacitor C 2  is connected to the output terminal of the fourth transistor PT 4 . 
         [0051]    The downlink module  15  comprises a ninth transistor PT 9  and an eleventh transistor PT 11 . The controlling terminal of the ninth transistor PT 9  is connected to the output terminal of the second transistor PT 2 . The input terminal of the ninth transistor PT 9  is connected to the output terminal of the eighth transistor PT 8 . The output terminal of the ninth transistor PT 9  outputs the current stage clock signal CK_N. 
         [0052]    The controlling terminal of the eleventh transistor PT  11  is connected to the output terminal of the second transistor PT 2 . The input terminal of the eleventh transistor PT  11  is connected to the output terminal of the ninth transistor PT 9 . The output terminal of the eleventh transistor PT  11  outputs the current stage pull-down control signal ST_N. 
         [0053]    One terminal of the first bootstrap capacitor C 1  is connected to the output terminal of the second transistor PT 2 . The other terminal of the first bootstrap capacitor C 1  is connected to the output terminal of the eighth transistor PT 8 . 
         [0054]    The leakage-proof module  16  comprises a tenth transistor PT 10 . The controlling terminal of the tenth transistor PT 10  is connected to the constant low voltage supply VGL. The input terminal of the tenth transistor PT 10  is connected to the output terminal of the second transistor PT 2 . The output terminal of the tenth transistor PT 10  is connected to the output terminal of the eighth transistor PT 8  through the first bootstrap capacitor C 1 . 
         [0055]    Please refer to  FIG. 2A  and  FIG. 2B  illustrating a concrete operational principle of the scan driver circuit  10  according to the preferred embodiment of the present invention. The clock signal CK_N in a cycle of every two sets is output. In other words, the waveform of the clock signal CK_N and the waveform of the clock signal CK_N+2 are identical. Firstly, the previous stage scanning signal G_N−1 outputs a low-voltage-level signal. At this time, the first transistor PT 1  of the pull-down control module  11  maintains conducted under the control of the scanning signal D 2 U at the low level. So the output terminal of the first transistor PT 1  inputs the previous stage scanning signal G_N−1 to the input terminal of the second transistor PT 2  of the pull-down module  12 . 
         [0056]    Meanwhile, the previous stage pull-down signal ST_N−1 also outputs a low-voltage-level signal. The second transistor PT 2  and the seventh transistor PT 7  are conducted. The output terminal of the second transistor PT 2  outputs a low-voltage-level signal G_N−1. 
         [0057]    The seventh transistor PT 7  of the recovering module  14  is conducted. The controlling terminal of the fifth transistor PT 5  and the controlling terminal of the eighth transistor PT 8  are connected to the constant high voltage supply VGH through the seventh transistor PT 7 . So the fifth transistor PT 5  and the eighth transistor PT 8  are disconnected. 
         [0058]    The tenth transistor PT  10  of the leakage-proof module  16  is conducted under the control of the constant low voltage supply VGL. The low-voltage-level signal G_N−1 output by the second transistor PT 2  of the pull-down module  12  works on the first bootstrap capacitor C 1  through the tenth transistor PT  10 , which makes the voltage level of a signal Q_N lower. Also, the scanning signal G_N outputs a low-voltage-level signal. Meanwhile, the ninth transistor PT 9  of the downlink module  15  is conducted under the control of the signal Q_N. The output terminal of the ninth transistor PT 9  outputs the current stage low-voltage-level clock signal CK_N to a driver circuit located at an previous stage scanning line. 
         [0059]    The eleventh transistor PT 11  is also conducted under the control of the signal Q_N. The output terminal of the eleventh transistor PT 11  outputs the current stage low-voltage-level pull-down control signal ST_N. The sixth transistor PT 6  is conducted under the control of the pull-down control signal ST_N. The controlling terminal of the fifth transistor PT 5  and the controlling terminal of the eighth transistor PT 8  are connected to the constant high voltage supply VGH through the sixth transistor PT 6 . So the fifth transistor PT 5  and the eighth transistor PT 8  keep disconnected, which makes sure that the low-voltage-level scanning signal G_N can be output and that the current leakage of the eighth transistor PT 8  cannot affect the output of the low-voltage-level scanning signal G_N. 
         [0060]    When the state of the next stage clock signal CK_N+1 turns into the low level, the third transistor PT 3  of the recovering control module  13  inputs the next stage clock signal CK_N+1 under the control of the low-voltage-level scanning signal U 2 D. The output terminal of the third transistor PT 3  outputs the clock signal CK_N+1, that is, outputting the recovering control signal to the recovering module  14 . 
         [0061]    The fourth transistor PT 4  of the recovering module  14  is conducted under the control of the recovering signal. At this time, the previous stage pull-down control signal ST_N−1 turns into a high-voltage-level signal. So the seventh transistor PT 7  is disconnected. At this time, the fifth transistor PT 5  and the eighth transistor P 18  are conducted under the control of the recovering signal. The high-voltage-level signal of the constant high voltage supply VGH is input to the Q_N point through the fifth transistor PT 5 , which pulls the signal Q_N up. Meanwhile, the high-voltage-level signal of the constant high voltage supply VGH is input to the scanning signal G_N, which pulls the scanning signal G_N up. Besides, the ninth transistor PT 9  and the eleventh transistor PT 11  are disconnected so the state of the clock signal CK_N and the state of the current stage pull-down control signal turn into the high level. The sixth transistor PT 6  is disconnected under the control of the current stage pull-down control signal ST_N, which prevents the fifth transistor PT 5  and the eighth transistor PT 8  from being affected by the constant high voltage supply VGH. 
         [0062]    At this stage, the process of outputting the low-voltage-level scanning signals in cascade for the scan driver circuit  10  is completed in this embodiment of the present invention. 
         [0063]    Preferably, the disposition of the second bootstrap capacitor C 2  in the recovering module  14  can pull up the level of the controlling terminal of the fifth transistor PT 5  and the level of the controlling terminal of the eighth transistor PT 8 . Therefore, the low level of the Q_N point is guaranteed. 
         [0064]    Preferably, the pull-down control module  11  further comprises a twelfth transistor PT 12 . The controlling terminal of the twelfth transistor PT 12  inputs the low-voltage-level scanning signal U 2 D. The input terminal of the twelfth transistor PT 12  inputs the next stage scanning signal G_N+1. The output terminal of the twelfth transistor PT 12  is connected to the pull-down module  12 . In this way, the pull-down control module  11  can receive the next stage scanning signal G_N+1 and generate the low-voltage-level scanning signal at the corresponding scanning line according to the next stage scanning signal G_N+1. Preferably, the recovering control module  13  further comprises a thirteenth transistor PT 13 . The controlling terminal of the thirteenth transistor PT 13  inputs the low-voltage-level scanning signal U 2 D. The input terminal of the thirteenth transistor PT 13  inputs the previous stage clock signal CK_N−1 (or CK_N+3). The output terminal of the thirteenth transistor PT 13  outputs the recovering signal at the scanning line. In this way, the recovering control module  13  can receive the previous stage clock signal CK_N−1 and generate the recovering signal at the corresponding scanning line according to the previous stage clock signal CK_N−1. 
         [0065]    Further, the scan driver circuit  10  has the function of backward scanning through the twelfth transistor PT 12  and the thirteenth transistor PT 13 . 
         [0066]    Preferably, the scan driver circuit  10  comprises a transistor with a P-type metal-oxide-semiconductor (MOS). The transistor with the P-type metal-oxide-semiconductor controls the pull-down control module  11 , the pull-down module  12 , the recovering control module  13 , and the recovering module  14 . It is also possible that a transistor with an N-type metal-oxide-semiconductor controls the pull-down control module  11 , the pull-down module  12 , the recovering control module  13 , and the recovering module  14 . 
         [0067]    The pull-down control signals ST_N and ST_N−1 control the sixth transistor PT 6  and the seventh transistor PT 7 , which makes sure that the fifth transistor PT 5  and the eighth transistor PT 8  keep disconnected when being charged at the Q_N point. It not only quarantines the low-voltage-level state of the Q_N point but also improves the reliability of the scan driver circuit. Also, the structure of the scan driver circuit is simplified. 
         [0068]      FIG. 3  is a circuit diagram of a scan driver circuit  20  according to a second preferred embodiment of the present invention. A recovering module  24  of the scan driver circuit  20  further comprises a fourteenth transistor PT 14  based on the first preferred embodiment. The controlling terminal of the fourteenth transistor PT 14  is connected to the output terminal of a fourth transistor PT 4 . The input terminal of the fourteenth transistor PT 14  is connected to a constant high voltage supply VGH. The output terminal of the fourteenth transistor PT 14  is connected to the controlling terminal of a sixth transistor PT 6 . 
         [0069]    The output of the fourteenth transistor PT 14  controls conduction and disconnection of the sixth transistor PT 6 , which prevents the sixth transistor PT 6  from leaking electricity due to the instability of the pull-down control signal. Therefore, the reliability of the scan driver circuit is improved further. 
         [0070]    Please refer to  FIG. 4A  and  FIG. 4B .  FIG. 4A  is a circuit diagram of a scan driver circuit  30  according to a third preferred embodiment of the present invention.  FIG. 4B  shows waveforms of signals applied in the scan driver circuit  30  according to the third preferred embodiment of the present invention. A recovering module  34  of the scan driver circuit  30  further comprises a fifteenth transistor PT 15  based on the second preferred embodiment. The controlling terminal of the fifteenth transistor PT 15  inputs an previous stage pull-down control signal ST_N−1. An input terminal of the fifteenth transistor PT 15  is connected to the constant high voltage supply VGH. The output terminal of the fifteenth transistor PT 15  is connected to the output terminal of the fourth transistor PT 4 . 
         [0071]    A clock signal CK_N in a cycle of every four sets is output. In other words, the waveform of the clock signal CK_N and the waveform of the clock signal CK_N+4 are identical. To ensure the reliability of a scanning level signal output by a second transistor PT 2 , the second transistor PT 2  is conducted under the control of an previous stage pull-down control signal ST_N−1, and the fifteenth transistor PT 15  is also conducted under the control of the previous stage pull-down control signal ST_N−1. So the disconnection of a fifth transistor PT 5  and an eighth transistor PT 8  is guaranteed. The low-voltage-level state of the Q_N point is guaranteed as well. Therefore, the reliability of the scan driver circuit is improved further based on the second embodiment. 
         [0072]    Please refer to  FIG. 5A  and  FIG. 5B .  FIG. 5A  is a circuit diagram of a scan driver circuit  40  according to a fourth preferred embodiment of the present invention.  FIG. 5B  shows waveforms of signals applied in the scan driver circuit  40  according to the fourth preferred embodiment of the present invention. A pull-down module  42  of the scan driver circuit  40  further comprises a sixteenth transistor PT 16  based on the third preferred embodiment. The controlling terminal of the sixteenth transistor PT 16  inputs a previous stage pull-down control signal ST_N−1. The input terminal of the sixteenth transistor PT 16  is connected to a constant low voltage supply VGL. The output terminal of the sixteenth transistor PT 16  is connected to the output terminal of a second transistor PT 2 . 
         [0073]    The level of the Q_N point of the constant low voltage supply VGL is lowered through the output of the sixteenth transistor PT 16 , which ensures the low-voltage-level state of the Q_N point. Therefore, the reliability of the scan driver circuit is improved further based on the third embodiment. 
         [0074]    The scan driver circuit in each of the preferred embodiments is a single-side driver circuit in cascade. That is, every driver circuit adopts the driving signal (e.g., a scanning signal or a pull-down control signal) of its previous stage driver circuit. However, the scan driver circuit in each of the preferred embodiments can be used a dual-side driver circuit in cascade. That is, every driver circuit adopts the driving signal of its upper-two-grade driver circuit. 
         [0075]    The feature of the present invention is the disposition of the pull-down control module and the disposition of the recovering control module, which not only improves the reliability of the scan driver circuit but simplifies the structure of the scan driver circuit. The scan driver circuit of the present invention can substitute for a conventional scan driver circuit of which the structure is complicated with low reliability. 
         [0076]    The present disclosure is described in detail in accordance with the above contents with the specific preferred examples. However, this present disclosure is not limited to the specific examples. For the ordinary technical personnel of the technical field of the present disclosure, on the premise of keeping the conception of the present disclosure, the technical personnel can also make simple deductions or replacements, and all of which should be considered to belong to the protection scope of the present disclosure.