Abstract:
In a driving circuit, one output circuit has a scanning signal line, a first transistor which controls electrical connection between the scanning signal line and a clock signal line which has a gate connected to a first node, the first node which is at an active potential in a first time period including a time period during which the active potential is output to the scanning signal line, a second transistor which electrically connects the first node and an inactive signal line which has a potential to open the transistor in a second time period other than the first time period, and the second transistor has a gate connected to a second node, wherein the second node has two kinds of timings to be charged for retaining the active potential.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority from Japanese application JP2011-193730 filed on Sep. 6, 2011, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a driving circuit and a display device using the driving circuit. 
         [0004]    2. Description of the Related Art 
         [0005]    As a display devices for an information communication terminal, such as a computer, or a television receiver, liquid crystal display device has been widely used. Moreover, organic EL display device (OLED), field emission display device (FED), and the like have also been known as flat-panel display devices. The liquid crystal display device is a device which changes the alignment of a liquid crystal material sealed between two substrates by changing an electric field to control the degree of transmittance of light passing through the two substrates and the liquid crystal material, thereby displaying an image. 
         [0006]    In display devices which apply a voltage corresponding to a certain gray scale value to each pixel of a screen, including the liquid crystal display device described above, each pixel has a pixel transistor for applying the voltage corresponding to the gray scale value. In general, gates of pixel transistors corresponding to one line of the screen are connected to one signal line (hereinafter referred to as “scanning signal line”). This scanning signal line is controlled by a driving circuit so as to output an active voltage which makes the pixel transistors conductive sequentially for each line. JP 2007-095190 A shows an example of a driving circuit which can operate more stably without the occurrence of a short-circuit current. 
       SUMMARY OF THE INVENTION 
       [0007]      FIG. 16  shows an output circuit  910  for outputting to a scanning signal line G n , as an example of one of a plurality of output circuits included in a driving circuit.  FIG. 17  is a timing diagram of operation of the output circuit  910  of  FIG. 16 . V n  represents a clock signal, and the potential of VGPL is fixed to Low potential. The clock signal V n  is an eight-phase clock signal which includes eight clock signals having the same period but different in timing. In this circuit, a scanning signal line G n−2  being at High potential is used as a trigger to change potentials of nodes N 1  and N 2 , and High potential of the clock signal V n  is output to the scanning signal line G n . 
         [0008]      FIG. 18  schematically shows a detailed change in voltage of the node N 2  at the time of operating the output circuit  910 . It is necessary for the node N 2  to be maintained at High potential for setting a transistor T 2  conductive in a time period in which High potential is not output to the scanning signal line G n . However, leakage occurs from transistors T 3 , T 4 , and T 7  to cause a gradual decrease in potential. To compensate for this, the node N 2  is charged via the transistor T 3 , which is diode-connected, at a timing at which a clock signal V n+4  is at High potential, thereby High potential of the node N 2  is maintained. 
         [0009]    In the output circuit of the driving circuit described above, it is considered to use a clock having more phases to lower the frequency of the clock signal V n  in order to decrease the number of times of charging and discharging of a transistor T 5  due to a change in potential on a drain side of the transistor T 5 . For example,  FIG. 19  shows a timing diagram where a 16-phase clock is used for the clock signal V n  of the output circuit  910  described above. In this case, since the interval of the clock signal V n+4  is increased, opportunities to perform charging to the node N 2  are decreased, so that the potential of the node N 2  may not be maintained as shown in  FIG. 20 . 
         [0010]    The invention has been made in view of the circumstances described above, and it is an object of the invention to provide a display device with high display quality in which a stable scanning signal is output even when a clock having more phases is used. 
         [0011]    According to an exemplary embodiment of the present invention, there is provided a driving circuit of a display device, the driving circuit outputting an active potential sequentially to a plurality of scanning signal lines, the active potential setting a transistor conductive. The driving circuit includes: a plurality of output circuits electrically connected respectively to the plurality of scanning signal lines, wherein one output circuit of the plurality of output circuits has a first transistor which controls electrical connection between one scanning signal line of the plurality of scanning signal lines and a clock signal line, a first node which is connected to a gate of the first transistor and is at the active potential in a first time period including a time period during which the active potential is output to the scanning signal line, a second transistor which controls to connect the first node and an inactive signal line electrically in a second time period other than the first time period, the inactive signal which retains an inactive potential which does not set the transistor conductive, and a second node which is connected to a gate of the second transistor, and the second node has two kinds of charging timings for retaining the active potential. 
         [0012]    Moreover, in the driving circuit according to the exemplary embodiment of the invention, the one output circuit further may have a first charging line which connects the second node via an element having a rectifying action and a second charging line which connects the second node via an element having a rectifying action in order to retain the active potential of the second node. 
         [0013]    Moreover, in the driving circuit according to the exemplary embodiment of the invention, one clock signal of a plurality of clock signals which have the same cycle and which are input to the plurality of output circuits may be input to any one of the first charging line and the second charging line, and one scanning signal line of another output circuit of the plurality of output circuits may be connected to the other of the first charging line and the second charging line. 
         [0014]    Moreover, in the driving circuit according to the exemplary embodiment of the invention, the one clock signal may be a clock signal of the plurality of clock signals which have the same cycle and which are input to the plurality of output circuits, the clock signal being at an active voltage during a period corresponding to half-cycle before a timing at which a clock signal to be input to the clock signal line connected to the first transistor is at the active voltage. The term “cycle” of the half-cycle used herein means a cycle of the clock signal. 
         [0015]    Moreover, in the driving circuit according to the exemplary embodiment of the invention, the one scanning signal line of the another output circuit may be input to any one output of three outputs which are sequentially output by the plurality of output circuits immediate after outputting to the scanning signal line of the one output circuit. 
         [0016]    Moreover, in the driving circuit according to the exemplary embodiment of the invention, two different clock signals of a plurality of clock signals which have the same cycle and which are input to the plurality of output circuits may be input to the first charging line and the second charging line. 
         [0017]    Another exemplary embodiment of the invention is directed to a display device having a plurality of pixels in a screen, including: the driving circuit according to any of the driving circuits described above; and pixel transistors arranged respectively in the plurality of pixels for retaining a voltage based on a gray scale value in each of the plurality of pixels, wherein the scanning signal lines of the driving circuit are each connected to gates of the pixel transistors of the pixels corresponding to one row of the screen. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  schematically shows a display device according to a first embodiment of the invention. 
           [0019]      FIG. 2  shows a configuration of a display panel of  FIG. 1 . 
           [0020]      FIG. 3  shows a circuit configuration of an output circuit of  FIG. 2 . 
           [0021]      FIG. 4  is a timing diagram of operation of the output circuit of  FIG. 3 . 
           [0022]      FIG. 5  schematically shows a detailed change in potential of a node N 2  in operation of the output circuit of  FIG. 3 . 
           [0023]      FIG. 6  shows a configuration of an output circuit according to a display device of a second embodiment. 
           [0024]      FIG. 7  schematically shows a detailed change in potential of the node N 2  in operation of the output circuit of  FIG. 6 . 
           [0025]      FIG. 8  shows a configuration of an output circuit according to a display device of a third embodiment. 
           [0026]      FIG. 9  schematically shows a detailed change in potential of the node N 2  in operation of the output circuit of  FIG. 8 . 
           [0027]      FIG. 10  shows a configuration of an output circuit according to a display device of a fourth embodiment. 
           [0028]      FIG. 11  is a timing diagram of operation of the output circuit of  FIG. 10 . 
           [0029]      FIG. 12  schematically shows a detailed change in potential of the node N 2  in operation of the output circuit of  FIG. 10 . 
           [0030]      FIG. 13  shows an output circuit as a modified example of the output circuit of  FIG. 10 . 
           [0031]      FIG. 14  is a timing diagram of operation of the output circuit of  FIG. 13 . 
           [0032]      FIG. 15  schematically shows a detailed change in potential of the node N 2  in operation of the output circuit of  FIG. 13 . 
           [0033]      FIG. 16  shows an example of an output circuit. 
           [0034]      FIG. 17  is a timing diagram of operation of the output circuit of  FIG. 16 . 
           [0035]      FIG. 18  schematically shows a detailed change in potential of the node N 2  in operation of the output circuit of  FIG. 16 . 
           [0036]      FIG. 19  is a timing diagram where a 16-phase clock is used.  FIG. 20  schematically shows a detailed change in potential of the node N 2  in the case of  FIG. 19 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]    Hereinafter, first to fourth embodiments of the invention will be described with reference to the drawings. In the drawing, the same or equivalent constituents are denoted by the same reference characters, and the repetitive description thereof is omitted. 
       First Embodiment 
       [0038]      FIG. 1  schematically shows a display device  100  according to a first embodiment of the invention. As shown in this drawing, the display device  100  includes a display panel  200  fixed so as to be interposed between an upper frame  110  and a lower frame  120 . In the embodiment, the display panel  200  is deemed to be a liquid crystal display panel. 
         [0039]      FIG. 2  shows a configuration of the display panel  200  of  FIG. 1 . The display panel  200  has two substrates, a TFT (Thin Film Transistor) substrate  220  and a color filter substrate  230 . Between these substrates, a liquid crystal material is sealed. The TFT substrate  220  has driving circuits  210  arranged on both sides of a display area  202  and a driving IC (Integrated Circuit)  260  controlling the driving circuits  210 . The driving circuits  210  applies a predetermined voltage sequentially to scanning signal lines G 1  to G 480 . The driving IC  260  applies a voltage corresponding to the gray scale value of a pixel to a plurality of data signal lines (not shown) extending so as to perpendicularly intersect the scanning signal lines G 1  to G 480  in the display area  202 . Moreover, the driving circuit  210  has output circuits  310  respectively connected to the scanning signal lines G n  (n=1 to 480). The output circuits  310  on one side of the display area  202  control odd-numbered scanning signal lines G n  (n: odd numbers), while the output circuits  310  on the other side control even-numbered scanning signal lines G n  (n: even numbers). 
         [0040]      FIG. 3  shows a circuit configuration of the output circuit  310 , and  FIG. 4  is a timing diagram of operation of the output circuit  310  of  FIG. 3 . The output circuit  310  operates with a 16-phase clock signal which includes 16 clock signals having the same cycle but different in timing. Since the driving circuit which drives the even-numbered scanning signal lines and the driving circuit which drives the odd-numbered scanning signal lines are respectively arranged on both sides of the display area  202 , one driving circuit  210  arranged on one side of the display area  202  operates substantially with an 8-phase clock. 
         [0041]    Next, operation of the output circuit  310  will be described. Here, V n  represents a clock signal, and the potential of VGPL is fixed to Low potential. All of these signals are input from the outside of the output circuit  310 . First, when a scanning signal line G n−2  is at High potential, a gate of a transistor T 7  is at High potential, so that the transistor T 7  becomes conductive. Therefore, a node N 2  is connected to VGPL to be at Low potential. Moreover, the scanning signal line G n−2  is also input to a diode-connected transistor T 1 . Therefore, a node N 1  connected to the transistor T 1  is at High potential (active potential), so that a potential difference is generated at a capacitance C 1  and a transistor T 5  becomes conductive. Since the node N 1  serves as the gate signal of a transistor T 4 , the node N 2  is connected to VGPL also through the transistor T 4  to be at Low potential. 
         [0042]    Next, when the clock signal V n  is at High potential, the potential of one of electrodes of the capacitance C 1  becomes High potential because the transistor T 5  is conductive, so that the gate potential of the transistor T 5  which is at the side of the other electrode of the capacitance C 1  is further raised due to so-called bootstrap. This ensures High potential of the scanning signal line G n . In a writing time period during which the scanning signal line G n  is at High potential, a data signal voltage based on the gray scale value of each pixel is applied to each of the data signal lines (not shown), and the applied voltage based on the gray scale value is retained in the pixel due to the drop of the scanning signal line G n , which will be described later. 
         [0043]    When the clock signal V n  is at Low potential, the scanning signal line G n  is also at Low potential. However, for further ensuring this, a clock signal V n+4  which is at High potential is input to a diode-connected transistor T 3 , so that the node N 2  is at High potential. A transistor T 6  whose gate is connected with the node N 2  at High potential connects the scanning signal line G n  and VGPL electrically, so that the scanning signal line G n  is at Low potential. On the other hand, High potential of a scanning signal line G n+4  after two horizontal driving periods is input to a gate of a transistor T 9  so as to connects the node N 1  and VGPL electrically, so that the node N 1  is at Low potential. 
         [0044]    Here, in the embodiment, the output circuit  310  has a first charging line  361  and a second charging line  362 . Here, the first charging line  361  is connected to the node N 2  via the diode-connected transistor T 3  acting as a rectifying element, and the clock signal V n+4  is applied to the first charging line  361 . Moreover, the second charging line  362  is connected to the node N 2  via a diode-connected transistor T 3 A, and a clock signal V n+12  is applied to the second charging line  362 . Accordingly, as shown in  FIG. 5 , charging is performed using not only the clock signal V n+4  but also the clock signal V n+12  which is at High potential in a time period during which the clock signal V n+4  is at Low potential. Therefore, High potential of the node N 2  can be maintained, and the driving circuit can output a more stable scanning signal, so that the display quality of the display device can be enhanced. Here, although the clock signal to be applied to the second charging line  362  is the clock signal V n+12 , any clock signal may be used as long as the clock signal is at an active potential in a time period of one-half cycle before the clock signal V n  is at High potential (active potential). 
       Second Embodiment 
       [0045]    A second embodiment of the invention will be described. Since a configuration of a display device according to the second embodiment is similar to that of the first embodiment shown in  FIGS. 1 and 2 , the repetitive description thereof is omitted.  FIG. 6  shows a configuration of an output circuit  320  according to the display device of the second embodiment. The output circuit  320  is different from the output circuit  310  in the first embodiment in that the signal to be input to the transistor T 3  is not the clock signal V n+4  but output of the scanning signal line G n+4 . 
         [0046]      FIG. 7  schematically shows a detailed timing of operation using the output circuit of  FIG. 6 . It is sufficient that High potential of the node N 2  not to set the transistor T 5  conductive is maintained when the clock signal V n  is at High potential. Therefore, as shown in  FIG. 7 , it is basically sufficient that charging is performed at a timing that the clock signal V n+12  is input to the second charging line  362 . However, since it is necessary to lower the node N 2  to Low potential at a timing after outputting to the scanning signal line G n , output of the scanning signal line G n+4  which is at High potential once in a vertical synchronizing period is applied to the first charging line  361 . This almost eliminates charging to the node N 2  at a timing other than the clock signal V n+12 . Therefore, for example, loads to the transistors T 2  and T 6  are decreased, so that the occurrence of threshold voltage shift or the like can be suppressed, and High potential of the node N 2  can be maintained when the clock signal V n  is at High potential. Accordingly, the driving circuit can output a more stable scanning signal, so that the display quality of the display device can be enhanced. 
       Third Embodiment 
       [0047]    A third embodiment of the invention will be described. Since a configuration of a display device according to the third embodiment is similar to that of the first embodiment shown in  FIGS. 1 and 2 , the repetitive description thereof is omitted.  FIG. 8  shows a configuration of an output circuit  330  according to the display device of the third embodiment. The output circuit  330  is different from the output circuit  320  in the second embodiment in that the signal to be input to the first charging line  361  and the gate of the transistor T 9  is not the output of the scanning signal line G n+4  but output of a scanning signal line G n+3 . 
         [0048]      FIG. 9  schematically shows a timing of operation using the circuit of  FIG. 8 . Similar to the second embodiment, the output of the scanning signal line G n+3  which is at High potential once in a vertical synchronizing time period is applied to the first charging line  361 . However, since the scanning signal line G n+3  is at High potential at a timing one step earlier than the scanning signal line G n+4  it is possible to raise the node N 2  to High potential as shown in  FIG. 9 , that is, lower the node N 1  to Low potential. This makes it possible to shorten a period during which the gate voltage of the transistor T 5  relating directly to the output of the scanning signal line G n  is high, so as to suppress threshold voltage shifting of the transistor T 5 . Moreover, since the node N 2  is rarely charged during a period in which the clock signal V n+12  is not provided, loads to the transistors T 2  and T 6  are also decreased, so that the occurrence of threshold voltage shift or the like can be suppressed also for these transistors. Moreover, since High potential of the node N 2  can be maintained when the clock signal V n  is at High potential, the driving circuit can output a more stable scanning signal, so that the display quality of the display device can be enhanced. Here, although output of the scanning signal line to be applied to the first charging line  361  is the output of the scanning signal line G n+3 , the output may be any one of three outputs of the other scanning signal lines immediately after the output of the scanning signal line G n . 
       Fourth Embodiment 
       [0049]    Since a configuration of a display device according to a fourth embodiment is similar to that of the first embodiment shown in  FIGS. 1 and 2 , the repetitive description thereof is omitted.  FIG. 10  shows a configuration of an output circuit  410  according to the display device of the fourth embodiment. Moreover,  FIG. 11  shows a timing diagram of operation using the output circuit  410 . The output circuit  410  is different from the output circuit  310  in the first embodiment in that the diode-connected transistor T 3 A is not used, and that an 8-phase clock signal V m+2  is input to the transistor T 3 . Even with the configuration described above, since High potential of the node N 2  can be maintained as shown in  FIG. 12 , the driving circuit can output a more stable scanning signal, so that the display quality of the display device can be enhanced. 
         [0050]      FIG. 13  shows an output circuit  420  as a modified example of the output circuit  410 , and  FIG. 14  shows a timing diagram of operation of the output circuit  420 . The output circuit  420  is different from the output circuit  410  in that the 8-phase clock signal to be input to the diode-connected transistor T 3  is a clock signal V m  which is different from the clock signal V m+2  in timing, and that the signal to be input to the gate of the transistor T 9  is an output signal to the scanning signal line G n+3 . In the case of the configuration described above, High potential of the node N 1  can be lowered earlier as shown in  FIG. 15 , and a period during which the gate voltage of the transistor T 5  is high and is relating directly to the output of the scanning signal line G n  can be reduced, so as to suppress threshold voltage shifting of the transistor T 5 . Moreover, since High potential of the node N 2  can be maintained, the driving circuit can output a more stable scanning signal, so that the display quality of the display device can be enhanced. 
         [0051]    Moreover, although, in each of the display devices of the embodiments described above, an 8-phase or 16-phase clock signal is used, a clock signal other than these can also be used. 
         [0052]    Moreover, the liquid crystal display device of each of the embodiments described above is not limited to a liquid crystal display device. The embodiments can be used for organic EL display devices, field emission display devices (FEDs), and other display devices using a shift register as a driving circuit. 
         [0053]    While there have been described what are at present considered to be certain embodiments of the invention, it will be understood that various modifications may be made thereto, and it is intended that the appended claims cover all such modifications as fall within the true spirit and scope of the invention.