Patent Publication Number: US-7221197-B2

Title: Driver circuit of display device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2004-242535 filed on Aug. 23, 2004; the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a driver circuit for driving scan lines or signal lines in a display device. 
   2. Description of the Related Art 
   Plane display devices, typified by liquid crystal display devices, feature thinness, light weight, and low power consumption, and accordingly are in use for displays of various types of electric appliances. Out of the plane display devices, active-matrix liquid crystal display devices with a transistor arranged in each pixel have been increasingly in use for note personal computers and portable information terminal devices. Recently, a technique has been established, which forms polysilicon thin film transistors (p-Si TFTs) with high electron mobility by means of a process to be performed at a temperature relatively lower than that which is applied in order to form amorphous silicon thin film transistors, which have been in use for conventional liquid crystal display devices. This establishment has made it possible to fabricate transistors in a smaller size, which are in use for liquid crystal display devices. This has made it possible to integrally form a pixel part and driver circuits on a transparent glass substrate through a single manufacturing process: the pixel part is configured by arranging a transistor at an intersection between each of a plurality of scan lines and its corresponding one of a plurality of signal lines, and the driver circuits respectively drive a group of scan lines and a group of signal lines. 
   In addition, as disclosed in Japanese Patent Laid-open No. 2003-344873, development has been in progress for a bootstrap circuit for the purpose of setting electric potential for output signals from a driver circuit at a sufficient level. 
   An electric potential difference between the high-level voltage and the low-level voltage of output signals from a driver circuit is used in order for the output signals from the driver circuit to control the writing of video signals into each pixel. For this reason, from the viewpoint of enabling the writing to be stable, it is desirable that the electric potential difference be as large as possible. However, a larger electric potential difference between the high-level voltage and the low-level voltage means a higher voltage stress being on the transistors. This is likely to reduce reliability in operation of each of the transistors that constitute the driver circuits. Furthermore, the larger electric potential difference leads driving the driver circuits at a higher voltage. This brings about a problem of requiring larger power consumption. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to prevent voltage stress on each transistor from increasing, to concurrently make it possible to set larger an electric potential difference between the high-level voltage and the low-level voltage of output signals. 
   A driver circuit for a display device according to the present invention is characterized by including shift registers and buffers. Each of the shift registers shifts the phase of input signals to be inputted thereinto, and outputs the signals with the shifted phase. Each of the buffers amplifies the amplitude of the output signals from the shift registers corresponding to the buffer by use of enable signals, and outputs the signals with the amplified amplitude to a scan line or a signal line. The driver circuit is characterized in that an electric potential difference between the high-level voltage and the low-level voltage of the enable signals is used as an electric potential difference with which to drive the scan lines or the signal lines, and in that an electric potential difference between the high-level power supply voltage and the low-level power supply voltage respectively in the shift registers and the buffers is set smaller than the electric potential difference between the high-level voltage and the low-level voltage of the enable signals. 
   In the present invention, an electric potential difference between the high-level power supply voltage and the low-level power supply voltage respectively for the shift registers and the buffers are set smaller than the electric potential difference between the high-level voltage and the low-level voltage of the enable signals. Thereby, the electric potential difference between the high-level voltage and the low-level voltage is made larger with regard to output signals from each of the buffers, which are amplified by use of the enable signals. On the other hand, the shift registers and the buffers are caused to operate by use of the high-level power supply voltage and the low-level power supply voltage, between which an electric potential difference is small. These help reduce voltage stress on each transistor. This makes it possible to set larger an electric potential difference between the high-level voltage and the low-level voltage of output signals from each of the buffers, thus enabling video signals to be written into the pixel corresponding to the buffer stably. Moreover, this makes it possible to reduce voltage stress on each transistor inside the shift registers and the buffers, thus achieving highly reliable operations. Additionally, this enables the driver circuit to operate at a lower voltage, thus enabling power consumption to be reduced. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a circuit diagram of a display device according to an embodiment. 
       FIG. 2  shows a circuit diagram of a driver circuit in the display device. 
       FIG. 3  shows a circuit diagram of a shift register and a buffer in the driver circuit. 
       FIG. 4  shows a timing chart of the driver circuit operations. 
       FIG. 5  shows diagram of output signals from the shift registers and output signals from the buffer in an overlapping manner. 
       FIG. 6  shows a circuit diagram of a shift register and a buffer in a driver circuit as a comparative example. 
       FIG. 7  shows a timing chart of the driver circuit as the comparative example. 
       FIG. 8  shows a diagram of output signals from the shift register and output signals from the buffer in the comparative example in an overlapping manner. 
       FIG. 9  shows a circuit diagram of other configurations respectively of the shift register and the buffer in the driver circuit. 
   

   DESCRIPTION OF THE EMBODIMENT 
   As shown in the circuit diagram of  FIG. 1 , a display device according to the embodiment includes a pixel part  11 , a scan line driver circuit  21  and a signal line driver circuit  31  on the surface of a transparent glass substrate  10 . In this case, descriptions will be provided for a configuration of the display device, giving an example of active-matrix liquid crystal display devices. 
   In the pixel part  11 , a plurality of scan lines G 1 , G 2 , . . . , Gn (hereinafter generically termed as “G”) and a plurality of signal lines S 1 , S 2 , . . . , Sm (hereinafter generically termed as “S”) are arranged in a way that the plurality of scan lines and the plurality of signal lines intersect each other. A switching element  12 , a pixel electrode  13 , a liquid crystal capacitor  14  and an auxiliary capacitor  15  are arranged at each intersection. As an example, a polysilicon thin film transistor is used for the switching element  12 . 
   The scan line driver circuit  21  includes a vertical shift register  22  and a buffer  23 . The vertical shift register  22  is formed of a plurality of shift registers electrically connected to one another longitudinally. The buffer is connected to the output stage of the vertical shift register  22 . The signal line driver circuit  31  includes a horizontal shift register  32 , a buffer  33 , a video signal line  34  and a plurality of analog switches  35 . The horizontal shift register  32  is formed of a plurality of shift registers electrically connected to one another longitudinally. The buffer  33  is connected to the output stage of the horizontal shift register  32 . The video signal line  34  is one through which video signals are supplied. The plurality of analog switches  35  are ones through which the video signal line  34  is connected with each of the signal lines S. 
   Both start pulse signals (STP) and clock signals (CK) are inputted into the vertical shift register  22  and the horizontal shift register  33 . In this case, the start pulse signals to be inputted into the vertical shift register  22  are termed as STV, and the start pulse signals to be inputted into the horizontal shift register  32  are termed as STH. In addition, the clock signals to be inputted into the vertical shift register  22  are termed as CKV, and the clock signals to be inputted into the horizontal shift register  32  are termed as CKH. 
   Each of the vertical shift register  22  and the horizontal shift register  32  shifts the phase of start pulse signals STP to be inputted thereinto, and outputs the signals with the shifted phase. In this respect, the scan line driver circuit  21  outputs vertical scan pulses from each of the shift registers therein to the respective scan lines G corresponding to the shift register while shifting phases of the vertical scan pulses one by one. The signal line driver circuit  31  outputs horizontal scan pulses from each of the shift registers therein to its corresponding one of the analog switches  35  provided to the respective signal lines S while shifting phases of the horizontal scan pulses one by one, thereby turning the analog switches  35  on. Accordingly, the signal line driver circuit  31  causes video signals, which have been supplied to the signal line driver circuit  31  through the video signal line  34  from the outside, to be outputted in each of the signal lines S respectively through the analog switches  35 . 
   The width W and the length L of the channel of each of the transistors in the buffers  23  and  24  are made larger than the width and the length of the channel of each of the transistors in the vertical shift register  22  and the horizontal shift register  32  in order that pulses capable of frilly driving the scan lines G and the analog switches  35  may be outputted. In addition, it is desirable that the driver circuits are respectively configured of only either a set of pMOS transistors or a set of nMOS transistors in order to cut back the manufacturing process and to enable costs to be reduced. 
     FIG. 2  shows a circuit diagram of the driver circuit. Basically, a configuration of the scan line driver circuit  21  is similar to that of the signal line driver circuit  31 . It is a matter of course that any one of the two driver circuits may be configured in a manner as shown in  FIG. 2 . Each of the driver circuits is configured to include the plurality of shift registers SR 1 , SR 2 , . . . , SRn (hereinafter genetically termed as SR), a clock line  36 , a plurality of buffers BUF 1 , BUF 2 , . . . , BUFn, and an output line  37 . The plurality of shift registers SR are electrically connected to one another longitudinally. The clock line  36  is one through which any two of three clock signals CK 1 , CK 2  and CK 3  with their phases shifted one by one are inputted to each of the shift registers SR. The plurality of buffers are connected respectively to the output stages of the shift registers SR 1 , SR 2 , . . . , SRn. The output line  37  is one through which enable signals OE (output enable) are supplied to each of the buffers BUF. The clock signals CK 1  to CK 3  constitutes vertical clock signals CKV in the vertical shift register  22 , and constitutes horizontal clock signals CKH in the horizontal shift register  32 . 
   The shift registers SR 1 , SR 2 , . . . , and SRn correspond respectively to a first stage, a second stage, . . . , and an nth stage. Each of the shift registers SR includes a first clock terminal  41  and a second clock terminal  42 . In the shift register SR 1 , for example, a first clock signal CK 1  is inputted into the first clock terminal  41 , and a second clock signal CK 2  is inputted into the second clock terminal  42 . In the shift register SR 2 , a second clock signal CK 2  is inputted into the first clock terminal  41 , and a third clock signal CK 3  is inputted into the second clock terminal  42 . 
   Each of the shift registers SR shifts the phase of input signals IN to be inputted thereinto in a way that the phase is synchronized to two of the three clock signals, and outputs the signals respectively with the phases thus shifted as output signals OUT. Start pulse signals STP are inputted, as the input signals IN, into a shift register SR 1  at a first stage. Output signals OUT from a shift register SR at any stage coming at or after the second stage are inputted into a shift register at the ensuing stage. Each of the buffers BUF amplifies the amplitude of output signals OUT, and outputs the signals with the amplified amplitude as output signals BOUT. 
   The scan line driver circuit  21  outputs output signals BOUT from each of the buffers BUF, as vertical scan pulses, to the scan lines G corresponding to the buffer. The signal line driver circuit  31  outputs output signals BOUT from each of the buffers BUF, as horizontal scan pulses, to the control electrode of the analog switch  35  corresponding to the buffer. 
     FIG. 3  shows a circuit diagram of the configurations respectively of one of the shift registers SR and one of the buffers BUF. Input signals IN are inputted into the input terminal  43 . In addition, any two of the three clock signals CK 1 , CK 2  and CK 3  with their phases shifted one by one are inputted to clock terminals. As an example, it is supposed in  FIG. 3  that a first clock signal CK 1  is inputted into the first clock terminal  41 , and that a second clock signal CK 2  is inputted into the second clock terminal  42 . As an example, it is also supposed that all of the transistors included in the shift registers SR and the buffers BUF are pMOS transistors. 
   Each of the shift registers SR is configured to include an output circuit, an input circuit and a reset circuit. The output circuit is configured of a first transistor T 1  and a second transistor T 2 . The drain and the source of the first transistor T 1  are electrically connected respectively to the first clock terminal  41  and the output terminal  44 . The source and the drain of the second transistor T 2  are electrically connected respectively to a power supply electrode  46  and the output terminal  44 . A first clock signal CK 1  is inputted into the first clock terminal  41 , and high-level power supply voltage VDD is supplied to the power supply electrode  46 . This output circuit outputs the first clock signal CK 1  to the output terminal  44  in a case where the first transistor T 1  is on and the second transistor T 2  is off This output circuit outputs the power supply voltage VDD to the output terminal  44  in a case where the first transistor T 1  is off and the second transistor T 2  is on. Here, a conductive path of the first transistor T 1  to the control electrode is denominated by a node n 1 , and a conductive path of the second transistor T 2  to the control electrode is denominated by a node n 2 . 
   The input circuit is configured of a third transistor T 3  and a fourth transistor T 4 . The drain and the gate of the third transistor T 3  are electrically connected to the input terminal  43 , and the source of the third transistor T 3  is electrically connected to the control electrode of the first transistor T 1 . In addition, the source, the drain and the gate of the fourth transistor T 4  are electrically connected respectively to the power supply electrode  46 , the control electrode of the second transistor T 2  and the input terminal  43 . The input circuit receives input signals IN through the input terminal  43 . 
   The reset circuit is configured of a fifth transistor T 5  and a sixth transistor T 6 . The drain and the gate of the fifth transistor T 5  are electrically connected to the second clock terminal  42 , and the source of the fifth transistor T 5  is electrically connected to the control electrode of the second transistor T 2 . In addition, the drain of the sixth transistor T 6  is electrically connected to the control electrode of the first transistor T 1 , and the gate of the sixth transistor T 6  is electrically connected to the drain of the fourth transistor T 4  and the control electrode of the second transistor T 2 , and the source of the sixth transistor T 6  is electrically connected to the power supply electrode  46 . A second clock signal CK 2  is inputted into the second clock terminal  42 . This reset circuit turns on one of the first and the second transistors T 1  and T 2 , and turns off the other of the first and second transistors T 1  and T 2 . 
   Each of the buffers BUF includes an inverter, an output circuit, an eleventh transistor T 11 . The inverter is configured of a seventh transistor T 7  and an eighth transistor T 8 . The output circuit is configured of a ninth transistor T 9  and a tenth transistor T 10 . The eleventh transistor T 11  is connected to the middle between the output terminal  44  of the shift register corresponding to the buffer and the control electrode of the ninth transistor T 9 . 
   More specifically, the gate and the source of the seventh transistor T 7  are connected respectively to the output terminal  44  of the shift register and the power supply electrode  46  to which the power supply voltage VDD is supplied. The gate and the drain of the eighth transistor T 8  is connected to a power supply electrode  47  to which low-level power supply voltage VSS 2  is supplied, and the source of the eighth transistor T 8  is connected to the drain of the seventh transistor T 7 . 
   The ninth transistor T 9  includes a conductive path between an enable terminal  48  and an out terminal  49 : the enable terminal  48  is one to which enable signals GE are inputted, and the output terminal  49  is one from which amplified signals are outputted. The tenth transistor T 10  includes a conductive path between the output terminal  49  and the power supply voltage VDD. More specifically, the drain and the source of the ninth transistor T 9  are connected respectively to the enable terminal  48  and the output terminal  49 , and the gate of the ninth transistor T 9  is connected to the output terminal  44  of the shift register corresponding to the buffer through the eleventh transistor T 11 . The gate of this eleventh transistor T 11  is supplied with the low-level power supply voltage VSS 2 . The drain and the source of the tenth transistor T 10  are connected respectively to the output terminal  49  and the power supply electrode  46  to which the power supply voltage VDD is supplied, and the gate of the tenth transistor T 10  is connected to a connecting point between the seventh transistor T 7  and the eighth transistor T 8 . Here, a conductive path of the tenth transistor T 10  to the control electrode is denominated by a node n 3 , and a conductive path of the ninth transistor T 9  to the control electrode is denominated by a node n 4 . 
   This driver circuit is characterized in that the electric potential difference between the high-level power supply voltage and the low-level power supply voltage respectively for the shift registers SR and the buffers BUF are set smaller than the electric potential difference between the high-level voltage and the low-level voltage for enable signals OE. 
   More specifically, the high-level power supply voltage VDD is caused to be maintained at a constant level, and concurrently the low-level power supply voltage VSS 2  used in the shift registers SR and the buffers BUF is caused to have an electric potential higher than the low-level power supply voltage VSS of enable signals OE. These will be described in detail with reference to  FIGS. 3 and 4 . 
     FIG. 4  is a timing chart showing a mutual relationship among input signals IN into the shift register SRI, clock signals CK 1  to CK 3 , enable signals OE, signals at the nodes n 1  to n 4 , output signals OUT from the shift register SR 1  and output signals BOUT from the buffer BUF corresponding to the shift register SR 1 . The output signals OUT from the shift register SR 1  are obtained by shifting the phase of the input signals IN. Incidentally, the other shift register SR operate according to the timing chart shown in  FIG. 4  in common with the shift register SR 1 . 
   As shown in  FIG. 4 , the low-level power supply voltage and the low-level voltage for the various types of signals such as the input signals IN, the clock signals CK 1  to CK 3  are set at VSS 2 , whose potential is higher than that of the low-level voltage VSS for the enable signals OE. This makes smaller an electric potential difference between the high-level voltage and the low-level voltage. As described later, the electric potential difference between the high-level voltage VDD and the low-level voltage VSS 2  suffices if the electric potential difference turns on the ninth transistor T 9 . The electric potential difference does not have to be larger than that. 
   In a time period between time t 1  and time t 2 , the electric potential of the input signals IN changes from the high-level voltage VDD to the low-level voltage VSS 2 , thus turning on the third transistor T 3  and the fourth transistor T 4 . The second clock signal CK 2  is at the high-level voltage, and accordingly the fifth transistor T 5  is in an off-state. The high-level power supply voltage is supplied to the node n 2  through the fourth transistor T 4 , thus raising the electric potential of the node n 2  to the high level. This turns off the second transistor T 2  and the sixth transistor T 6 . 
   The third transistor T 3  is on, and the sixth transistor T 6  is off. Accordingly, the input signals IN at the low-level voltage are supplied to the node n 1  through the third transistor T 3 , thus lowering the electric potential of the node n 1  to VSS 2 . This turns on the first transistor T 1 . As a consequence, the first clock signal CK 1  at the high-level voltage is supplied to the output terminal  44  of the shift register through the first transistor T 1 . This maintains the output signals OUT from the shift register at the high-level voltage. 
   In a time period between time t 2  and time t 3 , the electric potential of the input signals IN changes from the low-level voltage VSS 2  to the high-level voltage VDD, and concurrently the electric potential of the first clock signal CK 1  is reversed from the high-level voltage VDD to the low-level voltage VSS 2 . The rise of the electric potential of the input signals IN to the high level turns off the third transistor T 3  and the sixth transistor T 6 , thus turning the node n 1  to a floating state which does not apply electric voltage to the node n 1 . In addition, the node n 1  is influenced by the reversal of the electric potential of the first clock signal CK 1  to the low level through the transistor T 1 , thus lowering the electric potential of the node n 1  to a low electric potential (an LL level), which is lower than the low-level voltage VSS 2 . 
   This is because the existence of parasitic capacity between the gate and the source of the first transistor T 1  or between the gate and the drain of the first transistor T 1  changes the electric potential of the node  1  in conjunction with change in electric potential between the drain and the source of the first transistor T 1  in a case where the node n 1  is in the floating state. Bootstrap is a term for the phenomenon of fluctuation in electric potential of the node in the floating state under influence of fluctuation in electric potential in a transistor to which the node is connected. In addition, bootstrap node is a term for the node in this case. Here, the decrease of the electric potential of the node n 1  to the further lower level turns the first transistor T 1  to an on-state securely. This causes the output terminal  44  of the shift register to be supplied with the first clock signal CK 1  at the low-level voltage through the first transistor T 1 , thus turning the output signals OUT to the low-level voltage VSS 2 . 
   Furthermore, the second clock signal CK 2  is at the high-level voltage. For this reason, the fifth transistor T 5  is in an off-state, and the fourth transistor T 4  is also in an off-state. Accordingly, the node n 2  is supplied with no voltage, and is in a floating state. As a consequence, the parasitic capacity maintains the node n 2  at the high-level voltage. In other words, in this time period, the node n 1  is at the LL level while being in the floating state, and the node n 2  is at the high-level voltage VDD while being in the floating state. 
   If the widths of the channels respectively of the first transistor T 1  and the second transistor T 2  are beforehand set larger than the width of the channels respectively of the other transistors with the aforementioned case taken into consideration, this causes the parasitic capacities of the first transistor and the second transistor to be larger. Accordingly, the nodes n 1  and n 2  can maintain the LL-level electric potential and the high-level potential respectively. 
   At time t 3 , the electric potential of the first clock signal CK 1  rises to the high-level voltage VDD, the electric potential of the second clock signal CK 2  decreases to the low-level voltage VSS 2 . The decrease of the electric potential of the second clock signal CK 2  to the low-level voltage turns on the fifth transistor T 5 . At this time, the fourth transistor T 4  is in an off-state. For this reason, the electric potential of the node n 2  changes to the low-level voltage VSS 2  through the fifth transistor T 5 . As a consequence, the second transistor T 2  and the sixth transistor T 6  are turned on. Turning on the sixth transistor T 6  raises the node n 1  to the high-level voltage, and turns off the first transistor T 1 . Turning off the first transistor T 1  and turning on the second transistor T 2  in this manner causes the output terminal  44  to be supplied with the high-level power supply voltage VDD through the second transistor T 2 , thus raising the electric potential of the output signals OUT from the shift transistor to the high level. 
   After time t 3 , the electric potential of the output signals OUT from the shift register rises to the high-level voltage VDD. Thus, the electric potentials respectively of the nodes n 3  and n 4  are reversed, the electric potential of the node n 3  decreases to the low-level voltage VSS 2 , and the electric potential of the node n 4  rises to the high-level voltage VDD. As a consequence, the ninth and the tenth transistors T 9  and T 10  are turned off and on respectively. Accordingly, through the tenth transistor T 10 , the buffer is supplied with the power supply voltage VDD. Hence, the electric potential of the output signals BOUT is maintained at the high-level voltage, no matter what electric potential the enable signals OE may be at. For the reference, the output signals OUT and the output signals BOUT are shown in an overlapping manner in  FIG. 5 . 
   Next, descriptions will be provided for operations of the buffers BUF. As shown in  FIG. 4 , the high-level voltage and the low-level voltage of the enable signals OE are respectively VDD and VSS. This voltage VSS is a voltage lower than the aforementioned low-level voltage VSS 2 . 
   In a time period between time t 2  and time ta, when the output signals OUT from the shift register at the low-level voltage VSS 2  is inputted into the buffer BUF, the electric potential of the node n 3  turns to the high-level voltage VDD, since the seventh and the eighth transistors T 7  and T 8  constitute the inverter circuit. This turns off the tenth transistor T 10 . Furthermore, the output signals OUT at the low-level voltage, which are supplied from the shift register through the eleventh transistor T 11 , turns the node n 4  to the low-level voltage VSS 2 , thus turning on the ninth transistor T 9 . The enable signals GE at the high-level voltage are supplied to the output terminal  49  of the buffer BUF through the ninth transistor T 9 , accordingly maintaining the output signals BOUT at the high-level voltage VDD. 
   In a time period between time ta and time tb, the electric potential of the enable signals OE decreases to the low-level voltage VSS, thus causing the bootstrap to operate. Accordingly, the electric potential of the node N 4  in a floating state decreases to a level which is lower than VSS 2  by a voltage equivalent to (VDD-VSS), thus maintaining the ninth transistor T 9  in an on-state. Thereby, the electric potential of the output signals BOUT decreases to the low-level voltage VSS in conjunction with change in voltage of the enable signals OE. 
   In a time period between time tb and time t 3 , the electric potential of the enable signals OE rises to the high-level voltage VDD, thus returning the electric potential of the node n 4  to the normal low-level voltage VSS 2 . Thus, the ninth transistor T 9  is maintained in an on-state, and the electric potential of the output signals BOUT returns to the high-level voltage in conjunction with change in voltage of the enable signals OE. 
   After time t 3 , the electric potential of the output signals OUT from the shift resistor rises to the high-level voltage VDD. Thus, the electric potentials respectively of the nodes n 3  and n 4  are reversed, the electric potential of the node n 3  decreases to the low-level voltage VSS 2 , and the electric potential of the node n 4  rises to the high-level voltage VDD. As a consequence, the ninth and the tenth transistors T 9  and T 10  are turned off and on respectively. Accordingly, through the tenth transistor T 10 , the buffer is supplied with the power supply voltage VDD. Hence, the electric potential of the output signals BOUT is maintained at the high-level voltage, no matter what electric potential the enable signals OE may be at. For the reference, the output signals OUT and the output signals BOUT are shown in an overlapping manner in  FIG. 5 . 
   In the case of this driver circuit, as shown in  FIG. 5 , the output signals OUT from the shift register are signals at a voltage level in a range of VSS 2  to VDD, while the output signals BOUT are signals at a voltage level in a range of VSS to VDD, whose electric potential difference is larger than that between VSS 2  and VDD. This causes the buffer circuits to amplify the signals. That is because the electric potential difference between the high-level power supply voltage VDD and the low-level power supply voltage VSS 2  for the shift registers SR and the buffers BUF is set smaller than the electric potential difference between the high-level power supply voltage VDD and the low-level power supply voltage VSS for the enable signals OE. For example, all the parts of the shift registers and the parts other than the output circuit in each of the buffers are designed to operate at 18V, and the output circuit of each of the buffers is designed to operate at 22.5V. 
   In other words, the driver circuits in this display device can write the video signals into the pixels stably, since the buffers BUF can output the output signals BOUT with a larger electric potential difference between the high-level voltage and the low-level voltage. As far as the insides respectively of the shift registers SR and the buffers BUF are concerned, the circuits other than the output circuits of the buffers BUF are designed to operate by use of the high-level voltage and the low-level voltage, between which the electric potential difference is smaller. This makes it possible to reduce the voltage stress on each of the transistors, accordingly enabling the driver circuits to operate with higher reliability. In addition, the driver circuits are caused to operate at a lower voltage, thus enabling the power consumption to be reduced to a lower level. 
   Next, descriptions will be provided for a driver circuit as a comparative example.  FIG. 6  is a circuit diagram showing configurations respectively of a shift registor and a buffer on the driver circuit as the comparative example. In this comparative example, the low-level voltage of the various types of signals as well as the low-level power supply voltage in the shift registors and the buffers in addition to the low-level voltage of the enable signals OE are set at the single VSS. The other basic configurations respectively of the shift registor and the buffer are the same as those of the circuits show in  FIG. 3 . 
     FIG. 7  is a timing chart showing operations of the driver circuit in the comparative example. The driver circuit in the comparative example basically operates as shown by the timing chart of  FIG. 4 . However, all of the various types of signals are signals at VDD and VSS, between which the electric potential difference is larger. For this reason, an electric potential difference applied to the output signals BOUT is larger, thus enabling video signals to be written onto each of the pixels stably. On the other hand, the insides of a shift register and a buffer are supplied with the high-level voltage and the low-level voltage, between which the electric potential difference is larger. This causes the voltage stress on each of the transistors to be higher, thus reducing reliability in operations of the driver circuit. In addition, the driver circuit is operated at a higher voltage, thus consuming much more electric power. For the reference, the output signals OUT and the output signals BOUT are shown in overlapping manner in  FIG. 8 . 
   In contrast to this, in the case of the driver circuit according to this embodiment, the voltage stress on each of the transistors is lower, thus making higher the reliability in operations of the driver circuit. In addition, the driving of the driver circuit at the low-level voltage prevents an increase in the power consumption. 
   Consequently, according to this embodiment, the electric potential difference between the high-level power supply voltage and the low-level power supply voltage in the shift registers and the buffers is set smaller than the electric potential difference between the high-level voltage and the low-level voltage of enable signals OE, which the buffers use in their respective output circuits. This makes larger the electric potential difference between the high-level voltage and the low-level voltage of output signals BOUT, which output signals are amplified by use of enable signals OE. Accordingly, video signals can be written into each of the pixels stably. Furthermore, as far as the insides respectively of the shift registers and the buffers are concerned, all of the circuits, except for the output circuits of the buffers, operate by use of the high-level voltage VDD and the low-level voltage VSS 2 , between which the electric potential difference is smaller. This makes it possible to reduce the voltage stress on each of the transistors, accordingly enabling the operational reliability to be improved and concurrently power consumption to be reduced. 
   The configuration of the shift registers is not limited to the configuration shown in  FIG. 3 . No matter what configuration may be used for the shift registers, if the configuration enables phases of input signals to be shifted. 
   Furthermore, the configuration of the buffers is not limited to the configuration shown in  FIG. 3 . No matter what configuration may be used for the buffers, if the configuration makes it possible to amplify amplitudes of output signals OUT from the shift registers by use of enable signals OE. In this case, the electric potential difference between the high-level power supply voltage and the low-level power supply voltage of the shift registers and buffers is set smaller than the electric potential difference between the high-level voltage and the low-level voltage in the enable signals OE. Incidentally, it is desirable that the output circuits respectively of the buffers are configured to include the ninth transistor T 9  and the tenth transistor T 10 , and that the output circuits are configured to output the output signals BOUT by use of the bootstrap by the ninth transistor T 9 . 
   Finally, in the case of this embodiment, the driver circuit has been described, in which pMOS transistors are used for the shift registers and the buffers, and which start pulse signals STP with pulses in the downward direction are transmitted. However, the driver circuit is not limited to this. As shown in  FIG. 9 , for example, the shift registers and the buffers may be configured of nMOS transistors, in association with which the driver circuits are configured to transmit start pulse signals with pulses in the upward direction. This case can bring about the same effect as the aforementioned case brings about.