Abstract:
A display driver includes an input node for receiving display data, a level shift circuit configured to convert voltage level of the display data and output a first voltage and a second voltage based on the display data, an output node for outputting the output display data, a first P-channel MOS transistor coupled to the output node, whose gate is configured to input the first voltage, and a first N-channel MOS transistor coupled to the output node, whose gate is configured to input the second voltage, wherein a voltage difference between the second voltage and the first voltage varies based on the display data.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is a Continuation application of U.S. patent application Ser. No. 13/724,224, filed on Dec. 21, 2012, which is based on Japanese Patent Application No. 2011-281108 filed on Dec. 22, 2011, the entire contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to a drive circuit of a display device, and in particular to a level shift circuit that converts a signal level of a digital signal. 
         [0003]    The level shift circuit converts a digital signal having an amplitude between a reference supply voltage (VSS) and a low supply voltage (VDD 1 ) into a digital signal having an amplitude between the reference supply voltage and a high supply voltage (VDD 2 ). 
         [0004]    The level shift circuit is disclosed in, for example, Japanese Patent Laid-Open No. 1993-199101 (Patent Document 1). As shown in  FIG. 1 , the circuit includes resistances R 91  and R 92  between drains of transistors P 91  and N 91  and between drains of transistors P 92  and N 92 , respectively. The sources of the transistors P 91  and P 92  are coupled to the high supply voltage VDD 2 . The sources of the transistors N 91  and N 92  are coupled to the reference supply voltage VSS. The gate of the transistor P 91  is coupled to a connection node B of the resistance R 92  and the transistor N 92 . The gate of the transistor P 92  is coupled to a connection node A of the resistance R 91  and the transistor N 91 . An inverter  90  operates by being supplied with the reference supply voltage VSS and the low supply voltage VDD 1 , and logically inverts an input signal IN to thereby output the inverted signal as an input signal INB. The input signal IN is applied to the gate of the transistor N 91  and the input signal INB output from the inverter  90  is applied to the gate of the transistor N 92 . 
         [0005]    Inverters  91  and  92  operate by being supplied with the reference supply voltage VSS and the high supply voltage VDD 2 . The inverter  91  outputs an output signal Q 1  which is logically inverted based on a voltage of the node A. The inverter  92  outputs an output signal Q 2  which is logically inverted based on a voltage of the node B. The output signal Q 1  output from the inverter  91  serves as a normal output of the level shift circuit and the output signal Q 2  output from the inverter  92  serves as an inverted output of the level shift circuit. In this circuit, the gate of the inverter  91  coupled to the node A is used, as a resistance, for the resistance R 91  and the gate of the inverter  92  coupled to the node B is used, as a resistance, for the resistance R 92 . Thereby, it is not necessary to newly insert the resistances R 91  and R 92 , and thus the chip size of a circuit which shifts levels of many signals can be reduced. From the viewpoint of left-right symmetry of the circuit, it is preferable that the transistors N 91  and N 92  and the transistors P 91  and P 92  have the same dimensions (W/L) respectively and the resistances R 91  and R 92  have the same resistance value. 
         [0006]    An operation of the level shift circuit will be described. When a sufficient time has elapsed since the input signal IN became low level (VSS) and thus the circuit is stable, the transistor N 91  is turned off and the transistor N 92  is turned on. Therefore, the drain of the transistor N 92  (the node B) is at low level and the transistor P 91  is turned on. The transistor N 91  is turned off, and thus in a path from the high supply voltage VDD 2  to the transistor P 91 , the resistance R 91 , the transistor N 91 , and the reference supply voltage VSS in this order, no current other than an ignorable leakage current flows. Since no current flows, voltages at both ends of the resistance R 91  are the same and voltage of the node A is at high level near the high supply voltage VDD 2 . Since the node A is at high level, the transistor P 92  is turned off, and thus in a path from the high supply voltage VDD 2  to the transistor P 92 , the resistance R 92 , the transistor N 92 , and the reference supply voltage VSS in this order, no current other than an ignorable leakage current flows. Since no current flows, voltage of the node B is at low level near the reference supply voltage VSS. 
         [0007]    After that, when the input signal IN changes from low level to high level, the output of the inverter  90  quickly changes from high level to low level. If the voltage of the input signal IN is sufficiently higher than a threshold voltage Vtn of an N-channel MOS transistor, the transistor N 91  changes from off to on and the transistor N 92  changes from on to off. At this time, even if the transistor N 92  is turned off, the transistor P 92  is also in an off state, and thus the voltage of the node B does not change at low level. Therefore, the transistor P 91  still remains in an on state, and thus an initial current (a through-current) determined by on-resistance of the transistor N 91 , on-resistance of the transistor P 91 , and the resistance R 91  flows in the resistance R 91 . 
         [0008]    When the initial current flows in the resistance R 91 , a voltage drop occurs and the voltage of the node A drops by the voltage drop from the drain voltage of the transistor P 91 . When the voltage of the node A (V A ) is lower than a threshold voltage Vtp (negative value) of the transistor P 92  (V A ≦VDD 2 +Vtp), that is, when the gate-source voltage (the gate voltage) of the transistor P 92  is smaller than the threshold voltage Vtp, the transistor P 92  is turned on. At this time, the transistor N 92  is off, and thus the node B gradually becomes high level. Therefore, when the gate voltage of the transistor P 91  rises and the gate-source voltage (the gate voltage) of the transistor P 91  becomes greater than the threshold value Vtp, the transistor P 91  is turned off. Since the transistor N 91  is turned on, the node A becomes low level. When a sufficient time has elapsed in this state, the circuit is stabilized in a state opposite to the state where the input signal IN is low level. When the input signal IN changes from high level to low level, the level shift circuit operates on the same principle because the circuit is left-right symmetry. 
         [0009]    When the resistance R 91  (R 92 ) is 0Ω and the voltage drop by the initial current is 0 volt, that is, in a case of a level shift circuit which has no resistance R 91  (R 92 ) and in which transistors are directly coupled to each other, the transistor N 91  (N 92 ) requires a relatively large drive capability. That is, in a state in which the gate voltage is the voltage VDD 1  which is the maximum value of the input signal IN and the drain-source voltage Vds is a voltage near 0 volt, the transistor N 91  (N 92 ) is required to have a drive capability which can flow a current larger than a current of the transistor P 91  (P 92 ) turned on and draw out charge of the gate of the transistor P 92  (p 91 ) to turn on the transistor P 92  (P 91 ). The resistance R 91  (R 92 ) functions so as to limit the current of the transistor P 91  (P 92 ) and quickly drop the gate voltage of the transistor P 92  (P 91 ) by the voltage drop. Therefore, it is possible to quickly turn on the transistor P 92  (P 91 ). The drive capability of the transistor N 91  (N 92 ) need not be larger than necessity and the speed of the level shift circuit can be increased. 
         [0010]    Moreover, as shown in  FIG. 2 , Japanese Patent Laid-Open No. 1992-284021 (Patent Document 2) discloses an output circuit (level shift circuit) which uses diode-coupled transistors P 93  and P 94  instead of the resistances R 91  and R 92 . This document describes that an output OUT is taken out from the node B, but it is preferable to take out the output OUT by shaping waveform via an inverter in the same manner as in  FIG. 1 . The operation of the circuit is approximately the same as that of the circuit shown in  FIG. 1 . However, voltages of the node A and the node B (drain voltages of the transistors N 91  and N 92 ) drop by forward-direction voltage drops VF of the diode-coupled transistors P 93  and P 94 , and thus the transistors P 91  and P 92  can be turned on with relative ease. That is, the circuit can operate even when the maximum voltage VDD 1  of the input signal IN is further lower. 
         [0011]    In this way, the level shift circuit converts a level of an input signal supplied from a circuit operated by a low supply voltage so that the input signal can drive a circuit operated by a high supply voltage. However, a time period for a signal amplitude to change from high level to low level or from low level to high level becomes long. In the time period in which the level changes, a through-current flows in the inverter, and thus a consumption current increases and noise caused by a peak of the consumption current increases. 
       SUMMARY 
       [0012]    The present invention has been made in view of the above circumstances and provides a level shift circuit having a smaller through-current and a drive circuit of a display device including the level shift circuit. 
         [0013]    Hereinafter, means for solving the problems will be described by using reference numerals and signs used in “DETAILED DESCRIPTION”. The reference numerals and signs are added to clarify a correspondence relationship between descriptions in “What is claimed is:” and the “DETAILED DESCRIPTION”. However, the reference numerals and signs may not be used to interpret the technical scope of the invention described in the “What is claimed is:”. 
         [0014]    According to an aspect of the present invention, a level shift circuit includes a first and a second MOS transistors (N 1 /N 2 ) of a first conductivity type (N), a third and a fourth MOS transistors (P 1 /P 2 ) of a second conductivity type (P) complementary to the first conductivity type (N), a first and a second voltage generation circuits (Z 1 /Z 2 ), a fifth MOS transistor (P 4 ) of the second conductivity type (P), and a sixth MOS transistor (N 4 ) of the first conductivity type (N). An input signal (DI) having an amplitude between a third supply voltage (VDD 1 ) indicating a voltage between a first supply voltage (VSS) and a second supply voltage (VDD 2 ) and the first supply voltage (VSS) is input into the gate of the first MOS transistor (N 1 ). An inverted input signal (DIB) which is an inverted signal of the input signal (DI) is input into the gate of the second MOS transistor (N 2 ). The sources of the first and the second MOS transistors (N 1 /N 2 ) are commonly coupled to the first supply voltage (VSS). The sources of the third and the fourth MOS transistors (P 1 /P 2 ) are commonly coupled to the second supply voltage (VDD 2 ), the gate of the third MOS transistor (P 1 ) is coupled to the drain of the second MOS transistor (N 2 ), and the gate of the fourth MOS transistor (P 2 ) is coupled to the drain of the first MOS transistor (N 1 ). The first voltage generation circuit (Z 1 ) is coupled between the drain of the first MOS transistor (N 1 ) and the drain of the third MOS transistor (P 1 ), and the second-voltage generation circuit (Z 2 ) is coupled between the drain of the second MOS transistor (N 2 ) and the drain of the fourth MOS transistor (P 2 ). The gate of the fifth MOS transistor (P 4 ) is coupled to a connection node (NDB) of the drain of the third MOS transistor (P 1 ) and the first voltage generation circuit (Z 1 ), and the source of the fifth MOS transistor (P 4 ) is coupled to the second supply voltage (VDD 2 ). The gate of the sixth MOS transistor (N 4 ) is coupled to a connection node (NUB) of the drain of the first MOS transistor (N 1 ) and the first voltage generation circuit (Z 1 ). The source of the sixth MOS transistor (N 4 ) is coupled to the first supply voltage (VSS). The drain of the sixth MOS transistor (N 4 ) is coupled to the drain of the fifth MOS transistor (P 4 ). 
         [0015]    Moreover, in the level shift circuit, an output of an inverter ( 110 ) into which the input signal (DI) is input and which is operated by the first supply voltage (VSS) and the third supply voltage (VDD 1 ) may be input into the gate of the second MOS transistor (N 2 ) as the inverted input signal (DIB). 
         [0016]    Furthermore, the level shift circuit may include a seventh MOS transistor (P 3 ) of the second conductivity type (P), whose gate is coupled to a connection node (ND) of the drain of the fourth MOS transistor (P 2 ) and the second voltage generation circuit (Z 2 ) and whose source is coupled to the second supply voltage (VDD 2 ), and an eighth MOS transistor (N 3 ) of the first conductivity type (N), whose gate is coupled to a connection node (NU) of the drain of the second MOS transistor (N 2 ) and the second voltage generation circuit (Z 2 ), whose source is coupled to the first supply voltage (VSS), and whose drain is coupled to the drain of the seventh MOS transistor (P 3 ). 
         [0017]    From the viewpoint of another aspect of the present invention, a drive circuit of a display device includes a plurality of the level shift circuits. 
         [0018]    In addition, from the viewpoint of another aspect of the present invention, the level shift circuit includes a level conversion unit ( 100 ) and a buffer unit ( 200 ). The level conversion unit ( 100 ) includes the first and the second voltage generation circuits (Z 1 /Z 2 ), converts the input signals (DI/DIB) of a first signal level, which indicate a logic level by an amplitude between a reference supply voltage (VSS) and a first supply voltage (VDD 1 ) into conversion signals of a second signal level, which indicate a logic level by an amplitude between the reference supply voltage (VSS) and a second supply voltage (VDD 2 ), and outputs voltages of both ends (ND, NU/NDB, NDU) of the first and the second voltage generation circuits (Z 1 /Z 2 ) as the conversion signals, respectively. Furthermore, the level conversion unit ( 100 ) includes a first transistor (P 1 ) of the first conductivity type (P) and a second transistor (N 1 ) of the second conductivity type (N), coupled in series between the second supply voltage (VDD 2 ) and the reference supply voltage (VSS), and a third transistor (P 2 ) of the first conductivity type (P) and a fourth transistor (N 2 ) of the second conductivity type (N), coupled in series between the second supply voltage (VDD 2 ) and the reference supply voltage (VSS). The first voltage generation circuit (Z 1 ) is coupled between the drain of the first transistor (P 1 ) and the drain of the second transistor (N 1 ). The second voltage generation circuit (Z 2 ) is coupled between the drain of the third transistor (P 2 ) and the drain of the fourth transistor (N 2 ). The buffer unit ( 200 ) includes transistors (P 3 , P 4 /N 3 , N 4 ) of the first conductivity type (N) and the second conductivity type (P), where conversion signals are applied to the gates thereof, respectively. Output signals (DO/DOB) of the second signal level are output from connection nodes (DOB/DO) of the drains of the transistors (P 3 , P 4 /N 3 , N 4 ) of the first conductivity type (N) and the second conductivity type (P). 
       EFFECTS OF THE INVENTION 
       [0019]    According to the present invention, it is possible to provide a level shift circuit having a smaller through-current and a drive circuit of a display device including the level shift circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is an equivalent circuit diagram showing a configuration of a level shift circuit of a related art; 
           [0021]      FIG. 2  is an equivalent circuit diagram showing another configuration of a level shift circuit of a related art; 
           [0022]      FIG. 3  is an equivalent circuit diagram showing a configuration of a level shift circuit according to an embodiment of the present invention; 
           [0023]      FIG. 4  is an equivalent circuit diagram showing a first configuration example of the level shift circuit according to the embodiment of the present invention; 
           [0024]      FIG. 5  is a timing chart for explaining an operation of the level shift circuit according to the embodiment of the present invention; 
           [0025]      FIG. 6  is an equivalent circuit diagram showing a second configuration example of the level shift circuit according to the embodiment of the present invention; 
           [0026]      FIG. 7  is an equivalent circuit diagram showing a third configuration example of the level shift circuit according to the embodiment of the present invention; 
           [0027]      FIG. 8  is an equivalent circuit diagram showing a fourth configuration example of the level shift circuit according to the embodiment of the present invention; and 
           [0028]      FIG. 9  is a block diagram showing a configuration of a display device according to the embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    An embodiment of the present invention will be described with reference to the drawings. 
         [0030]      FIG. 3  is an equivalent circuit diagram showing a configuration of a level shift circuit according to the embodiment of the present invention. The level shift circuit includes a level conversion unit  100  and a buffer unit  200 . The level conversion unit  100  includes P-channel MOS transistors P 1  and P 2 , N-channel MOS transistors N 1  and N 2 , voltage generation circuits Z 1  and Z 2 , and an inverter  110 . The buffer unit  200  includes inverters  210  and  220 , the inverter  210  includes a P-channel MOS transistor P 3  and an N-channel MOS transistor N 3 , and the inverter  220  includes a P-channel MOS transistor P 4  and an N-channel MOS transistor N 4 . The inverters  210  and  220  are circuits that output an inverted output signal and a normal output signal of the level shift circuit, respectively. If one of the output signals is not necessary, the corresponding inverter can be omitted. 
         [0031]    The transistor P 1 , the voltage generation circuit Z 1 , and the transistor N 1  are coupled in series between a high supply voltage VDD 2  and a reference supply voltage VSS. The transistor P 2 , the voltage generation circuit Z 2 , and the transistor N 2  are coupled in series between the high supply voltage VDD 2  and the reference supply voltage VSS. The gate of the transistor P 1  is coupled to a connection node NU of the voltage generation circuit Z 2  and the drain of the transistor N 2 . The gate of the transistor P 2  is coupled to a connection node NUB of the voltage generation circuit Z 1  and the drain of the transistor N 1 . The node NU is coupled to the gate of the transistor N 3  of the inverter  210  and the node NUB is coupled to the gate of the transistor N 4  of the inverter  220 . A connection node ND of the drain of the transistor P 2  and the voltage generation circuit Z 2  is coupled to the gate of the transistor P 3  of the inverter  210 . A connection node NDB of the drain of the transistor P 1  and the voltage generation circuit Z 1  is coupled to the gate of the transistor P 4  of the inverter  220 . 
         [0032]    The inverter  110  logically inverts an input signal DI and outputs an input signal DIB. The input signal DI is applied to the gate of the transistor N 1  and the input signal DIB is applied to the gate of the transistor N 2 . A low supply voltage VDD 1  and the reference supply voltage VSS are applied to the inverter  110  and the input signals DI and DIB have an amplitude oscillating between the low supply voltage VDD 1  and the reference supply voltage VSS. Signals indicated by the voltages of the nodes NU, NUB, ND, and NDB have an amplitude oscillating between the high supply voltage VDD 2  and the reference supply voltage VSS. A set of the voltages of the nodes NU and ND drives the inverter  210  and a set of the voltages of the nodes NUB and NDB drives the inverter  220 . 
         [0033]    The transistor P 3  and the transistor N 3  are coupled in series between the high supply voltage VDD 2  and the reference supply voltage VSS and form the inverter  210 . The gate of the transistor P 3  is coupled to the node ND and the gate of the transistor N 3  is coupled to the node NU. An output signal DOB is output from a connection node of the drain of the transistor P 3  and the drain of the transistor N 3 . The output signal DOB is a signal obtained by logically inverting the input signal DI and converting the signal level. 
         [0034]    The transistor P 4  and the transistor N 4  are coupled in series between the high supply voltage VDD 2  and the reference supply voltage VSS and form the inverter  220 . The gate of the transistor P 4  is coupled to the node NDB and the gate of the transistor N 4  is coupled to the node NUB. An output signal DO is output from a connection node of the drain of the transistor P 4  and the drain of the transistor N 4 . The output signal DO is a signal obtained by maintaining the logic of the input signal DI and converting the signal level. 
         [0035]    From the viewpoint of the symmetry of the circuit, it is preferable that the transistors P 1  and P 2  and the transistors N 1  and N 2  have the same dimensions (W/L). It is also preferable that the voltage generation circuits Z 1  and Z 2  are circuits having the same circuit characteristics (or dimensions). In addition, it is desirable that the transistors P 3  and P 4  and the transistors N 3  and N 4  also have the same dimensions (W/L). 
         [0036]    As shown in  FIG. 4 , the voltage generation circuits Z 1  and Z 2  can use diode-coupled transistors P 5  and P 6  (First Example). In  FIG. 4 , each transistor is shown by a notation which clearly illustrates connection of the back gate. The transistors P 5  and P 6  are P-channel MOS transistors in the same manner as the transistors P 1  to P 4 . The back gates of the P-channel MOS transistors are coupled to the high supply voltage VDD 2 . The back gates of the transistors N 1  to N 4 , which are N-channel MOS transistors, are coupled to the reference supply voltage VSS. Here, the back gates of the transistors P 5  and P 6  are coupled to the high supply voltage VDD 2 , but may be coupled to their own sources. 
         [0037]    An operation of the level shift circuit shown in  FIG. 4  will be described with reference to  FIG. 5 . In this level shift circuit, the high supply voltage VDD 2  is applied to the back gates and the diode-coupled transistors P 5  and P 6  are used as the voltage generation circuits Z 1  and Z 2 . 
         [0038]    In a state in which a sufficient time has elapsed since the input signal DI indicated a low level (VSS) and thus the circuit is stable, the transistor N 1  is in an off state because the input signal DI (low level) is applied to the gate. The transistor N 2  is in an on state because the input signal DIB (high level) which is logically inverted by the inverter  110 , is applied to the gate. The transistor N 2  is in the on state, and thus the voltage of the node NU is low level ( FIG. 5  NU) and the transistor P 1  is in the on state. Although the transistor P 1  is in the on state, the transistor N 1  is in the off state, and thus no current flows in the transistor P 1  and the voltage of the node NDB becomes a high level near the high supply voltage VDD 2  ( FIG. 5  NDB). 
         [0039]    The diode-coupled transistor P 5  has a diode forward-direction voltage drop (forward voltage) VF having a positive value from the drain coupled to the node NUB to the source coupled to the node NDB. Therefore, the voltage of the node NUB is a voltage (VDD 2 −VF) which is lower than the voltage (≈VDD 2 ) of the node NDB by VF ( FIG. 5  NUB). At this time, the back gate of the transistor P 5  is coupled to the high supply voltage VDD 2  and has the same voltage as the voltage of the source VDD 2 ), and thus no back bias effect is generated in the transistor P 5 . Moreover, the forward-direction voltage drop VF of the diode-coupled transistor P 5  is the same as an absolute value of a threshold voltage Vtp. The absolute value of the threshold voltage Vtp of the transistor P 5 , that is, the forward-direction voltage drop VF, is sufficiently smaller than a voltage difference between the reference supply voltage VSS and the high supply voltage VDD 2 . The gate of the transistor P 2  is coupled to the node NUB (VDD 2 −VF), and thus the gate-source voltage Vgs (−VF) is lower than or equal to the threshold voltage Vtp and the transistor P 2  is in the off state. The transistor P 2  is in the off state and the transistor N 2  is in the on state, and thus the voltage of the node NU is low level (≈VSS) ( FIG. 5  NU). 
         [0040]    The diode-coupled transistor P 6  has a diode forward-direction voltage drop (forward voltage) VF having a positive value from the drain to the source. Therefore, the voltage of the node ND is a voltage (VSS+VF) which is higher than the voltage (≈VSS) of the node NU by VF ( FIG. 5  ND). The back gate of the transistor P 6  is coupled to the high supply voltage VDD 2 . At this time, the source has a voltage that is VF higher than the reference supply voltage VSS, that is, a voltage lower than the high supply voltage VDD 2 , and thus, in the transistor P 6 , an absolute value of the threshold voltage Vtp (the forward-direction voltage drop VF as a diode) increases by the back bias effect and the forward-direction voltage drop VF of the diode-coupled transistor P 6  becomes equal to the absolute value of the threshold voltage Vtp with the back bias effect. Also at this time, the absolute value of the threshold voltage Vtp of the transistor P 6 , that is, the forward-direction voltage drop VF, is a value sufficiently smaller than the voltage difference between the reference supply voltage VSS and the high supply voltage VDD 2 . Therefore, the voltage of the node ND is a low level indicating the forward-direction voltage drop VF with the back bias effect ( FIG. 5  ND). 
         [0041]    In such a stable condition, when the input signal DI changes from the low level (VSS) to the high level (VDD 1 ), the input signal DIB which is logically inverted via the inverter  110 , changes to the low level (VSS) immediately after the change of the input signal DI ( FIG. 5 ). The voltage that indicates the high level of the input signal DI is sufficiently higher than the threshold voltage Vtn of the transistor N 1 , and thus the transistor N 1  changes to the on state. Moreover, the voltage that indicates the low level of the input signal DIB is lower than the threshold voltage Vtn of the transistor N 2 , and thus the transistor N 2  changes to the off state. Immediately after the transistor N 2  is turned off, the transistor P 2  is also in the off state, and thus the voltages of the nodes NU and ND are maintained at the low level. Therefore, the transistor P 1  is still in the on state, and a through-current Ii 1  flows from the high supply voltage VDD 2  to the reference supply voltage VSS via the diode-coupled transistor P 5 . The initial current value of the through-current is determined on the basis of on-resistances of the transistors P 1 , P 5 , and N 1 . 
         [0042]    The voltage of the node NUB is determined by ratios of the on-resistances of the transistors P 1 , P 5 , and N 1 . When the voltage of the node NUB (V NUB ) becomes lower than a voltage obtained by adding the threshold voltage Vtp (negative value) to the high supply voltage VDD 2 , which is the source voltage of the transistor P 2 , (V NUB &lt;VDD 2 +Vtp), the transistor P 2  becomes on state. At this time, the transistor N 2  is in the off state, and thus the voltage of the node ND becomes high level (≈VDD 2 ). Therefore, the voltage of the node NU becomes high level at a voltage lower than the voltage of the node ND by the forward-direction voltage drop (forward voltage) VF of the transistor P 6 . The source of the transistor P 1  is coupled to the high supply voltage VDD 2  and the gate is coupled to the node NU, and thus when the voltage of the node NU (V NU ) becomes higher than a voltage obtained by adding the threshold voltage Vtp (negative value) of the transistor P 1  to the high supply voltage VDD 2  (V NU ≧VDD 2 +Vtp), the transistor P 1  becomes off state. When the transistor P 1  becomes off state, the voltages of the nodes NDB and NUB become low level with a difference corresponding to the forward-direction voltage drop (forward voltage) VF of the transistor P 5 . Therefore, the voltage of the node NUB becomes approximately equal to the reference supply voltage VSS and the voltage of the node NDB becomes approximately a voltage of the forward-direction voltage drop VF of the transistor P 5  ( FIG. 5  NDB, NUB). At this time, by the back bias effect of the transistor P 5 , the forward-direction voltage drop VF becomes larger than that when there is no back bias effect. When a sufficient time has elapsed, the circuit is stabilized in a state opposite to the state of when the input signal DI is low level. Also when the input signal DI changes from high level to low level, the level shift circuit operates in the same principle because the circuit is symmetrical. 
         [0043]    The transistors P 3 , N 3 , P 4 , and N 4  in the buffer unit  200  operate responding to the changes of the voltages of the nodes ND, NU, NDB, and NUB. The source of the diode-coupled transistor P 5  (corresponding to an anode of a diode) is the node NDB and the drain (corresponding to a cathode of a diode) is the node NUB. The source of the diode-coupled transistor P 6  (anode) is the node ND and the drain (cathode) is the node NU. The diode-coupled transistors P 5  and P 6  are biased in the forward direction at all times. In this state, there is a voltage difference corresponding to the forward-direction voltage drop VF between the nodes ND and NU and between the nodes NDB and NUB. The forward-direction voltage drop VF of the diode-coupled P-channel MOS transistors is approximately equal to the absolute value of the threshold voltage Vtp. The back gate is coupled to the high supply voltage VDD 2 , and thus the back bias effect affects the value of the Vtp, that is, the forward-direction voltage drop VF. Specifically, as shown in  FIG. 5 , the forward-direction voltage drop VF when the voltages of the nodes ND and NDB are low becomes larger than that when the voltages of the nodes ND and NDB are high. This is because the nodes ND and NDB are the sources of the transistors P 6  and P 5  whose back gates are coupled to the high supply voltage VDD 2 , and thus when the voltages of the nodes ND and NDB become lower than the high supply voltage VDD 2 , the threshold voltage Vtp (the forward-direction voltage drop VF as a diode) increases by the back bias effect. 
         [0044]    In the inverter  210  including the transistor P 3  and the transistor N 3 , there is a voltage difference corresponding to the forward-direction voltage drop VF between the input voltages applied to the gates of the transistor P 3  and the transistor N 3 . The gate voltages change while there is a voltage difference between them, and thus one transistor is turned off earlier than the other transistor. Since the one transistor is turned off earlier, a time period in which both transistors are in the on state at the same time decreases and the through-current decreases. Furthermore, the back gate of the diode-coupled P-channel MOS transistor P 6  is coupled to the high supply voltage VDD 2 , and thus when the voltage of the output node ND of the level conversion unit  100  is lower than the high supply voltage VDD 2 , the source voltage of the diode-coupled transistor P 6  decreases and the back bias effect increases. Thereby, the absolute value of the threshold voltage Vtp, that is, the value of the forward-direction voltage drop VF, increases, and thus the through-current can be further reduced. This also applies to the inverter  220  which includes the transistor P 4  and the transistor N 4 . 
         [0045]    The diode-coupled transistor P 5  limits a drive current of the transistor P 1  and functions so as to quickly lower the gate voltage of the transistor P 2  by the forward-direction voltage drop VF. Therefore, it is possible to turn on the transistor P 2  more quickly. Thus, the drive capability of the N-channel MOS transistor is not required to be large beyond necessity and the speed of the level shift circuit can be increased. 
         [0046]    In other words, the voltage of the node NUB coupled to the gate of the P-channel MOS transistor P 2  decreases by the forward-direction voltage drop VF of the diode-coupled P-channel MOS transistor P 5 . Therefore, the P-channel MOS transistor P 2  can be relatively easily turned on and even when the input voltage VDD 1  is low, that is, even when the drive current of the N-channel MOS transistor N 1  is small, a stable operation can be performed. Then, the low level voltages applied to the gates of the P-channel MOS transistors P 3  and P 4  are the voltage VF including a back bias effect higher than that of the reference supply voltage VSS, and thus it is possible to reduce the consumption current caused by the through-current. Moreover, the diode-coupled P-channel MOS transistor P 5  does not require a large drive capability, and thus it is possible to use a transistor having a small channel width W. As described above, regarding the level shift circuit according to the embodiment, the layout area can be reduced and the input voltage can be lowered while the value of the through-current is reduced. 
         [0047]    In the above description, the diode-coupled P-channel MOS transistors P 5  and P 6  are exemplified as the voltage generation circuits Z 1  and Z 2 . However, as shown in  FIG. 6 , the diode-coupled N-channel MOS transistors N 5  and N 6  may be the voltage generation circuits Z 1  and Z 2  (Second Example). The drain and the gate of the transistor N 5  are coupled to each other. The drain and the gate of the transistor N 6  are coupled to each other. The back gates of the transistors N 5  and N 6  are coupled to the reference supply voltage VSS. The operation of this level shift circuit is the same as that of the first configuration example, but in this case, the back bias effect appears in a high level voltage on the side of the high supply voltage VDD 2 . The effect is the same as that of the P-channel MOS transistors. 
         [0048]    Moreover, as shown in  FIG. 7 , the diodes D 1  and D 2  may be used as the voltage generation circuits Z 1  and Z 2  (Third Example). The diode D 1  such as a PN junction is coupled between the transistor P 1  and the transistor N 1 , and the diode D 2  such as a PN junction is coupled between the transistor P 2  and the transistor N 2 . 
         [0049]    It is known that the forward-direction voltage drop VF of the PN junction is about 0.7 V when the material of the semiconductor is silicon, about 0.5 V when the material is germanium, and about 0.2 V when the PN junction is a Schottky barrier diode in which one side is metal. When a plurality of diodes is coupled in series as the voltage generation circuits Z 1  and Z 2 , the forward-direction voltage drop VF_A of the voltage generation circuits Z 1  and Z 2  is calculated as VF_A=VF×the number of diodes. When the value of the forward-direction voltage drop VF_A is greater than the absolute value of the threshold voltage Vtp, the speed of the level shift circuit increases, but the P-channel MOS transistors P 1  and P 2  are not turned off completely. Therefore, a path in which current flows constantly is formed in the level shift circuit, and thus the power consumption increases. When the forward-direction voltage drop VF_A is excessively smaller than the threshold voltage Vtp, the P-channel MOS transistors P 1  and P 2  are completely turned off, and thus the power consumption at that time can be ignorable, but the effect of the present invention decreases. Therefore, it is preferable that the value of the forward-direction voltage drop VF_A is about the absolute value of the threshold voltage Vtp of the P-channel MOS transistors P 1  and P 2 . 
         [0050]    Moreover, as shown in  FIG. 8 , the resistances R 1  and R 2  may be used as the voltage generation circuits Z 1  and Z 2  (Fourth Example). When the resistances R 1  and R 2  are used, the voltage drop by each resistance depends on a current flowing in the resistance, and thus it is necessary to increase the resistance value in order to ensure a voltage in an area where the current value is small. However, in a state in which a level change of a signal is completed and the signal is stabilized, no current flows in the resistances R 1  and R 2 , and thus the voltages of the node ND and the node NU are the same and the voltages of the node NDB and the node NUB are the same. When the signal level is high, the voltage is approximately the same as the high supply voltage VDD 2 , and when the signal level is low, the voltage is approximately the same as the reference supply voltage VSS, and thus it is possible to reliably turn on and off the transistors. 
         [0051]      FIG. 9  is a block diagram showing a configuration of a display device according to the embodiment of the present invention. A display device  300  includes a controller  310 , a data driver unit  320 , a gate driver unit  330 , and a display panel  340 . The display panel  340  is a liquid crystal display panel on which pixels are arranged in a matrix, and displays a display image. The controller  310  supplies a control signal and a data signal to the data driver unit  320  and the gate driver unit  330  on the basis of a signal which is supplied from the outside and which represents a display image and a control signal of the signal representing the display image. A clock signal CLK indicating a timing, a data strobe signal DSTB, and a display data signal DATA are supplied to the data driver unit  320 . A gate clock signal GCLK and a gate strobe signal GSTB are supplied to the gate driver unit  330 . 
         [0052]    The data driver unit  320  outputs data line signals D&lt; 1 &gt; to D&lt;X&gt; to pixel lines in a column direction on the display panel  340  on the basis of the display data signal DATA, the clock signal CLK, and the data strobe signal DSTB. A plurality of drive circuits  329  that drive the data line signals D&lt; 1 &gt; to D&lt;X&gt; is voltage-follower-coupled differential amplifier circuits. The gate driver unit  330  outputs gate line signals G&lt; 1 &gt; to G&lt;Y&gt; to pixel lines in a line direction on the display panel  340  on the basis of the gate clock signal GCLK and the gate strobe signal GSTB. In the data driver unit  320 , the level shift circuits  327  described above, for generating the data line signals D&lt; 1 &gt; to D&lt;X&gt; corresponding to a high voltage signal (VDD 2 ), which is a level of the data signal, from DATA of a low voltage signal (VDD 1 ) and the number of which is a number obtained by multiplying the number of data lines (X) by a bit width (N) of the DATA, are incorporated; and all the level shift circuits  327  operate at approximately the same time at the timing of line display. Outputs of the level shift circuits  327  are input into digital-analog conversion circuits (DACs)  328  and output analog signals of the DACs  328  are amplified by the drive circuits  329  and output to data lines (not shown) of the display panel  340  as the data line signals D&lt; 1 &gt; to D&lt;X&gt;. 
         [0053]    For example, when the display panel  340  is a full high vision vertical stripe color panel of three primary colors (FHD: 1,920 columns×1,080 lines), the number of outputs (X) of the data driver unit  320  becomes 5,760 (=1,920×three primary colors). At this time, when the bit width of the DATA signal is 8 bits (full color: about 16,770,000 colors), the number of level shifters mounted on the data driver unit  320  becomes 46,080. On the display device, when there is displayed a pattern in which black (00000000) lines and white (11111111) lines of the DATA for each line are alternately repeated, such as a white/black column checkered pattern and a white/black horizontal lines display, 46,080 level shifters  327  operate at approximately the same time for each line. Furthermore, the above operation is repeated for 1,080 lines in one frame, and one frame is displayed 60 times per second (normal speed display) to 240 times per second (quad-speed display or 3D of double speed display). This means that 46,080 level shift circuits  327  change value 25,920 times per second, that is, the through-current flows 25,920 times per second. As described above, the level shift circuits of the present invention can significantly reduce the consumption current and the noise caused by a current peak in the entire display device even though each level shift circuit reduces a small amount of through-current. 
         [0054]    Although the present invention has been described with reference to the embodiment, Examples of the embodiment can be combined and implemented as long as no contradiction occurs. Furthermore, the present invention is not limited to the embodiment described above, and various modifications that can be understood by those skilled in the art can be made in the configurations and details of the present invention within the scope of the present invention.