Abstract:
In a two-stage inverter circuit including an inverter circuit constituted by first and second FETs and an inverter circuit constituted by two FETs, a source and a gate of a third FET are connected to a first power source and a second power source, respectively. A drain of the third FET is connected to a source of the first FET. A source and a gate of a fourth FET are connected to the first power source and the second power source, respectively. A drain of the fourth FET is connected to a source of a seventh FET. A gate of the seventh FET is connected to the second power source, and a drain of the seventh FET is connected to a back gates of the first, third, fourth, seventh and fifth FETs. The drain of the third FET is connected to the drain of the fourth FET.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This Nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2005-15768 filed in Japan on Jan. 24, 2005, the entire contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a buffer circuit equipped with power sources having different voltages and including CMOS inverter circuits as logical gate circuits, or to an integrated circuit equipped with such a buffer circuit.  
         [0004]     2. Description of Related Art  
         [0005]     Since portable apparatuses, such as portable telephones and notebook-sized personal computers, are generally powered by batteries, such apparatuses are requested to have low power consumption so as to withstand extended use. Hence, each of integrated circuits being used in these apparatuses is divided into blocks for each function. Depending on the usage condition of such an apparatus, power is supplied only to a functional block required for the operation of the apparatus at the time, whereby power saving is attained.  
         [0006]     Such an integrated circuit is configured to use power sources having voltages different depending on the function of the integrated circuit to lower the power voltages to the utmost extent and to reduce power consumption. Hence, the integrated circuit is equipped with a buffer circuit for signal interface between circuits that uses power sources having different voltages.  
         [0007]      FIG. 1A  is a circuit diagram showing the configuration of a conventional buffer circuit (designated by numeral  200 ). For example, an output signal from a functional block (not shown), operating on a drive voltage VDD 1 , for attaining a predetermined function inside an integrated circuit  100  is inputted to the buffer circuit  200 . On the other hand, the buffer circuit  200  outputs an output signal to another functional block (not shown) operating on a drive voltage VDD 2  lower than VDD 1 . In this case, the buffer circuit  200  has a voltage step-down circuit for stepping down the drive voltage of the integrated circuit  100  from the high voltage VDD 1  to the low voltage VDD 2 . In this kind of buffer circuit  200 , the output of an inverter circuit  210  connected to a power source VD 1  having the high voltage VDD 1  is connected to the input of an inverter circuit  220  connected to a power source VD 2  having the low voltage VDD 2 .  
         [0008]      FIG. 1B  is a circuit diagram showing the specific circuit configurations of the two inverter circuits  210  and  220  shown in  FIG. 1A . The inverter circuit  210  is composed of a P-channel FET  211  whose source is connected to the power source VD 1 , and an N-channel FET  212  whose source is connected to a drain of the FET  211 . A drain of the FET  212  is grounded. Gates of the two FETs  211  and  212  are connected to each other and serve as the input terminal of the inverter circuit  210 . Furthermore, a connection node of the drain of the FET  211  and the source of the FET  212  serves as the output terminal of the inverter circuit  210 .  
         [0009]     Similarly, the inverter circuit  220  is composed of a P-channel FET  221  whose source is connected to the power source VD 2 , and an N-channel FET  222  whose source is connected to a drain of the FET  221 . The drain of the FET  222  is grounded. Gates of the two FETs  221  and  222  are connected to each other and serves as the input terminal of the inverter circuit  220 . Furthermore, a connection node of the drain of the FET  221  and the source of the FET  222  serves as the output terminal of the inverter circuit  220 .  
         [0010]     In the integrated circuit  100  equipped with such buffer circuit  200 , the power sources VD 1  and VD 2  are repeatedly turned ON and OFF depending on the operation state of the integrated circuit  100 , for example, in the case that the power source VD 1  is turned OFF to stop the operation of the functional block operating on the power source VD 1  or in the case that the power source VD 2  is turned OFF to stop the operation of the functional block operating on the power source VD 2 . This reduces the power consumption of the circuit.  
         [0011]     Furthermore, Japanese Patent Application Laid-open No. 2000-341110 has proposed a CMOS transistor circuit in which an output of a first inverter circuit powered by a first power source is connected to an input of a second inverter circuit powered by a second power source, characterized in that an output of a third inverter circuit to which the first power source is inputted and which is powered by the second power source is connected to a gate of an N-channel FET, a drain of the N-channel FET is connected to the output of the first inverter circuit, and a source of the N-channel FET is grounded. In this kind of conventional technology, even in the case that the voltage of the first power source is lowered to the ground level, through current flows the second inverter circuit is prevented, whereby power consumption can be reduced.  
         [0012]     However, since the power sources are turned ON or OFF depending on the operation state of the functional block inside the integrated circuit in the above-mentioned conventional technology, an unnecessary current is generated in the CMOS inverter circuit when the power sources are turned ON or OFF, thereby causing a problem of increasing power consumption.  
         [0013]      FIG. 2A ,  FIG. 2B  and  FIG. 2C  are time charts showing operation states of the conventional buffer circuit  200  at the time of power-ON.  FIG. 2A  shows the changes in the voltages of the two power sources VD 1  and VD 2 ,  FIG. 2B  shows the ON/OFF states of the two FETs  221  and  222  of the inverter circuit  220 , and  FIG. 2C  shows the state of through current flowing the two FETs  221  and  222 .  
         [0014]     In the case that the power source VD 1  is turned ON while the power source VD 2  (voltage VDD 2 ) is supplied to the buffer circuit  200  shown in  FIG. 1 , there is a period in which the voltage of the power source VD 1  is lower than the voltage of the power source VD 2 . For example, when the voltage at the input terminal of the inverter circuit  210  is 0V, the FET  211  turns ON, whereby the voltage Va at the input terminal of the inverter circuit  220  becomes equal to the voltage of the power source VD 1  and rises as the voltage of the power source VD 1  rises.  
         [0015]     At time ta, the voltage of the power source VD 1  reaches a threshold voltage Vth of the FET  222 , and the FET  222  turns ON. Next, at time tb, the voltage of the power source VD 1  reaches a voltage obtained by subtracting the threshold voltage Vth of the FET  221  from the voltage VDD 2 , and the FET  221  turns OFF. Hence, the two FETs  221  and  222  are in the ON state in the period between time ta and time tb, whereby an unnecessary through current flows the two FETs  221  and  222 , and the power consumption increases.  
         [0016]      FIG. 3A ,  FIG. 3B  and  FIG. 3C  are time charts showing other operation states of the conventional buffer circuit  200  at the time of power-ON.  FIG. 3A  shows the changes in the voltages of the two power sources VD 1  and VD 2 ,  FIG. 3B  shows the ON/OFF states of the two FETs  221  and  222  of the inverter circuit  220 , and  FIG. 3C  shows the state of through current flowing the two FETs  221  and  222 .  
         [0017]     In the case that the power source VD 1  (voltage VDD 1 ) and the power source VD 2  (voltage VDD 2 ) are turned ON simultaneously and the voltages are supplied to the buffer circuit  200  shown in  FIG. 1 , since the rising of the voltage of the power source VD 2  is faster than the rising of the voltage of the power source VD 1 , there is a period in which the voltage of the power source VD 1  is lower than the voltage of the power source VD 2 . For example, when the voltage at the input terminal of the inverter circuit  210  is 0V, the FET  211  turns ON, whereby the voltage Va at the input terminal of the inverter circuit  220  becomes equal to the voltage of the power source VD 1  and rises as the voltage of the power source VD 1  rises.  
         [0018]     At time tc, the voltage of the power source VD 1  reaches the threshold voltage Vth of the FET  222 , and the FET  222  turns ON. Next, at time td, the voltage of the power source VD 1  reaches the voltage obtained by subtracting the threshold voltage Vth of the FET  221  from the voltage VDD 2 , and the FET  221  turns OFF. Hence, both the two FETs  221  and  222  are in the ON state in the period between time tc and time td, whereby an unnecessary through current flows the two FETs  221  and  222 , and the power consumption increases.  
         [0019]     Furthermore, in the CMOS transistor circuit disclosed in the above-mentioned Japanese Patent Application Laid-open No. 2000-341110, in the case that the voltage of the first power source is lowered to the ground level, the second power source connected to the third inverter circuit is grounded via the third inverter circuit and the N-channel FET both being in the ON state, whereby an extra current may flow when the first power source is turned ON or OFF.  
       BRIEF SUMMARY OF THE INVENTION  
       [0020]     In consideration of the above-mentioned circumstances, it is an object of the present invention to provide a buffer circuit capable of reducing power consumption, wherein when a first power source or a second power source is turned ON or OFF, unnecessary current flow is prevented by switching the power source for a first logical gate circuit to the first power source or the second power source depending on high or low of the voltage of the first power source and the voltage of the second power source. It is another object of the present invention to provide a buffer circuit capable of improving operation speed by providing a P-channel FET and an N-channel FET whose sources, drains and gates are respectively connected to each other. It is still another object of the present invention to provide a buffer circuit capable of stabilizing back gate voltages and to provide an integrated circuit equipped with such a buffer circuit.  
         [0021]     A buffer circuit according to a first invention is a buffer circuit comprising: a first power source; a second power source different from the first power source; and a first logical gate circuit including CMOS inverter circuit and a second logical gate circuit; wherein the first logical gate circuit is powered by the first power source, and the second logical gate circuit is powered by the second power source and whose input terminal is connected to an output terminal of the first logical gate circuit; characterized by further comprising a power source switching circuit for switching the power source for the first logical gate circuit to the second power source when the voltage of the first power source is lower than the voltage of the second power source, and to the first power source when the voltage of the first power source is higher than the voltage of the second power source.  
         [0022]     A buffer circuit according to a second invention is a buffer circuit in accordance with the first invention, characterized in that the power source switching circuit comprises a first switching element for supplying the voltage of the first power source to the first logical gate circuit and a second switching element for supplying the voltage of the second power source to the first logical gate circuit, and turns ON or OFF the first switching element or the second switching element depending on high or low of the voltage of the first power source and the second power source.  
         [0023]     A buffer circuit according to a third invention is a buffer circuit in accordance with the second invention, characterized in that the CMOS inverter circuit including a first FET of a P-channel FET, and a second FET of an N-channel FET whose source and gate are connected to a drain and a gate of the first FET, respectively; the first switching element is a third FET of a P-channel FET whose source is connected to the first power source and whose gate is connected to the second power source; the second switching element is a fourth FET of a P-channel FET whose source is connected to the second power source, whose gate is connected to the first power source and whose drain is connected to a drain of the third FET; and the drain of the fourth switching element is connected to a source of the first FET of the first logical gate circuit.  
         [0024]     A buffer circuit according to a fourth invention is a buffer circuit in accordance with the second invention, characterized in that the CMOS inverter circuit including a first FET of a P-channel FET, and a second FET of an N-channel FET whose source and gate are connected to a drain and a gate of the first FET, respectively; the first switching element is a third FET of a P-channel FET whose source is connected to the first power source and whose gate is connected to the second power source; and the second switching element is a fourth FET of a P-channel FET whose source is connected to the second power source and whose drain is connected to the source of the first FET of the first logical gate circuit and a drain of the third FET; the buffer circuit further comprising a fifth FET of a P-channel FET and a sixth FET of an N-channel FET whose respective sources are connected to each other and also connected to the first power source, whose respective drains are connected to each other and also connected to a gate of the fourth FET and whose respective gates are connected to each other and also connected to the second power source.  
         [0025]     A buffer circuit according to a fifth invention is a buffer circuit in accordance with the third or fourth invention, characterized by further comprising a seventh FET of a P-channel FET whose source is connected to the respective drains of the third FET and the fourth FET and whose gate is connected to the second power source, wherein a drain and a back gate of the seventh FET are connected to back gates of the first, third, fourth and fifth FETs.  
         [0026]     A buffer circuit according to a sixth invention is a buffer circuit in accordance with the third or fourth invention, characterized by further comprising: a seventh FET of a P-channel FET whose source is connected to the respective drains of the third FET and the fourth FET; and an eighth FET of an N-channel FET whose gate and drain are connected to the second power source and whose source is connected to a gate of the seventh FET; wherein a drain and a back gate of the seventh FET are connected to back gates of the first, third, fourth and fifth FETs.  
         [0027]     An integrated circuit according to a seventh invention comprises a functional block powered by a first power source and a functional block powered by a second power source different from the first power source, characterized by further comprising the buffer circuit in accordance with any one of the first invention through the sixth invention.  
         [0028]     In the buffer circuit according to the first invention, in the case that the voltage of the first power source becomes lower than the voltage of the second power source, the power source switching circuit switches the power source for the first logical gate circuit to the second power source. In the case that the voltage at the input terminal of the first logical gate circuit including the CMOS inverter circuit is 0V, the voltage at the output terminal of the first logical gate circuit becomes equal to the voltage of the second power source. Since the input terminal of the second logical gate circuit is connected to the output terminal of the first logical gate circuit, the voltage at the input terminal of the second logical gate circuit becomes equal to the voltage of the second power source. Hence, no through current is generated in the second logical gate circuit.  
         [0029]     In the case that, the voltage of the first power source becomes higher than the voltage of the second power source, the power source switching circuit switches the power source for the first logical gate circuit to the first power source. In the case that the voltage at the input terminal of the first logical gate circuit is 0V, the voltage at the output terminal of the first logical gate circuit becomes equal to the voltage of the first power source. Since the input terminal of the second logical gate circuit is connected to the output terminal of the first logical gate circuit, the voltage at the input terminal of the second logical gate circuit becomes equal to the voltage of the first power source. Since the voltage of the first power source is higher than the voltage of the second power source, no through current is generated in the second logical gate circuit.  
         [0030]     In the buffer circuit according to the second invention, in the case that the voltage of the first power source becomes lower than the voltage of the second power source, the first switching element turns OFF, and the second switching element turns ON. Hence, the power source for the first logical gate circuit is switched to the second power source. On the other hand, in the case that the voltage of the first power source becomes higher than the voltage of the second power source, the first switching element turns ON, and the second switching element turns OFF. Hence, the power source for the first logical gate circuit is switched to the first power source.  
         [0031]     In the buffer circuit according to the third invention, in the case that the voltage of the first power source becomes lower than the voltage of the second power source, the third FET turns OFF, and the fourth FET turns ON. The second power source becomes the power source for the first logical gate circuit via the source and drain of the fourth FET having been turned ON. In the case that the voltage at the input terminal of the first logical gate circuit is 0V, the first FET of a P-channel FET turns ON, and the voltage at the output terminal of the first logical gate circuit becomes equal to the voltage of the second power source. Since the input terminal of the second logical gate circuit is connected to the output terminal of the first logical gate circuit, the voltage at the input terminal of the second logical gate circuit becomes equal to the voltage of the second power source. Hence, the gate voltage of the P-channel FET of the inverter circuit included in the second logical gate circuit becomes equal to the source voltage thereof, and the FET turns OFF, whereby no through current is generated in the second logical gate circuit.  
         [0032]     In the case that the voltage of the first power source is higher than the voltage of the second power source, the third FET turns ON, and the fourth FET turns OFF. The first power source becomes the power source for the first logical gate circuit via the source and drain of the third FET having been turned ON. In the case that the voltage at the input terminal of the first logical gate circuit is 0V, the first FET of a P-channel FET turns ON, and the voltage at the output terminal of the first logical gate circuit becomes equal to the voltage of the first power source. Since the input terminal of the second logical gate circuit is connected to the output terminal of the first logical gate circuit, the voltage at the input terminal of the second logical gate circuit becomes equal to the voltage of the first power source. Hence, since the voltage of the first power source is higher than the voltage of the second power source, the gate voltage of the P-channel FET of the inverter circuit included in the second logical gate circuit becomes higher than the source voltage thereof, and the FET turns OFF, whereby no through current is generated in the second logical gate circuit.  
         [0033]     In the buffer circuit according to the fourth invention, in the case that the voltage of the first power source becomes lower than the voltage of the second power source, the fifth FET of a P-channel FET whose gate is connected to the second power source and whose source is connected to the first power source turns OFF. On the other hand, the sixth FET of an N-channel FET whose gate is connected to the second power source and whose source is connected to the first power source is saturated in a range wherein the voltage of the first power source is higher than the voltage obtained by subtracting the threshold voltage of the sixth FET from the voltage of the second power source. Hence, the gate voltage of the fourth FET is the voltage obtained by subtracting the threshold voltage of the sixth FET from the voltage of the second power source. Since the voltage lower than the voltage of the second power source is applied to the gate, the ON operation of the fourth FET is speeded up.  
         [0034]     In the case that the voltage of the first power source is higher than the voltage of the second power source, the fifth FET of a P-channel FET whose gate is connected to the second power source and whose source is connected to the first power source turns ON. Hence, the gate voltage of the fourth FET becomes equal to the voltage of the first power source.  
         [0035]     In the buffer circuit according to the fifth invention, in the case that the voltage of the first power source becomes lower than the voltage of the second power source, the third FET turns OFF, and the fourth FET turns ON. Since the gate voltage and the source voltage of the seventh FET become equal to the voltage of the second power source, the back gate voltages of the first, third, fourth, fifth and seventh FETs become almost equal to the voltage of the second power source by virtue of the PN forward bias from the source of the seventh FET.  
         [0036]     On the other hand, in the case that the voltage of the first power source is higher than the voltage of the second power source, the third FET turns ON, and the fourth FET turns OFF. In addition, since the gate voltage of the seventh FET becomes lower than the source voltage thereof, the seventh FET turns ON. Hence, the drain voltage of the seventh FET becomes equal to the voltage of the first power source. Since the drain of the seventh FET is connected to the back gates of the first, third, fourth, fifth and seventh FETs, the back gate voltages of the first, third, fourth, fifth and seventh FETs become equal to the voltage of the first power source.  
         [0037]     In the buffer circuit according to the sixth invention, in the case that the voltage of the first power source becomes lower than the voltage of the second power source, the fifth FET of a P-channel FET whose gate is connected to the second power source and whose source is connected to the first power source turns OFF. On the other hand, the sixth FET of an N-channel FET whose gate is connected to the second power source and whose source is connected to the first power source turns ON, the third FET turns OFF, and the fourth FET turns ON. The eighth FET whose gate and drain are connected to the second power source is saturated. Hence, the gate voltage of the seventh FET becomes the voltage obtained by subtracting the threshold voltage of the eighth FET from the voltage of the second power source. Since the voltage lower than the voltage of the second power source is applied to the gate of the seventh FET, the ON operation of the seventh FET is speeded up.  
         [0038]     In the integrated circuit according to the seventh invention, the output signal of the functional block powered by the first power source is used as the input signal of the functional block powered by the second power source via such a buffer circuit as described above.  
         [0039]     The first, second and seventh invention comprise the power source switching circuit for switching the power source for the first logical gate circuit to either the first power source or the second power source in accordance with the voltages of the first and second power sources. Hence, even in the case that high or low of the voltage of the first power source and the voltage of the second power source is changed when the first power source or the second power source is turned ON or OFF, unnecessary through current flow is prevented, and power consumption can be reduced. Furthermore, even in the case that the first power source is turned OFF, the second power source is supplied to the first logical gate circuit. Hence, the function of the first logical gate circuit does not stop, and a signal inputted to the first logical gate circuit is outputted to the second logical gate circuit.  
         [0040]     The third invention comprises the third FET whose source is connected to the first power source and whose gate is connected to the second power source, and the fourth FET whose source is connected to the second power source and whose gate is connected to the first power source. Hence, even in the case that high or low of the voltage of the first power source and the voltage of the second power source is changed when the first power source or the second power source is turned ON or OFF, unnecessary through current flow is prevented, and power consumption can be reduced.  
         [0041]     The fourth invention comprises the third FET whose source is connected to the first power source and whose gate is connected to the second power source, the fourth FET whose source is connected to the second power source, and the fifth FET of a P-channel FET and the sixth FET of an N-channel FET whose sources, drains and gates are respectively connected to each other. Hence, the ON operation of the fourth FET can be speeded up.  
         [0042]     The fifth invention comprises the seventh FET whose gate is connected to the second power source and whose source is connected to the respective drains of the third FET and the fourth FET. Hence, even in the case that switching is carried out between the first power source and the second power source, the back gate voltages of the first, third, fourth, fifth and seventh FETs can be stabilized.  
         [0043]     The sixth invention comprises the eighth FET of an N-channel FET whose gate and drain are connected to the second power source, and the seventh FET whose gate is connected to the source of the eighth FET. Hence, the ON operation of the seventh FET can be speeded up.  
         [0044]     The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0045]      FIG. 1A  is a circuit diagram showing the configuration of the conventional buffer circuit, and  FIG. 1B  is a circuit diagram showing the specific circuit configurations of the two inverter circuits shown in  FIG. 1A ;  
         [0046]      FIG. 2A ,  FIG. 2B  and  FIG. 2C  are time charts showing operation states of the conventional buffer circuit at the time of power-ON;  
         [0047]      FIG. 3A ,  FIG. 3B  and  FIG. 3C  are time charts showing other operation states of the conventional buffer circuit at the time of power-ON;  
         [0048]      FIG. 4  is a block diagram showing the configuration of a buffer circuit according to the present invention;  
         [0049]      FIG. 5  is a circuit diagram showing the specific circuit configurations of two inverter circuits and a power source switching circuit constituting the buffer circuit according to the present invention;  
         [0050]      FIG. 6A ,  FIG. 6B ,  FIG. 6C  and  FIG. 6D  are time charts showing operation states of the buffer circuit according to the present invention at the time of power-ON;  
         [0051]      FIG. 7A ,  FIG. 7B ,  FIG. 7C  and  FIG. 7D  are time charts showing operation states of the buffer circuit according to the present invention at the time of power-OFF;  
         [0052]      FIG. 8A ,  FIG. 8B ,  FIG. 8C  and  FIG. 8D  are time charts showing other operation states of the buffer circuit according to the present invention at the time of power-ON;  
         [0053]      FIG. 9A ,  FIG. 9B ,  FIG. 9C  and  FIG. 9D  are time charts showing other operation states of the buffer circuit according to the present invention at the time of power-OFF;  
         [0054]      FIG. 10  is a circuit diagram showing the specific circuit configuration of a buffer circuit according to Embodiment 2 of the present invention; and  
         [0055]      FIG. 11  is a circuit diagram showing the specific circuit configuration of a buffer circuit according to Embodiment 3 of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     Embodiment 1  
       [0056]     The present invention will be described below on the basis of the drawings showing the embodiments thereof.  FIG. 4  is a block diagram showing the configuration of a buffer circuit (designated by numeral  2 ) according to the present invention. An output signal from a functional block (not shown), operating on a drive voltage VDD 1 , for attaining a predetermined function inside an integrated circuit  1  is inputted to the buffer circuit  2 . On the other hand, the buffer circuit  2  outputs an output signal to another functional block (not shown) operating on a drive voltage VDD 2  lower than VDD 1 . In this case, the buffer circuit  2  steps down the drive voltage of the integrated circuit  1  from the high voltage VDD 1  to the low voltage VDD 2 .  
         [0057]     The buffer circuit  2  has CMOS inverter circuits  4  and  5  as logical gate circuits and a power source switching circuit  3 . A power source VD 1  having the voltage VDD 1  and a power source VD 2  having the voltage VDD 2  lower than the voltage VDD 1  are connected to the power source switching circuit  3 . The power source switching circuit  3  carries out switching so that either the power source VD 1  or the power source VD 2  is connected to the inverter circuit  4 . An output of the inverter circuit  4  is connected to an input of the inverter circuit  5 . The power source VD 2  is connected to the inverter circuit  5 .  
         [0058]      FIG. 5  is a circuit diagram showing the specific circuit configurations of the inverter circuits  4  and  5  and the power source switching circuit  3  constituting the buffer circuit  2 . The inverter circuit  4  is composed of a P-channel FET  41  and an N-channel FET  42 . Gates of the two FETs  41  and  42  are connected to each other, and the connection node thereof serves as an input terminal of the inverter circuit  4 . A drain of the FET  41  is connected to a source of the FET  42 , and a connection node thereof serves as an output terminal of the inverter circuit  4 . A drain of the FET  42  is grounded. A source of the FET  41  is connected to the power source switching circuit  3 .  
         [0059]     The power source switching circuit  3  is composed of P-channel FETs  31 ,  32  and  33 . A source and a gate of the FET  31  are connected to the power source VD 1  and the power source VD 2 , respectively. A drain of the FET  31  is connected to the source of the FET  41 . A source and a gate of the FET  32  are connected to the power source VD 2  and the power source VD 1 , respectively. A drain of the FET  32  is connected to a source of the FET  33 . A gate of the FET  33  is connected to the power source VD 2 . A drain of the FET  33  is connected to back gates of the FETs  31 ,  32 ,  33  and  41 . The connection node of the drain of the FET  31  and a source of the FET  41  is connected to a connection node of the drain of the FET  32  and the source of the FET  33 .  
         [0060]     The output terminal of the inverter circuit  4  is connected to an input terminal of the inverter circuit  5 . The inverter circuit  5  is composed of a P-channel FET  51  and an N-channel FET  52 . Gates of the FETs  51  and  52  are connected to each other, and the connection node thereof serves as the input terminal of the inverter circuit  5 . A source of the FET  51  is connected to the power source VD 2 . A drain of the FET  51  is connected to a source of the FET  52 , and the connection node thereof serves as an output terminal of the inverter circuit  5 . A drain of the FET  52  is grounded.  
         [0061]     Next, the operation of the above-mentioned buffer circuit  2  according to the present invention will be described below.  FIG. 6A ,  FIG. 6B ,  FIG. 6C  and  FIG. 6D  are time charts showing operation states of the buffer circuit  2  at the time of power-ON.  FIG. 6A  shows the changes in the voltages of the two power sources VD 1  and VD 2 ,  FIG. 6B  shows the ON/OFF states of the two FETs  31  and  32  of the power source switching circuit  3 ,  FIG. 6C  shows the state at the connection node of the power source switching circuit  3  and the inverter circuit  4 , more specifically, the state of the voltage Vb at the connection node of the drain of the FET  31  of the power source switching circuit  3  and the source of the FET  41  of the inverter circuit  4 , and  FIG. 6D  shows the state of the voltage Va at the input terminal of the inverter circuit  5 .  
         [0062]     As shown in  FIG. 6A , in the case that the power source VD 1  is turned ON while the power source VD 2  (voltage VDD 2 ) is supplied to the buffer circuit  2 , the rising of the voltage of the power source VD 1  takes time, therefore there is a period in which the voltage of the power source VD 1  is lower than the voltage of the power source VD 2 . In this period, since the voltage of the power source VD 1  is lower than the voltage of the power source VD 2 , the FET  32  turns ON and the FET  31  turns OFF as shown in  FIG. 6B . Hence, the voltage Vb at the source of the FET  41  becomes equal to the voltage VDD 2  of the power source VD 2  as shown in  FIG. 6C .  
         [0063]     In this case, for example, when the voltage at the input terminal of the inverter circuit  4  is 0V, the FET  41  is in the ON state, and the voltage at the output terminal of the inverter circuit  4  becomes equal to the voltage VDD 2  of the power source VD 2 . Hence, the voltage Va at the input terminal of the inverter circuit  5  also becomes equal to the voltage VDD 2  as shown in  FIG. 6D .  
         [0064]     When the voltage of the power source VD 1  rises with the passage of time and reaches a voltage lower than the voltage VDD 2  of the power source VD 2  by the threshold voltage Vth of the FET  32  at time t 1  as shown in  FIG. 6A , the FET  32  turns from ON to OFF as shown in  FIG. 6B . Furthermore, when the voltage of the power source VD 1  reaches a voltage higher than the voltage VDD 2  of the power source VD 2  by the threshold voltage Vth of the FET  31  at time t 2 , the FET  31  turns from OFF to ON as shown in  FIG. 6B . Since the FET  31  turns ON at this time, the voltage of the power source VD 1  is applied to the source of the FET  41  via the source and drain of the FET  31 . Hence, the voltage Vb at the source of the FET  41  rises gradually and reaches the voltage VDD 1  at time t 3  as shown in  FIG. 6C . It is assumed that the voltage Vb at time t 2  when the FET  31  turns from OFF to ON is VDD 2  for the sake of simplicity.  
         [0065]     The voltage Va at the input terminal of the inverter circuit  5  changes in a manner similar to the voltage Vb at the source of the FET  41  by virtue of the above-mentioned operation as shown in  FIG. 6D . Hence, even in the period in which the voltage of the power source VD 1  is lower than the voltage of the power source VD 2 , the gate voltage of the FET  51  of the inverter circuit  5  can be prevented from becoming lower than the source voltage (VDD 2 ). As a result, since the FET  51  does not turn ON, it is thus possible to suppress the generation of through current flowing the FETs  51  and  52 .  
         [0066]     Furthermore, even in the case that the power source VD 1  is turned OFF and the voltage of the power source VD 1  becomes 0V, the power source VD 2  is connected to the inverter circuit  4  by the power source switching circuit  3 . Hence, the inverter circuit  4  can output a signal to the inverter circuit  5  in accordance with a signal inputted to the inverter circuit  4  itself.  
         [0067]     In the case that the voltage of the power source VD 1  becomes lower than the voltage of the power source VD 2 , the source voltage Vb of the FET  41  becomes equal to the voltage of the power source VD 2 . Hence, the voltage at the source of the FET  33  connected to the source of the FET  41  also becomes equal to the voltage of the power source VD 2 . On the other hand, since the gate of the FET  33  is connected to the power source VD 2 , the gate voltage and the source voltage of the FET  33  become equal, and the FET  33  turns OFF. Since the drain of the FET  33  is connected to the back gates of the FETs  31 ,  32 ,  33  and  41 , the back gate voltages of the FETs  31 ,  32 ,  33  and  41  become almost equal to the voltage of the power source VD 2  by virtue of the PN forward bias from the source of the FET  33 , whereby the circuit can be operated stably.  
         [0068]     Furthermore, in the case that the voltage of the power source VD 1  is higher than the voltage of the power source VD 2 , the source voltage Vb of the FET  41  becomes equal to the voltage of the power source VD 1 , and the source voltage of the FET  33  connected to the source of the FET  41  also becomes equal to the voltage of the power source VD 1 . On the other hand, since the gate of the FET  33  is connected to the power source VD 2 , the FET  33  turns ON. Since the drain of the FET  33  is connected to the back gates of the FETs  31 ,  32 ,  33  and  41 , the back gate voltages of the FETs  31 ,  32 ,  33  and  41  become equal to the voltage of the power source VD 1 , whereby the circuit can be operated stably.  
         [0069]      FIG. 7A ,  FIG. 7B ,  FIG. 7C  and  FIG. 7D  are time charts showing operation states of the buffer circuit  2  at the time of power-OFF.  FIG. 7A  shows the changes in the voltages of the two power sources VD 1  and VD 2 ,  FIG. 7B  shows the ON/OFF states of the FETs  31  and  32  of the power source switching circuit  3 ,  FIG. 7C  shows the state at the connection node of the power source switching circuit  3  and the inverter circuit  4 , more specifically, the state of the voltage Vb at the connection node of the drain of the FET  31  of the power source switching circuit  3  and the source of the FET  41  of the inverter circuit  4 , and  FIG. 7D  shows the state of the voltage Va at the input terminal of the inverter circuit  5 .  
         [0070]     As shown in  FIG. 7A , in the case that the power source VD 1  is turned OFF while the power source VD 2  (voltage VDD 2 ) is supplied to the buffer circuit  2 , the lowering of the voltage of the power source VD 1  takes time, therefore there is a transient period in which the voltage of the power source VD 1  is higher than the voltage of the power source VD 2 . However, the voltage of the power source VD 1  eventually becomes lower than the voltage of the power source VD 2 . In the case that the voltage of the power source VD 1  becomes lower than the voltage of the power source VD 2 , the FET  32  turns ON and the FET  31  turns OFF. Hence, the source voltage Vb of the FET  41  becomes equal to the voltage VDD 2  of the power source VD 2 .  
         [0071]     In this case, for example, when the voltage at the input terminal of the inverter circuit  4  is 0V, the FET  41  is in the ON state. Hence, the voltage at the output terminal of the inverter circuit  4  becomes equal to the voltage VDD 2  of the power source VD 2 , and the voltage Va at the input terminal of the inverter circuit  5  also becomes equal to the voltage VDD 2 .  
         [0072]     In the transient period in which the voltage of the power source VD 1  is higher than the voltage of the power source VD 2 , when the voltage of the power source VD 1  reaches the voltage higher than the voltage VDD 2  of the power source VD 2  by the threshold voltage Vth of the FET  31  at time t 5  after the power source VD 1  is turned OFF at time t 4  as shown in  FIG. 7A , the FET  31  turns from ON to OFF as shown in  FIG. 7B . Furthermore, when the voltage of the power source VD 1  reaches the voltage lower than the voltage VDD 2  of the power source VD 2  by the threshold voltage Vth of the FET  32  at time t 6 , the FET  32  turns from OFF to ON. Since the FET  32  turns ON, the voltage of the power source VD 2  is applied to the source of the FET  41  via the source and drain of the FET  32 . Hence, the source voltage Vb of the FET  41  is fixed at the voltage of the power source VD 2  as shown in  FIG. 7C .  
         [0073]     Consequently, the voltage Va at the input terminal of the inverter circuit  5  changes in a manner similar to the source voltage Vb of the FET  41  as shown in  FIG. 7D . Hence, even in the period in which the voltage of the power source VD 1  is lower than the voltage of the power source VD 2 , the gate voltage of the FET  51  of the inverter circuit  5  can be prevented from becoming lower than the source voltage (VDD 2 ). As a result, the FET  51  does not turn ON, and it is thus possible to suppress the generation of thorough current flowing the FETs  51  and  52 .  
         [0074]      FIG. 8A ,  FIG. 8B ,  FIG. 8C  and  FIG. 8D  are time charts showing other operation states of the buffer circuit  2  at the time of power-ON.  FIG. 8A  shows the changes in the voltages of the two power sources VD 1  and VD 2 ,  FIG. 8B  shows the ON/OFF states of the FETs  31  and  32  of the power source switching circuit  3 ,  FIG. 8C  shows the state at the connection node of the power source switching circuit  3  and the inverter circuit  4 , more specifically, the state of the voltage Vb at the connection node of the drain of the FET  31  of the power source switching circuit  3  and the source of the FET  41  of the inverter circuit  4 , and  FIG. 8D  shows the state of the voltage Va at the input terminal of the inverter circuit  5 .  
         [0075]     As shown in  FIG. 8A , in the case that the power source VD 1  (voltage VDD 1 ) and the power source VD 2  (voltage VDD 2 ) are turned on simultaneously at time t 11 , since the rising of the voltage of the power source VD 2  is faster than the rising of the voltage of the power source VD 1 , there is a transient period in which the voltage of the power source VD 1  is lower than the voltage of the power source VD 2 . However, the voltage of the power source VD 1  eventually becomes higher than the voltage of the power source VD 2 . On the other hand, in the transient period in which the voltage of the power source VD 1  is lower than the voltage of the power source VD 2 , the FET  32  turns ON and the FET  31  turns OFF. Hence, the source voltage Vb of the FET  41  becomes equal to the voltage VDD 2  of the power source VD 2 .  
         [0076]     In this case, for example, when the voltage at the input terminal of the inverter circuit  4  is 0V, the FET  41  is in the ON state. Hence, the voltage at the output terminal of the inverter circuit  4  becomes equal to the voltage VDD 2  of the power source VD 2 , and the voltage Va at the input terminal of the inverter circuit  5  also becomes equal to the voltage VDD 2 .  
         [0077]     When the voltage of the power source VD 2  reaches VDD 2  at time t 12  as shown in  FIG. 8A , since the FET  32  is in the ON state as shown in  FIG. 8B , the source voltage Vb of the FET  41  and the voltage Va at the input terminal of the inverter circuit  5  become the voltage VDD 2  of the power source VD 2 . Next, when the voltage of the power source VD 1  reaches the voltage lower than the voltage VDD 2  of the power source VD 2  by the threshold voltage Vth of the FET  32  at time t 13 , the FET  32  turns from ON to OFF as shown in  FIG. 8B . Furthermore, when the voltage of the power source VD 1  reaches the voltage higher than the voltage VDD 2  of the power source VD 2  by the threshold voltage Vth of the FET  31  at time t 14  as shown in  FIG. 8A , the FET  31  turns from OFF to ON as shown in  FIG. 8B . Since the FET  31  turns ON, the voltage of the power source VD 1  is applied to the source of the FET  41  via the source and drain of the FET  31 . Hence, the source voltage Vb of the FET  41  rises depending on the voltage of the power source VD 1  and reaches the voltage VDD 1  at time t 15  as shown in  FIG. 8C .  
         [0078]     Consequently, the voltage Va at the input terminal of the inverter circuit  5  changes in a manner similar to the voltage Vb at the source of the FET  41  as shown in  FIG. 8D . Hence, even in the period in which the voltage of the power source VD 1  is lower than the voltage of the power source VD 2 , the gate voltage of the FET  51  of the inverter circuit  5  can be prevented from becoming lower than the source voltage (VDD 2 ). As a result, the FET  51  does not turn ON, and it is thus possible to suppress the generation of through current flowing the FETs  51  and  52 .  
         [0079]      FIG. 9A ,  FIG. 9B ,  FIG. 9C  and  FIG. 9D  are time charts showing other operation states of the buffer circuit  2  at the time of power-OFF.  FIG. 9A  shows the changes in the voltages of the two power sources VD 1  and VD 2 ,  FIG. 9B  shows the ON/OFF states of the FETs  31  and  32  of the power source switching circuit  3 ,  FIG. 9C  shows the state at the connection node of the power source switching circuit  3  and the inverter circuit  4 , more specifically, the state of the voltage Vb at the connection node of the drain of the FET  31  of the power source switching circuit  3  and the source of the FET  41  of the inverter circuit  4 , and  FIG. 9D  shows the state of the voltage Va at the input terminal of the inverter circuit  5 .  
         [0080]     In the case that the power source VD 1  (voltage VDD 1 ) is turned OFF at time t 16  and when the power source VD 2  (voltage VDD 2 ) is turned OFF at time t 19  as shown in  FIG. 9A , the voltage of the power source VD 1  begins to lower earlier. Hence, there is a transient period in which the voltage of the power source VD 1  is lower than the voltage of the power source VD 2 . However, both the voltage of the power source VD 1  and the voltage of the power source VD 2  become 0V eventually. On the other hand, in the transient period in which the voltage of the power source VD 1  is lower than the voltage of the power source VD 2 , the FET  32  turns ON and the FET  31  turns OFF. Hence, the source voltage Vb of the FET  41  becomes equal to the voltage VDD 2  of the power source VD 2 .  
         [0081]     In this case, for example, when the voltage at the input terminal of the inverter circuit  4  is 0V, the FET  41  is in the ON state. Hence, the voltage at the output terminal of the inverter circuit  4  becomes equal to the voltage VDD 2  of the power source VD 2 , and the voltage Va at the input terminal of the inverter circuit  5  also becomes equal to the voltage VDD 2 .  
         [0082]     When the voltage of the power source VD 1  reaches the voltage higher than the voltage VDD 2  of the power source VD 2  by the threshold voltage Vth of the FET  31  at time t 17  after the power source VD 1  is turned OFF at time t 16  as shown in  FIG. 9A , the FET  31  turns from ON to OFF as shown in  FIG. 9B . Furthermore, when the voltage of the power source VD 1  reaches the voltage lower than the voltage VDD 2  of the power source VD 2  by the threshold voltage Vth of the FET  32  at time t 18  as shown in  FIG. 9A , the FET  32  turns from OFF to ON as shown in  FIG. 9B . Since the FET  32  turns ON, the voltage of the power source VD 2  is applied to the source of the FET  41  via the source and drain of the FET  32 . Hence, the source voltage Vb of the FET  41  changes depending on the voltage of the power source VD 2  as shown in  FIG. 9C .  
         [0083]     Consequently, the voltage Va at the input terminal of the inverter circuit  5  changes in a manner similar to the voltage Vb at the source of the FET  41  as shown in  FIG. 9D . Hence, even in the period in which the voltage of the power source VD 1  is lower than the voltage of the power source VD 2 , the gate voltage of the FET  51  of the inverter circuit  5  can be prevented from becoming lower than the source voltage (VDD 2 ). As a result, the FET  51  does not turn ON, and it is thus possible to suppress the generation of through current flowing the FETs  51  and  52 .  
         [0084]     As described above, in the buffer circuit  2  equipped with the power sources (VD 1  and VD 2 ) having different voltages according to the present invention, in the case that the power sources VD 1  and VD 2  are turned ON or OFF, switching is carried out between the power sources VD 1  and VD 2  depending on the high or low of the voltage between the power source VD 1  and the power source VD 2 , whereby the voltage at the input terminal of the inverter circuit  5  is prevented from becoming lower than the voltage of the power source VD 2 . With this configuration, through current flows the inverter circuit  5  is prevented, whereby the power consumption of the buffer circuit can be made lower than that of this kind of conventional circuit. Furthermore, even in the case that the power source VD 1  is turned OFF, the voltage of the power source VD 2  is applied to the first logical gate circuit, whereby the function of the inverter circuit  4  does not stop. Hence, even in this case, a signal inputted to the inverter circuit  4  can be outputted to the inverter circuit  5 , and signal transmission can thus be carried out inside the integrated circuit  1 .  
       Embodiment 2  
       [0085]      FIG. 10  is a circuit diagram showing the specific circuit configuration of a buffer circuit according to Embodiment 2 of the present invention. A buffer circuit  21  according to Embodiment 2 is equipped with a transmission gate which is composed of a P-channel FET  34  and an N-channel FET  35  and is inserted between the power source VD 1  and the gate of the FET  32  of a power source switching circuit  3 , in addition to the buffer circuit according to Embodiment 1.  
         [0086]     A source of the FET  34  is connected to a source of the FET  35  and also connected to the power source VD 1 . A gate of the FET  34  is connected to a gate of the FET  34  and also connected to the power source VD 2  and the gate of the FET  33 . A drain of the FET  34  is connected to a drain of the FET  35  and also connected to the gate of the FET  32 . A back gate of the FET  34  is connected to the back gates of the FETs  31 ,  32 ,  33  and  41 . Other components similar to those of Embodiment 1 are designated by the same numerals and their descriptions are omitted.  
         [0087]     Next, the operation of the buffer circuit  21  according to Embodiment 2 will be described referring to the above-mentioned time charts in  FIG. 6  to  FIG. 9 . The FET  35  constituting the transmission gate becomes saturation at about time t 1  to time t 2  shown in  FIG. 6A , at about time t 5  to time t 6  shown in  FIG. 7A , at about time t 13  to time t 14  shown in  FIG. 8A  and at about time t 17  to time t 18  shown in  FIG. 9A . As a result, the gate voltage of the FET  32  becomes the voltage obtained by subtracting the threshold voltage of the FET  35  from the voltage of the power source VD 2 , and the voltage lower than the voltage of the power source VD 2  is applied to the gate of the FET  32 . Hence, in comparison with the case wherein the voltage of the power source VD 2  is applied to the gate of the FET  32 , time t 1  in  FIG. 6A  and time t 13  in  FIG. 8A  can be delayed, and time t 6  in  FIG. 7A  and time t 18  in  FIG. 9A  can be advanced.  
         [0088]     Furthermore, since the FET  34  and the FET  31  carry out the same operation, for example, in the case that the voltage of the power source VD 1  becomes higher than the voltage of the power source VD 2 , the FET  34  turns ON at the same time when the FET  31  turns ON. Hence, the gate voltage of the FET  32  becomes equal to the voltage of the power source VD 1 , and the FET  32  turns OFF.  
         [0089]     As described above, in Embodiment 2, the transmission gate which is composed of the FETs  34  and  35  is inserted between the power source VD 1  and the gate of the FET  32 , whereby the time required for power source switching can be shortened in comparison with the case wherein the voltage of the power source VD 2  is applied to the gate of the FET  32 .  
       Embodiment 3  
       [0090]      FIG. 11  is a circuit diagram showing the specific circuit configuration of a buffer circuit according to Embodiment 3 of the present invention. A buffer circuit  22  according to Embodiment 3 is equipped with an N-channel FET  36  connected between the power source VD 2  and the gate of the FET  33  of a power source switching circuit  3 , in addition to the buffer circuit according to Embodiment 2.  
         [0091]     A gate and a drain of the FET  36  are connected to the power source VD 2 . A source of the FET  36  is connected to the gate of the FET  33 . Other components similar to those of Embodiment 2 are designated by the same numerals and their descriptions are omitted.  
         [0092]     Next, the operation of the buffer circuit  22  according to Embodiment 3 will be described. In the case that the voltage of the power source VD 1  becomes lower than the voltage of the power source VD 2  when the power sources VD 1  and VD 2  are turned ON or OFF, the P-channel FET  34  turns OFF since its gate voltage becomes higher than its source voltage. The N-channel FET  35  turns ON since its gate voltage becomes higher than its source voltage. In addition, the FET  31  turns OFF since its gate voltage becomes higher than its source voltage. Furthermore, the FET  32  turns ON since its gate voltage becomes lower than its source voltage.  
         [0093]     Since the gate voltage and the source voltage of the FET  36  are equal to the voltage of the power source VD 2 , the FET  36  becomes saturation. Hence, the gate voltage of the FET  33  becomes the voltage obtained by subtracting the threshold voltage of the FET  36  from the voltage of the power source VD 2 , and the voltage lower than the voltage of the power source VD 2  is applied to the gate of the FET  33 . As a result, in comparison with the case wherein the voltage of the power source VD 2  is applied to the gate of the FET  33 , the ON operation of the FET  33  can be hastened.  
         [0094]     When the voltage of the power source VD 1  is higher than the voltage of the power source VD 2 , the source voltage Vb of the FET  41  becomes equal to the voltage of the power source VD 1 . Hence, the source voltage of the FET  33  connected to the source of the FET  41  also becomes equal to the voltage of the power source VD 1 . On the other hand, since the voltage almost equal to the voltage of the power source VD 2  is applied to the gate of the FET  33  via the FET  36 , the FET  33  turns ON. Since the drain of the FET  33  is connected to the back gates of the FETs  31 ,  32 ,  33  and  41 , the back gate voltages of the FETs  31 ,  32 ,  33  and  41  become equal to the voltage of the power source VD 1 , whereby the circuit can be operated stably.  
         [0095]     As described above, the ON operation of the FET  33  can be hastened by connecting the FET  36  to the gate of the FET  33 . Hence, the time required for making the back gate voltages of the FETs  31 ,  32 ,  33  and  41  equal to the voltage of the power source VD 1  is shortened. As a result, the circuit operation can be stabilized in a short time.  
         [0096]     The configuration using the inverter circuits  4  and  5 , wherein a P-channel FET and an N-channel FET are connected in series as an example of a logical gate circuit, has been described in the above-mentioned Embodiments 1, 2 and 3. However, the configuration of the logical gate circuit is not required to be limited to this kind of circuit. Other logical gate circuits, such as NAND and NOR, may also be used, provided that an inverter circuit is included.  
         [0097]     Furthermore, the configuration example wherein the voltages of the power sources VD 1  and VD 2  are positive voltages with respect to the ground level has been described in the above-mentioned Embodiments 1, 2 and 3. However, the configuration is not required to be limited to such a configuration. A configuration wherein the voltages of the power sources VD 1  and VD 2  are negative voltages with respect to the ground level may also be used. In this kind of configuration, the power source switching circuit  3  detects the absolute voltage values of the power sources VD 1  and VD 2  and can thus carry out power source switching depending on the high or low between the detected absolute voltage values.  
         [0098]     As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiment are therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds there-of are therefore intended to be embraced by the claims.