Patent Publication Number: US-2023132469-A1

Title: Level shift circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to International Application No. PCT/JP2021/039969 filed on Oct. 29, 2021. The entire disclosure of this application is incorporated by reference herein. 
    
    
     BACKGROUND 
     The present disclosure relates to a level shift circuit that converts the potential of a signal, which is required when the signal is passed between circuits to which different power supply voltages are supplied. 
     A level shift circuit is provided in an interface part in which a signal is passed from a relatively low-voltage circuit inside a semiconductor chip to a relatively high-voltage circuit outside the semiconductor chip, for example, and used at the time of conversion of the potential of the signal. 
     For example, Japanese Unexamined Patent Publication No. H07-202650 describes a level shift circuit that generates an output signal of a desired level depending on the level of an input signal and supplies the signal to a next-stage semiconductor device. 
     Specifically, in  FIG.  1    of the cited patent document, when the initial state is Φ21=L, the node N 23 =H (Vcc). Therefore, the transistor  31  is OFF, the transistor  33  is ON, the nodes N 32 , N 25 =L, and the transistor  42  is OFF. Also, since the node N 24 =L, the transistor  41  is ON, the transistor  43  is OFF, the nodes N 42 , N 26 =H (Vpp), and the transistor  32  is ON. 
     Once Φ21 transitions from L to H, the transistor  31  turns ON and the transistor  33  turns OFF, whereby Vcc and the nodes N 32  and N 25  are connected through the transistors  31  and  32 , causing the node N 25  to rise. In this way, the level shift operation is assisted in the level shift circuit of the cited patent document. 
     In  FIG.  1    of the cited patent document, however, when Φ21 transitions from L to H, causing the node N 23  to become L, the node N 24  becomes H (Vcc), whereby the transistor  43  turns ON and the nodes N 42  and N 26  changes from H (Vpp) to L. This turns OFF the transistor  32 , shutting off the connection between Vcc and the nodes N 25  and N 32  through the transistors  31  and  32 . As a result, as shown by the dashed-line circle in  FIG.  6   , the assist operation of the level shift is discontinued midway through the rising of the signal to Vcc, and this may end up in insufficient speedup. 
     Also, in the initial state described above in  FIG.  1    of the cited patent document, since the transistor  41  is ON and the transistor  42  is OFF, the node between the transistor  41  and the transistor  42  is Vcc. When Φ21 transitions from L to H in this state, the node N 24  transitions from L to H (Vcc), whereby the transistor  41  turns OFF and the transistor  43  turns ON. Since the transistor  42  maintains the OFF state, the node between the transistor  41  and the transistor  42  is boosted from Vcc due to a gate-drain coupling capacitance of the transistor  41 . This causes a problem that a stress is applied across the source-drain of the transistor  42 . Such a voltage stress was not a so large problem in conventional transistors, but with the recent miniaturization of the manufacturing processes, has become a problem required to be cared for. 
     An objective of the present disclosure is providing a level shift circuit in which the effectiveness of the assist operation is further enhanced and the voltage stress to transistors is eased. 
     SUMMARY 
     According to one mode of the present disclosure, a level shift circuit includes: an input node receiving an input signal transitioning between a first power supply and a second power supply lower in potential than the first power supply; an inverted input node receiving an inverted input signal inverted from the input signal; a first n-type transistor having a gate connected to the input node, a source connected to the second power supply or the inverted input node, and a drain connected to an inverted output node; a first p-type transistor having a drain connected to the inverted output node, a source connected to a third power supply higher in potential than the first power supply, and a gate connected to an output node; a second n-type transistor having a gate connected to the inverted input node, a source connected to the second power supply or the input node, and a drain connected to the output node; a second p-type transistor having a drain connected to the output node, a source connected to the third power supply, and a gate connected to the inverted output node; a third n-type transistor having a source connected to the inverted input node and a drain connected to the inverted output node; a first inverter having an input connected to the inverted output node and an output connected to a gate of the third n-type transistor; a fourth n-type transistor having a source connected to the input node and a drain connected to the output node; and a second inverter having an input connected to the output node and an output connected to a gate of the fourth n-type transistor. 
     In the level shift circuit of the above mode, in the third transistor that switches ON/OFF of charge supply, the assist operation is turned OFF after a lapse of a predetermined delay time of the first inverter from the change of the inverted output signal. Similarly, in the fourth transistor that switches ON/OFF of charge supply, the assist operation is turned OFF after a lapse of a predetermined delay time of the second inverter from the change of the output signal. Therefore, in comparison with the prior art, since a sufficient assist operation time can be secured, the effectiveness of the assist operation can be further enhanced. 
     Moreover, in the technique of the cited patent document, in  FIG.  1    of this document, when the node N 24  becomes H, both the transistors  41  and  42  are OFF, causing a problem of boosting the voltage at the node between the transistor  41  and the transistor  42 . By contrast, according to this mode, there is no such place where transistors on both sides of a given node become OFF simultaneously, causing a floating state, in the process of the assist operation. Therefore, no problem about boosting of the voltage at a given node will occur. 
     According to the present disclosure, in a level shift circuit, the effectiveness of the assist operation can be further enhanced, and the voltage stress to transistors can be eased. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a circuit diagram showing a configuration of a level shift circuit according to the first embodiment. 
         FIG.  2 A  is a view showing an example of voltage waveforms at nodes of the level shift circuit. 
         FIG.  2 B  is a view showing another example of voltage waveforms at nodes of the level shift circuit. 
         FIG.  3 A  is a circuit diagram showing an alteration of the level shift circuit according to the first embodiment. 
         FIG.  3 B  is a circuit diagram showing another alteration of the level shift circuit according to the first embodiment. 
         FIG.  4    is a circuit diagram showing a configuration of a level shift circuit according to the second embodiment. 
         FIG.  5    is a circuit diagram showing an alteration of the level shift circuit according to the second embodiment. 
         FIG.  6    is a view showing an example of voltage waveforms at nodes of the conventional level shift circuit. 
         FIG.  7    is a circuit diagram showing yet another alteration of the level shift circuit according to the first embodiment. 
         FIG.  8    is a circuit diagram showing another alteration of the level shift circuit according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will be described hereinafter. Note that any specific numerical values presented in the following embodiments are mere examples for easy understanding of the invention, and by no means intended to limit the scope of the invention. In the following description, a node of a circuit, a signal passing through the node, and the voltage at the node may be referred to under the same reference character. Also, a power supply itself and the voltage of the power supply may be referred to under the same reference character. 
     First Embodiment 
     A level shift circuit  1  receives an input signal IN and an inverted input signal NIN that transition between a first power supply VDD and a ground VSS, and outputs an output signal OUT and an inverted output signal NOUT that transition between a third power supply VDDIO and the ground VSS. In other words, the input signal IN and the inverted input signal NIN are signals having an amplitude of VDD, and the output signal OUT and the inverted output signal NOUT are signals having an amplitude of VDDIO. The first power supply VDD is 0.8 [V] and the third power supply VDDIO is 1.3 [V], for example. 
       FIG.  1    shows an example of the circuit diagram of the level shift circuit  1  according to the first embodiment. 
     The level shift circuit  1  includes a basic circuit constituted by first and second n-type transistors N 1  and N 2  and first and second p-type transistors P 1  and P 2  and a rise assist circuit  2 . 
     —Basic Circuit— 
     The first n-type transistor N 1  has a gate connected to an input node IN, a source connected to the ground VSS, and a drain connected to an inverted output node NOUT. The gate of the first n-type transistor N 1  receives the input signal IN through the input node IN. Note that the ground VSS corresponds to a second power supply. The second power supply is not limited to the ground VSS but may be a power supply of another potential lower than that of the first power supply VDD. 
     The second n-type transistor N 2  has a gate connected to an inverted input node NIN, a source connected to the ground VSS, and a drain connected to an output node OUT. The gate of the second n-type transistor N 2  receives the inverted input signal NIN through the inverted input node NIN. 
     In the illustrated example, the inverted input signal NIN is a signal obtained by inverting an input signal XIN through an inverter  81 , and the input signal IN is a signal obtained by inverting the input signal XIN twice through the inverter  81  and an inverter  82 . Although not illustrated, the power supply terminal of the inverter  81  is connected to the first power supply VDD and the ground terminal thereof is connected to the ground VSS. Similarly, the power supply terminal of the inverter  82  is connected to the first power supply VDD and the ground terminal thereof is connected to the ground VSS. Note that the inverters  81  and  82  may be omitted in  FIG.  1    and instead the input signal IN and the inverted input signal NIN may be generated in a circuit (not shown) preceding the level shift circuit  1  and input into the level shift circuit  1 . 
     The first p-type transistor P 1  has a gate connected to the output node OUT, a source connected to the third power supply VDDIO, and a drain connected to the inverted output node NOUT. In other words, the first p-type transistor P 1  and the first n-type transistor N 1  are serially connected between the third power supply VDDIO and the ground VSS. The third power supply VDDIO is higher in potential than the first power supply VDD. 
     The second p-type transistor P 2  has a gate connected to the inverted output node NOUT, a source connected to the third power supply VDDIO, and a drain connected to the output node OUT. In other words, the second p-type transistor P 2  and the second n-type transistor N 2  are serially connected between the third power supply VDDIO and the ground VSS. 
     —Rise Assist Circuit— 
     The rise assist circuit  2  is a circuit that assists the rise of the output signal OUT or the rise of the inverted output signal NOUT. 
     In the illustrated example, the rise assist circuit  2  includes third and fourth n-type transistors N 3  and N 4  and first and second inverters  21  and  22 . 
     The third n-type transistor N 3  has a source connected to the inverted input node NIN and a drain connected to the inverted output node NOUT. 
     The first inverter  21  has an input connected to the inverted output node NOUT and an output connected to the gate of the third n-type transistor N 3 . Although not illustrated, the first inverter  21  has a circuit configuration similar to that of the second inverter  22  shown in  FIG.  1   , in which, for example, a p-type transistor and an n-type transistor are serially connected between the third power supply VDDIO and the ground VSS. The output signal of the first inverter  21  is hereinafter denoted by S 1 . 
     The fourth n-type transistor N 4  has a source connected to the input node IN and a drain connected to the output node OUT. 
     The second inverter  22  has an input connected to the output node OUT and an output connected to the gate of the fourth n-type transistor N 4 . In the second inverter  22 , for example, a p-type transistor P 11  (a high-potential side switching element) and an n-type transistor N 11  (a low-potential side switching element) are serially connected between the third power supply VDDIO and the ground VSS. The output signal of the second inverter  22  is hereinafter denoted by S 2 . 
     —Operation of Level Shift Circuit— 
     Next, the operation of the level shift circuit  1  will be described. 
     &lt;Operation when Input Signal IN Transitions from L to H&gt; 
     The operation of the level shift circuit  1  when the input signal IN transitions from L to H will be described with reference to  FIG.  2 A . 
     As shown in  FIG.  2 A , in the initial state (at time T 0 ), the input signal XIN=L, the input signal IN=L, and the inverted input signal NIN=H. Since the second n-type transistor N 2  is ON and the second p-type transistor P 2  is OFF, the output signal OUT=L. The output signal S 2  of the second inverter  22  is H, and thus the fourth n-type transistor N 4  is ON. 
     Also, since the first n-type transistor N 1  is OFF and the first p-type transistor P 1  is ON, the inverted output signal NOUT=H. The output signal Si of the first inverter  21  is L, and thus the third n-type transistor N 3  is OFF. 
     At time T 1 , once the input signal XIN rises, the inverted input signal NIN changes from H to L, and the input signal IN changes from L to H. 
     With the inverted input signal NIN becoming L, the second n-type transistor N 2  turns OFF. Since the fourth n-type transistor N 4  is ON in the initial state, charge is supplied from the first power supply VDD to the output node OUT through the fourth n-type transistor N 4  in the process of rising of the input signal IN from L to H. This charge supply assists the rising of the output signal OUT whereby the rising of the output signal OUT can be advanced. 
     Also, with the input signal IN becoming H, the first n-type transistor N 1  turns ON, causing the inverted output signal NOUT to start falling. When the inverted output signal NOUT falls, the second p-type transistor P 2  turns ON, causing start of charge supply from the third power supply VDDIO to the output node OUT. 
     Since the second inverter  22  is a circuit operating with the third power supply VDDIO as described above, when the voltage of the output signal OUT rises up to a threshold determined based on the third power supply, the output signal S 2  changes from H to L. That is, the second inverter  22  changes from H to L after a lapse of a predetermined delay time. The fourth n-type transistor N 4  then turns OFF, terminating the assist operation by the rise assist circuit  2 . 
     Since the second p-type transistor P 2  is ON, the output signal OUT finally rises up to the third power supply VDDIO. 
     As described above, in this embodiment, the termination time of the assist operation can be extended depending on the delay time of the second inverter  22 . Specifically, in the example of  FIG.  2 A , the assist operation of the rise assist circuit  2  continues until the output signal OUT becomes near the first power supply VDD. Since the inverter  82  is a circuit operating with the first power supply VDD, this means that the assist operation has been achieved up to near this power supply voltage. That is, the effectiveness of the assist operation can be further enhanced. 
     Note that, when the inverted output signal NOUT falls, causing the output signal S 1  of the first inverter  21  to become H, the third n-type transistor N 3  turns ON. However, since both the inverted input signal NIN and the inverted output signal NOUT are L, i.e., both the source and drain of the third n-type transistor N 3  are L, the third n-type transistor N 3 , though it is ON, will have no influence on the operations of other circuits. 
     &lt;Operation when Input Signal IN Transitions from H to L&gt; 
     The operation of the level shift circuit  1  when the input signal IN transitions from H to L will be described with reference to  FIG.  2 B . 
     As shown in  FIG.  2 B , in the initial state (at time T 0 ), the input signal XIN=H, i.e., the input signal IN=H and the inverted input signal NIN=L. Since the second n-type transistor N 2  is OFF and the second p-type transistor P 2  is ON, the output signal OUT=H. The output signal S 2  of the second inverter  22  is L, and thus the fourth n-type transistor N 4  is OFF. 
     Also, since the first n-type transistor N 1  is ON and the first p-type transistor P 1  is OFF, the inverted output signal NOUT=L. The output signal Si of the first inverter  21  is H, and thus the third n-type transistor N 3  is ON. 
     At time T 1 , once the input signal XIN falls, the inverted input signal NIN changes from L to H, and the input signal IN changes from H to L. 
     With the input signal IN becoming L, the first n-type transistor N 1  turns OFF. Since the third n-type transistor N 3  is ON in the initial state, charge is supplied from the first power supply VDD to the inverted output node NOUT through the third n-type transistor N 3  in the process of rising of the inverted input signal NIN from L to H. This charge supply assists the rising of the inverted output signal NOUT whereby the rising of the inverted output signal NOUT can be advanced. 
     Also, with the inverted input signal NIN becoming H, the second n-type transistor N 2  turns ON, causing the output signal OUT to start falling. When the output signal OUT falls, the first p-type transistor P 1  turns ON, causing start of charge supply from the third power supply VDDIO to the inverted output node NOUT. 
     Since the first inverter  21  is a circuit operating with the third power supply VDDIO as described above, when the voltage of the inverted output signal NOUT rises up to a threshold determined based on the third power supply VDDIO, the output signal S 1  changes from H to L. That is, the first inverter  21  changes from H to L after a lapse of a predetermined delay time. The third n-type transistor N 3  then turns OFF, terminating the assist operation by the rise assist circuit  2 . 
     As described above, in this embodiment, the termination time of the assist operation can be extended depending on the delay time of the first inverter  21 . As a result, in the example of  FIG.  2 B , for example, the assist operation continues until the inverted output signal NOUT becomes near the first power supply VDD. Since the inverter  81  is a circuit operating with the first power supply VDD, this means that the assist operation has been achieved up to near this power supply voltage. That is, the effectiveness of the assist operation can be further enhanced. 
     Since the first p-type transistor P 1  is ON, the inverted output signal NOUT finally rises up to the third power supply VDDIO. 
     Note that, when the output signal OUT falls, causing the output signal S 2  of the second inverter  22  to become H, the fourth n-type transistor N 4  turns ON. However, since both the input signal IN and the output signal OUT are L, i.e., both the source and drain of the fourth n-type transistor N 4  are L, the fourth n-type transistor N 4 , though it is ON, will have no influence on the operations of other circuits. 
     —Functions and Effects of First Embodiment— 
     As described above, the technique of the cited patent document (hereinafter simply called the “prior art”) has room for improvement because the assist operation is discontinued at a comparatively early stage of the rising of the output signal of the level shift circuit due to the change in the output signal (see  FIG.  6   ). 
     By contrast, in the level shift circuit  1  of the first embodiment, even when the output state of the output signal OUT changes, it is after a lapse of the delay time of the second inverter  22  from this change that the rise assist circuit  2  stops its operation. The same holds true for the output signal NOUT. In comparison with the prior art, therefore, sufficient assist operation can be obtained. 
     Also, as described above, in the prior art, there is a case where the transistor  41  and the transistor  42  are OFF simultaneously, causing a problem that the voltage at the node midway between these transistors is boosted at such a time. 
     By contrast, in this embodiment, unlike the prior art, there is no such place where transistors on both sides of a given node become OFF simultaneously, causing a floating state, in the process of the assist operation of the output signal. Therefore, no problem about boosting of the voltage at a given node will occur. To state more specifically, in this embodiment, the fourth n-type transistor N 4  corresponds to the transistor  42  in the prior art, the input node IN corresponds to the node midway between the transistor  41  and the transistor  42 , and a p-type transistor (not shown) of the inverter  82  corresponds to the transistor  41  in the prior art. In this embodiment, an n-type transistor of the inverter  82  is also connected to the input node IN that connects the fourth n-type transistor N 4  and the p-type transistor of the inverter  82 . With the function of this n-type transistor, unlike the prior art, the voltage at the input node IN will not be boosted in accordance with the transition of the input signal, whereby the problem of the prior art is solved. 
     (Alterations) 
       FIGS.  3 A and  3 B  show alterations of the level shift circuit  1  according to the first embodiment. In  FIGS.  3 A and  3 B , components corresponding to those in  FIG.  1    are denoted by the same reference characters. These alterations will be described hereinafter focusing mainly on different points from the first embodiment. Basic operations are similar to those of the first embodiment. 
     These alterations are different from the configuration of  FIG.  1    in the circuit configurations of the first inverter  21  and the second inverter  22 . Note that, while the circuit configuration of the second inverter  22  is shown in  FIGS.  3 A and  3 B , the first inverter  21  also uses a circuit of the same configuration. 
     Specifically, in the second inverter  22  of the level shift circuit  1  of each of  FIGS.  3 A and  3 B , the low-potential side switching element has two n-type transistors N 12  and N 13  serially connected between the node S 2  and the ground VSS. In other words, the second inverter  22  has a configuration of the p-type transistor P 11  and two n-type transistors N 12  and N 13  serially connected between the third power supply VDDIO and the ground VSS. The same holds true for the first inverter  21 : i.e., the low-potential side switching element has two n-type transistors N 12  and N 13  serially connected between the node  51  and the ground VSS. 
     In  FIG.  3 A , in the second inverter  22 , the gates of the n-type transistors N 12  and N 13  are both connected to the output node OUT. The drain of the n-type transistor N 12  is connected to the node S 2  and the source thereof is connected to the drain of the n-type transistor N 13 . The source of the n-type transistor N 13  is connected to the ground VSS. 
     Similarly, in the first inverter  21  in  FIG.  3 A , the gates of the n-type transistors N 12  and N 13  are both connected to the inverted output node NOUT. The drain of the n-type transistor N 12  is connected to the node  51  and the source thereof is connected to the drain of the n-type transistor N 13 . The source of the n-type transistor N 13  is connected to the ground VSS. 
     In  FIG.  3 B , in comparison with  FIG.  3 A , the difference is that the gate of the n-type transistor N 12  is connected to the third power supply VDDIO. The other configuration is the same as that of  FIG.  3 A . In other words, the low-potential side switching element of the second inverter  22  in  FIG.  3 B  has a configuration in which the n-type transistor N 12  having a gate connected to the third power supply VDDIO (corresponding to the seventh n-type transistor) and the n-type transistor N 13  having a gate connected to the output node OUT (corresponding to the eighth n-type transistor) are serially connected. Similarly, the low-potential side switching element of the first inverter  21  has a configuration in which the n-type transistor N 12  having a gate connected to the third power supply VDDIO (corresponding to the fifth n-type transistor) and the n-type transistor N 13  having a gate connected to the inverted output node NOUT (corresponding to the sixth n-type transistor) are serially connected. 
     In these alterations ( FIGS.  3 A and  3 B ), the high-potential side switching element of the second inverter  22  is constituted by the p-type transistor P 11  having a gate connected to the output node OUT, a source connected to the third power supply VDDIO, and a drain connected to the node S 2 , as in  FIG.  1   . Similarly, the high-potential side switching element of the first inverter  21  is constituted by the p-type transistor P 11  having a gate connected to the inverted output node NOUT, a source connected to the third power supply VDDIO, and a drain connected to the node S 1 . 
     With the configuration in these alterations, in the low-potential side switching elements of the first inverter  21  and the second inverter  22 , the voltage stress applied across the source-drain of a single transistor can be reduced. 
     A specific example will be described along the “Operation when input signal IN transitions from L to H” described above. When the input signal IN rises from L in the initial state (at time T 0 ) to H, the voltage at the node S 2  is boosted due to the gate-source coupling capacitance of the fourth n-type transistor N 4 . This causes the possibility that an overvoltage may be applied between the node S 2  and the ground VSS (e.g., across the source-drain of the low-potential side switching element constituting the second inverter  22 ). Similarly, in the “Operation when input signal IN transitions from H to L,” when the input signal IN falls from H in the initial state (at time T 0 ) to L, the voltage at the node S 1  is boosted due to the gate-source coupling capacitance of the third n-type transistor N 3 . This causes the possibility that an overvoltage may be applied between the node S 1  and the ground VSS (e.g., across the source-drain of the low-potential side switching element constituting the first inverter  21 ). 
     In view of the above, as in these alterations, by configuring the low-potential side switching element to have two serially-connected n-type transistors, the voltage stress applied to one n-type transistor can be reduced. That is, overvoltage-caused degradation of the n-type transistors can be prevented. 
     In the configurations of  FIGS.  1 ,  3 A, and  3 B , the source of the first n-type transistor N 1  may be connected to the inverted input node NIN, and the source of the second n-type transistor N 2  may be connected to the input node IN.  FIG.  7    shows this alteration from the configuration of  FIG.  1   . In this case, also, substantially the same operations as those in the first embodiment and the alterations described above are performed, and similar effects are obtained. 
     This alteration will be described along the “Operation when input signal IN changes from L to H” in  FIG.  7   , focusing mainly on an operation related to the different point from the configuration of  FIG.  1   . 
     First, in the configuration of  FIG.  7   , in the initial state, since the inverted input signal NIN is H, the source of the first n-type transistor N 1  is H, which is different from the case of  FIG.  1   . However, since the first n-type transistor N 1  is OFF at this time, there is no substantial difference in operation between  FIG.  1    and  FIG.  7   . 
     Once the input signal XIN rises in  FIG.  7   , the input signal IN changes from L to H, causing the source of the second n-type transistor N 2  to become H, which is different from the case of  FIG.  1   . However, since the second n-type transistor N 2  is OFF at this time, there is no substantial difference from the operation in the above embodiment. 
     The operations other than the above are similar to those in the above embodiment described above. That is, while some connections are different between  FIG.  1    and  FIG.  7   , there is no substantial difference in operation between them. 
     The same holds true when the “Operation when input signal IN transitions from H to L” is performed and when changes similar to those in  FIG.  7    are applied to the alterations of  FIGS.  3 A and  3 B . 
     Second Embodiment 
       FIG.  4    shows an example of the circuit diagram of a level shift circuit  1  according to the second embodiment. In  FIG.  4   , components corresponding to those in  FIG.  1    are denoted by the same reference characters. This embodiment will be described hereinafter focusing mainly on different points from the first embodiment. Note that, by denoting elements (e.g., transistors and inverters) by the same reference characters, it is not intended to impose a restriction that these elements are identical in design parameters, process parameters, etc. That is, the technical scope of the present disclosure includes configurations in which elements denoted by the same reference characters in  FIGS.  1  and  4    have different parameters from each other between  FIG.  1    and  FIG.  4   . The same holds true for the relations between other figures. 
     The configuration of  FIG.  4    is different from the configuration of  FIG.  1    in that the gates of the first n-type transistor N 1  and the second n-type transistor N 2  are connected to the first power supply VDD. Also, the configuration of  FIG.  4    is different from the configuration of  FIG.  1    in that the source of the first n-type transistor N 1  is connected to the inverted input node NIN and the source of the second n-type transistor N 2  is connected to the input node IN. 
     —Operation of Level Shift Circuit— 
     Next, the operation of the level shift circuit  1  of this embodiment will be described. 
     &lt;Operation when Input Signal IN Transitions from L to H&gt; 
     The operation of the level shift circuit  1  when the input signal IN transitions from L to H will be described. The signal waveforms are similar to those in  FIG.  2 A . 
     In the initial state (at time T 0 ), the input signal XIN=L, the input signal IN=L, and the inverted input signal NIN=H. Since VDD is applied to the gates of the first n-type transistor N 1  and the second n-type transistor N 2 , the inverted output signal NOUT=H and the output signal OUT=L. Therefore, the output signal S 1  of the first inverter  21  is L, and thus the third n-type transistor N 3  is OFF. Also, the output signal S 2  of the second inverter  22  is H, and thus the fourth n-type transistor N 4  is ON. 
     At time T 1 , once the input signal XIN rises, the input signal IN starts rising from L to H. The gate-source voltage of the second n-type transistor N 2  in the initial state is VDD. Therefore, in the process of rising of the input signal IN, charge is supplied from the first power supply VDD to the output node OUT through the second n-type transistor N 2  until the output signal OUT becomes VDD−Vt (Vt is the threshold voltage of N 2 ). Moreover, since the fourth n-type transistor N 4  remains ON, charge is supplied from the first power supply VDD to the output node OUT through the fourth n-type transistor N 4 . This charge supply through the second n-type transistor N 2  and the fourth n-type transistor N 4  assists the rising of the output signal OUT whereby the rising can be advanced. 
     Also, when the inverted input signal NIN becomes L, the inverted output signal NOUT starts falling through the first n-type transistor N 1 . When the inverted output signal NOUT falls, the second p-type transistor P 2  turns ON, causing start of charge supply from the third power supply VDDIO to the output node OUT. 
     Since the second inverter  22  is a circuit operating with the third power supply VDDIO, when the voltage of the output signal OUT rises up to a threshold determined based on the third power supply VDDIO, the output signal S 2  changes from H to L. That is, the second inverter  22  changes from H to L after a lapse of a predetermined delay time. The fourth n-type transistor N 4  then turns OFF, terminating the assist operation by the rise assist circuit  2 . 
     Since the second p-type transistor P 2  is ON, the output signal OUT finally rises up to the third power supply VDDIO. 
     As described above, in this embodiment, as in the first embodiment, the termination time of the assist operation can be extended depending on the delay time of the second inverter  22 . 
     Note that, when the inverted output signal NOUT falls, causing the output signal S 1  of the first inverter  21  to become H, the third n-type transistor N 3  turns ON. However, since both the inverted input signal NIN and the inverted output signal NOUT are L, i.e., both the source and drain of the third n-type transistor N 3  are L, the third n-type transistor N 3 , though it is ON, will have no influence on the operations of other circuits. 
     &lt;Operation when Input Signal IN Transitions from H to L&gt; 
     The operation of the level shift circuit  1  when the input signal IN transitions from H to L will be described. The signal waveforms are similar to those in  FIG.  2 B . 
     In the initial state (at time T 0 ), the input signal XIN=H, the input signal IN=H, and the inverted input signal NIN=L. Since VDD is applied to the gates of the first n-type transistor N 1  and the second n-type transistor N 2 , the inverted output signal NOUT=L and the output signal OUT=H. Therefore, the output signal Si of the first inverter  21  is H, and thus the third n-type transistor N 3  is ON. Also, the output signal S 2  of the second inverter  22  is L, and thus the fourth n-type transistor N 4  is OFF. 
     At time T 1 , once the input signal XIN falls, the inverted input signal NIN starts rising from L to H. The gate-source voltage of the first n-type transistor N 1  in the initial state is VDD. Therefore, in the process of rising of the inverted input signal NIN, charge is supplied from the first power supply VDD to the inverted output node NOUT through the first n-type transistor N 1  until the inverted output signal NOUT becomes VDD−Vt (Vt is the threshold voltage of N 1 ). Moreover, since the third n-type transistor N 3  remains ON, charge is supplied from the first power supply VDD to the inverted output node NOUT through the third n-type transistor N 3 . This charge supply through the first n-type transistor N 1  and the third n-type transistor N 3  assists the rising of the inverted output signal NOUT whereby the rising can be advanced. 
     Also, when the input signal IN becomes L, the output signal OUT starts falling through the second n-type transistor N 2 . When the output signal OUT falls, the first p-type transistor P 1  turns ON, causing start of charge supply from the third power supply VDDIO to the inverted output node NOUT. 
     Since the first inverter  21  is a circuit operating with the third power supply VDDIO, when the voltage of the inverted output signal NOUT rises up to a threshold determined based on the third power supply VDDIO, the output signal S 1  changes from H to L. That is, the first inverter  21  changes from H to L after a lapse of a predetermined delay time. The third n-type transistor N 3  then turns OFF, terminating the assist operation by the rise assist circuit  2 . 
     Since the first p-type transistor P 1  is ON, the inverted output signal NOUT finally rises up to the third power supply VDDIO. 
     As described above, in this embodiment, as in the first embodiment, the termination time of the assist operation can be extended depending on the delay time of the inverter  21 ,  22 . 
     Note that, when the output signal OUT falls, causing the output signal S 2  of the second inverter  22  to become H, the fourth n-type transistor N 4  turns ON. However, since both the input signal IN and the output signal OUT are L, i.e., both the source and drain of the fourth n-type transistor N 4  are L, the fourth n-type transistor N 4 , though it is ON, will have no influence on the operations of other circuits. 
     —Functions and Effects of Second Embodiment— 
     As described above, in the level shift circuit  1  of the second embodiment, as in the first embodiment, even when the output state of the output signal OUT changes, it is after a lapse of the delay time of the second inverter  22  from this change that the rise assist circuit  2  stops its operation. The same holds true for the output signal NOUT. In comparison with the prior art, therefore, sufficient assist operation can be obtained. Also, as in the first embodiment, there is no such place where transistors on both sides of a given node become OFF simultaneously, causing a floating state, in the process of the assist operation. Therefore, no problem about boosting of the voltage at a given node will occur. 
     Moreover, in this embodiment, the gates of the first and second n-type transistors N 1  and N 2  are connected to the first power supply VDD, the source of the first n-type transistor N 1  is connected to the inverted input node NIN, and the source of the second n-type transistor N 2  is connected to the input node IN. Therefore, in each of the first and second n-type transistors N 1  and N 2 , the voltage applied across the source/drain/gate can be reduced in comparison with the case of the first embodiment. Specifically, in the first embodiment, there can be a case where, in the first n-type transistor N 1 , the gate voltage is VSS, the source voltage is VSS, and the drain voltage is VDDIO, for example. In this case, the source-drain voltage is VDDIO. By contrast, in the second embodiment, in the first n-type transistor N 1  or the second n-type transistor N 2 , when the drain voltage is VDDIO, the gate voltage and the source voltage are VDD. The source-drain voltage is therefore VDDIO−VDD, which is low in comparison with that in the first embodiment. In this embodiment, therefore, since a transistor with a low breakdown voltage but high speed can be used as the first n-type transistor N 1 , the circuit operation can be sped up in comparison with the first embodiment. 
     (Alteration 1) 
       FIG.  5    shows an alteration of the level shift circuit  1  according to the second embodiment. In  FIG.  5   , components corresponding to those in  FIG.  4    are denoted by the same reference characters. This alteration will be described hereinafter focusing mainly on different points from the second embodiment. Basic operations are similar to those of the second embodiment. 
     This alteration is different from the configuration of  FIG.  4    in that a fifth n-type transistor N 5  is provided between the inverted output node NOUT and the first n-type transistor N 1 , and a sixth n-type transistor N 6  is provided between the output node OUT and the second n-type transistor N 2 . 
     Specifically, the fifth n-type transistor N 5  has a gate connected to the third power supply VDDIO and a source connected to the drain of the first n-type transistor N 1 . The drain of the fifth n-type transistor N 5  is connected to the inverted output node NOUT, the input of the first inverter  21 , and the drain of the third n-type transistor N 3 . 
     The sixth n-type transistor N 6  has a gate connected to the third power supply VDDIO and a source connected to the drain of the second n-type transistor N 2 . The drain of the sixth n-type transistor N 6  is connected to the output node OUT, the input of the second inverter  22 , and the drain of the fourth n-type transistor N 4 . 
     According to this alteration, the maximum value of the source voltage of the fifth n-type transistor N 5 , i.e., the drain voltage of the first n-type transistor N 1  can be VDDIO−Vt (Vt is the threshold voltage of the fifth n-type transistor N 5 ). The same holds true for the drain voltage of the second n-type transistor N 2 . It is therefore possible to reduce the potential difference between the source and drain of each of the first and second n-type transistors N 1  and N 2 , and thus transistors low in breakdown voltage can be used as the first and second n-type transistors N 1  and N 2 . 
     The embodiments and the alterations described above can be combined together appropriately. For example, in  FIGS.  4  and  5   , as the first inverter  21  and the second inverter  22 , the configuration shown in  FIG.  3 A  may be used, or the configuration shown in  FIG.  3 B  may be used, in place of the configuration shown in  FIG.  1   . In this case, also, effects similar to those obtained in the first embodiment can be obtained. 
     Specifically, in the configurations of  FIGS.  4  and  5   , the circuit shown in X of  FIG.  3 A  may be adopted, so that the low-potential side switching element of the first inverter  21  may have a configuration in which two n-type transistors each having a gate connected to the inverted output node NOUT are serially connected. Also, the low-potential side switching element of the second inverter  22  may have a configuration in which two n-type transistors each having a gate connected to the output node OUT are serially connected. 
     Similarly, in the configurations of  FIGS.  4  and  5   , the circuit shown in X of  FIG.  3 B  may be adopted, so that the low-potential side switching element of the first inverter  21  may have a configuration in which an n-type transistor having a gate connected to the third power supply VDDIO (corresponding to the fifth n-type transistor) and an n-type transistor having a gate connected to the inverted output node NOUT (corresponding to the sixth n-type transistor) are serially connected. Also, the low-potential side switching element of the second inverter  22  may have a configuration in which an n-type transistor having a gate connected to the third power supply VDDIO (corresponding to the seventh n-type transistor) and an n-type transistor having a gate connected to the output node OUT (corresponding to the eighth n-type transistor) are serially connected. 
     (Alteration 2) 
       FIG.  8    shows another alteration of the level shift circuit  1  according to the second embodiment. In  FIG.  8   , components corresponding to those in  FIG.  4    are denoted by the same reference characters. This alteration will be described hereinafter focusing mainly on different points from the second embodiment. Basic operations are similar to those of the second embodiment. 
     In  FIG.  8   , a power supply detection circuit  4  is provided in addition to the configuration of  FIG.  4   . The power supply detection circuit  4  includes two n-type transistors N 41  and N 42  and two inverters  43  and  44 . 
     The n-type transistor N 41  is provided between the third power supply VDDIO and a node Xp, and has a gate connected to the first power supply VDD. The n-type transistor N 42  has a gate connected to the third power supply VDDIO, a source connected to the node Xp, and a drain connected to the first power supply VDD. The inverters  43  and  44  are serially connected between the node Xp and the output of the power supply detection circuit  4 . Although not shown, the inverters  43  and  44  each have a power supply terminal connected to the first power supply VDD and a ground terminal connected to the ground VSS. 
     In this alteration, in place of the first power supply VDD, the output of the power supply detection circuit  4  is connected to the gates of the first n-type transistor N 1  and the second n-type transistor N 2 . In other words, the gates of the first n-type transistor N 1  and the second n-type transistor N 2  are connected to the first power supply VDD through the power supply detection circuit  4 . 
     The power supply detection circuit  4  outputs the first power supply VDD when both the first power supply VDD and the third power supply VDDIO are ON, and the output becomes the ground VSS when the first power supply VDD is ON and the third power supply VDDIO is OFF. 
     Specifically, (1) in the case of both the first power supply VDD and the third power supply VDDIO being ON, when VDDIO−Vt2&lt;VDD (Vt2 is the threshold voltage of the n-type transistor N 42 ), the voltage Xp at the node Xp is VDDIO−Vt2 by the function of the n-type transistor N 42 . Similarly, (2) in the case of both the first power supply VDD and the third power supply VDDIO being ON, when VDDIO−Vt2&gt;VDD, the voltage Xp at the node Xp is VDD. In whichever case, (1) or (2), the input of the inverter  43  is H, and therefore the output of the inverter  44  is also H. That is, the first power supply VDD is supplied from the power supply detection circuit  4  to the gates of the first n-type transistor N 1  and the second n-type transistor N 2 . 
     When the first power supply VDD is ON and the third power supply VDDIO is OFF (VDDIO=VSS), in which the n-type transistor N 41  is ON and the n-type transistor N 42  is OFF, the voltage Xp at the node Xp is VSS. In this case, the input of the inverter  43  is L, and therefore the output of the inverter  44  is also L. That is, the ground VSS is supplied from the power supply detection circuit  4  to the gates of the first n-type transistor N 1  and the second n-type transistor N 2 . 
     Having the above configuration, it is ensured that there will be no place where a flow-through current flows in the level shift circuit  1  whichever power supply, the first power supply VDD or the third power supply VDDIO, rises first. 
     The level shift circuit according to the present disclosure is very useful since it can further enhance the effectiveness of the assist operation and also ease the voltage stress to transistors.