Abstract:
A voltage dropping circuit generating a second power source voltage to output to a second node by dropping a first power source voltage supplied to a first node, includes: an output transistor having a first terminal to which the first power source voltage is supplied and a second terminal connected to the second node turns on or off according to a difference between the second power source voltage and a reference voltage; and a back gate variable diode circuit including a diode-connected transistor connected between the first node and the first terminal and to configured to turn on or off according to a voltage difference between the first and second power sources, wherein the first power source voltage is applied to the back gate of the diode-connected transistor when it is higher than the second power source voltage, and the second power source voltage is applied in other case.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2015-016008, filed on Jan. 29, 2015, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The technique disclosed herein relates to a voltage dropping circuit and an integrated circuit. 
     BACKGROUND 
     In recent years, a reduction in power consumption of electronic equipment has been desired and the voltage for operating a transistor mounted in an integrated circuit is controlled precisely for each kind of circuit. An integrated circuit has a circuit portion that operates on a base voltage that is supplied from the outside and a circuit portion that operates on a voltage other than the base voltage. The voltage other than the base voltage is generated from the base voltage by using a charge pump circuit or the like, or from the base voltage or a power source voltage that is generated separately by using a low drop out circuit. The power source circuit of the integrated circuit such as this is called an adaptive supply voltage (ASV) system. 
     Further, in order to reduce the power consumption of an integrated circuit, it is effective to reduce the leak current of a transistor that is mounted in the integrated circuit. As one of method for reducing the leak current of a transistor, an adapting body bias (ABB) system that controls the back gate potential of a transistor is known. The back gate voltage that controls the back gate potential of a transistor is generated by, for example, a low drop out circuit because the current-carrying capacity is small. 
     In the case where a low drop out circuit that implements the ABB system is provided in an integrated circuit adopting the above-described ASV system, the back gate voltage that is higher than the base voltage is generated from the base voltage whose power source supply capacity is high while the back gate voltage is equal to or less than the base voltage. Specifically, a capacitive element that holds the back gate voltage is charged up to the back gate voltage by the low drop out circuit after being changed up to the base voltage by the base power source. In the low drop out circuit, for example, a transistor is connected between the terminal to which the high-voltage power source voltage is supplied and the terminal from which the back gate voltage is output, and the turning-on/off of the transistor is controlled in accordance with the results of comparison between the back gate voltage and a reference potential. 
     As described previously, in the power source sequence in the ASV system, the high-voltage power source voltage is generated by the charge pump or the like, and therefore, the supply of the high-voltage power source voltage to each unit within the integrated circuit is delayed from the supply of the base voltage. Consequently, the supply of the high-voltage power source voltage to the low drop out circuit is delayed from the supply of the base voltage. Due to this, in the low drop out circuit, the base voltage is applied to the terminal from which the back gate voltage is output before the high-voltage power source voltage is supplied, and therefore, a current flows backward. 
     In order to prevent such a backflow of a current in the low drop out circuit, a transistor is diode-connected between the transistor of the low drop out circuit and the supply terminal of the high-voltage power source voltage. 
     RELATED DOCUMENTS 
     
         
         [Patent Document 1] Japanese Laid Open Patent Publication No. 2004-260052 
         [Patent Document 2] Japanese Laid Open Patent Publication No. S62-109114 
         [Patent Document 3] Japanese Laid Open Patent Publication NO. 2013-025695 
       
    
     SUMMARY 
     According to a first aspect of embodiments, a voltage dropping circuit configured to generate a second power source voltage by dropping a first power source voltage that is supplied to a first node, and to output the second power source voltage to a second node, includes: an output stage transistor, the first power source voltage being configured to be supplied to a first terminal of the output stage transistor, a second terminal of the output stage transistor being connected to the second node, the output stage transistor being configured to turn on or off in accordance with a magnitude relationship between the second power source voltage and a reference voltage; and a back gate variable diode circuit including a diode-connected transistor that is connected between the first node and the first terminal and configured to turn on or off in accordance with a magnitude relationship between the first power source voltage and the second power source voltage, wherein the first power source voltage is applied to the back gate of the diode-connected transistor when the first power source voltage is higher than the second power source voltage, and the second power source voltage is applied to the back gate of the diode-connected transistor when the second power source voltage is higher than the first power source voltage. 
     According to a second aspect of embodiments, an integrated circuit includes: a first power source circuit configured to generate a first power source voltage from a base voltage that is supplied from the outside; a voltage dropping circuit configured to generate a second power source voltage by dropping the first power source voltage and to output the second power source voltage to a second node; and a logic circuit configured to operate based on the second power source voltage, wherein the second power source voltage is generated from the base voltage when the second power source voltage is lower than the base voltage, and is generated by the voltage dropping circuit after the second power source voltage reaches the base voltage, and the voltage dropping circuit includes: a first node to which the first power source voltage is supplied; an output stage transistor, the first power source voltage being supplied to a first terminal of the output stage transistor, a second terminal of the output stage transistor being connected to the second node, the output stage transistor being configured to turn on or off in accordance with a magnitude relationship between the second power source voltage and a reference voltage; and a back gate variable diode circuit including a diode-connected transistor that is connected between the first node and the first terminal and configured to turn on or off in accordance with a magnitude relationship between the first power source voltage and the second power source voltage, wherein the first power source voltage is applied to the back gate of the diode-connected transistor when the first power source voltage is higher than the second power source voltage, and the second power source voltage is applied to the back, gate of the diode-connected transistor when the second power source voltage is higher than the first power source voltage. 
     The object and advantages of the embodiments will be realized and attained by means of the elements and combination particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a configuration example of a circuit and a power source system within an integrated circuit; 
         FIG. 2A  illustrates a circuit configuration example Of the low drop out (LDO) circuit; 
         FIG. 2B  illustrates a change in the Pch back gate voltage (VNW) due to the power source sequence when starting the power source in the circuit in  FIG. 2A ; 
         FIG. 2C  illustrates a sectional structure of an output stage transistor of the LDO; 
         FIG. 3A  illustrates a first circuit example of the low drop out circuit (LDO) that prevents the backflow of a current; 
         FIG. 3B  illustrates a second circuit example of the low drop out circuit (LDO) that prevents the backflow of a current; 
         FIG. 3C  illustrates a sectional structure of a backflow preventing transistor that is added to the second circuit example; 
         FIG. 4A  illustrates a low drop out circuit (LDO) of a first embodiment; 
         FIG. 4B  illustrates an equivalent circuit of the LDO of the first embodiment when VNW&gt;VDE; 
         FIG. 4C  illustrates an equivalent circuit of the LDO of the first embodiment when VNW&lt;VDE; 
         FIG. 4D  illustrates a sectional structure of a transistor that forms a back gate variable diode circuit; 
         FIG. 5A  illustrates a low drop out circuit (LDO) of a second embodiment; 
         FIG. 5B  illustrates an equivalent circuit of the LDO of the second embodiment when VNW&gt;VDE; 
         FIG. 5C  illustrates an equivalent circuit of the LDO of the second embodiment when VNW&lt;VDE; 
         FIG. 6A  illustrates a low-drop DC/DC converter of a third embodiment; 
         FIG. 6B  illustrates an equivalent circuit of the low-drop DC/DC converter of the third embodiment when Vout&gt;VDE; 
         FIG. 6C  illustrates an equivalent circuit of the low-drop DC/DC converter of the third embodiment when Vout&lt;VDE. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Before explaining a low drop out circuit and an integrated circuit that makes use of the low drop out circuit of an embodiment, a general integrated circuit adopting the ABB and ASV systems and a low drop out circuit are explained. 
       FIG. 1  illustrates a configuration example of a circuit and a power source system within an integrated circuit. 
     An integrated circuit  10  has a P-type substrate (Psub)  11 . On the P-type substrate  11 , an I/O circuit  12 , a PLL circuit  13 , an AD/DA conversion circuit  14 , a USB interface circuit  15 , a DDR circuit  16 , an ABB+ASV circuit unit  20 , and a well  30  that forms a logic circuit are formed. 
     The I/O circuit  12  inputs and outputs data and signals from and to the outside. The PLL circuit  13  generates an operation clock. The AD/DA conversion circuit  14  converts an analog signal into digital data and converts digital data into an analog signal. The USB interface circuit IS interfaces with a USB memory. The DDR (Double Data Rate) circuit  16  inputs and outputs data at high speed to and from an external DRAM board. 
     The ABB+ASV circuit unit  20  is a power source circuit of the integrated circuit  10  and protects the power source and implements the ABB system and the ASV system. The ABB+ASV circuit unit  20  has a charge pump  21 , a low drop out (LDO)  22 , a thermometer  23 , a process monitor  24 , and an electrically programmable fuse element (E-Fuse)  25 . The LDO circuit is an example of voltage dropping circuits. 
     In the well  30 , a first logic circuit (Logic1)  31 , a second logic circuit (Logic2)  32 , and an SRAM  33  are formed. The supply of a base power source voltage to the second logic circuit is controlled by the ASV system by using a power switch  17  provided outside the well  30 . 
     The configuration illustrated in  FIG. 1  is an example and is designed appropriately in accordance with specifications. 
     The integrated circuit having the configuration such as described above has a power source wire through which a power source voltage necessary for the operation of each circuit portion is supplied. In the configuration illustrated in  FIG. 1 , the integrated circuit has a base power source (VDD) wire  40 , a ground (VSS) wire  41 , a high-voltage power source (VDE) wire  42 , a Pch back gate voltage (VNW) wire  43 , and an Nch back gate voltage (VPW) wire  44 . To the VDD wire  40  and the VSS wire  41 , the base power source (VDD) is supplied from an external power source  1 . The external power source  1  is, for example, a IV power source and the VSS wire  41  becomes the GND (0 V) and the VDD wire  40  becomes 1 V. The high-voltage power source (VDE) is, for example, a 3.3V power source and is used for inputting/outputting with already-existing external equipment. The VDE is generated from the VDD power source by the CP  21  and for stabilization of the power source, a capacitive element  45  and a Schottky barrier diode (SBD)  46  are connected in parallel between the VDE wire  43  and the VSS wire  41 . There is a case where the capacitive element  45  and the SBD  46  are provided within the integrated circuit  10 , but the size is large, and therefore, it is common for them to be attached externally to the integrated circuit  10  as illustrated in  FIG. 1 . The VNW is a voltage that controls the back gate potential of the Pch transistor by the ABB system and is generated by the LDO  22  from the VDE power source at a voltage between the VDE power source voltage and the VDD power source voltage. The VPW is a voltage that controls the back gate potential of the Nch transistor by the ABB system and is a negative voltage, and is generated from the VDD power source by the CP  21 . In the case of the VPW also, for stabilization of the power source, between the VPW wire  44  and the VSS wire  41 , an external capacitive element  47  and an SBD  48  are connected in parallel. 
     The power source wire is formed on the P-type substrate  11 , but in  FIG. 1 , the power source wire is illustrated separate from the P-type substrate  11  for making the power source wire easy-to-see. 
     The circuit configuration and the power source configuration of the integrated circuit illustrated in  FIG. 1  are widely known, and therefore, more explanation is omitted. 
       FIG. 2A  illustrates a circuit configuration example of the low drop out (LDO) circuit.  FIG. 2B  illustrates a change in the Pch back gate voltage (VNW) due to the power source sequence when starting the power source in the circuit in  FIG. 2A .  FIG. 2C  illustrates a sectional structure of an output stage transistor of the LDO. The LDO circuit is an example of voltage dropping circuits. 
     The LDO  22  has an output stage transistor PTr 1 , an amplifier (AMP) that functions as a comparison circuit, a voltage-dividing circuit of the VDD, a voltage-dividing circuit of the VNW, a charging circuit between the VNW and the VDD. In  FIG. 2A , as an example of a logic circuit to which the VDD, VSS, VNW, and VPW are supplied, an inverter circuit is also illustrated. 
     The output stage transistor PTr 1  is connected between the VDE wire  42  and the VNW wire  43  and the back gate is connected to the VDE wire  42 . Here, the controlled terminal (source) that is connected to the VDE wire  42  of the PTr 1  is referred to as a first terminal and the controlled terminal (drain) that is connected to the VNW wire  42  of the PTr 1  is referred to as a second terminal. Further, there is a case where the VDE wire  42  that is connected to the PTr 1  is referred to as a first node and the VNW wire  43  that is connected to the PTr 1  as a second node. Furthermore, there is a case where the VDE (high-voltage power source, voltage) is referred to as a first power source voltage, the VNW (Pch back gate voltage) as a second power source voltage, and the GND (ground) as a third power source voltage. 
     The voltage-dividing circuit of the VDD has two resistors R 11  and R 12  connected in series between the VDD wire  40  and the VSS wire  41  and generates a reference voltage by dividing the VDD in a ratio between the resistances of R 11  and R 12 . The voltage-dividing circuit of the VNW has two resistors R 21  and R 22  connected in series between the VNW wire  43  and the VSS wire  41  and generates the divided voltage VNW by dividing the VNW in a ratio between the resistances of R 21  and R 22 . The AMP compares the reference voltage with the divided voltage VNW and increases the output voltage in the case where the divided voltage VNW is higher than the reference voltage, and reduces the output voltage in the case where the divided voltage VNW is lower than the reference voltage. Due to this, the amount of the current flowing through the PTr 1  is reduced in the case where the VNW is higher than a predetermined voltage and the amount of the current flowing through the PTr 1  increases in the case where the VNW is lower than the predetermined, voltage, and thereby, the VNW is controlled to be a predetermined voltage. 
     In the case where the VNW is higher than the VDD and lower than the VDE, if all the charges for charging the capacitive element  45  of the VNW are generated by dropping the VDE when starting the power source, the burden of the CP  21  is too heavy, and therefore, it is necessary to increase the drive force of the CP  21  in order to shorten the time taken for the power source to start. Consequently, when starting the power source, the capacitive element  45  is charged through the VDD power source wire  40  until the VNW reaches the VDD and after the VNW reaches the VDD, the VNW is increased to a predetermined voltage by the LDO  22 . Because of this, as illustrated in  FIG. 2A , there are provided a diode D 1  and a switch SW connected in parallel between the VDD power source wire  40  and the VNW power source wire  43 . The diode D 1  is connected so that the forward direction is from the VDD toward the VNW. The switch SW is controlled by the VDD and turns on when the VDD reaches about 1 V and turns off when the VNW becomes higher than the VDD. 
     To the back gate of the PMOS that is formed in the first logic circuit  31  and the second logic circuit  32 , the VNW is applied and to the back gate of the NMOS, the VPW is applied, when the values of the VNW and VPW are changed, the power consumption of the PMOS and the NMOS changes. 
     If the supply of power source from the external power source  1  is started to the integrated circuit  10  at the time of startup, the VDD begins to increase as illustrated in  FIG. 2B . When, the VDD reaches about 0.3 V, a current flows through the Schottky barrier diode (SBD) D 1 , and therefore, the VNW increases along the broken line indicated by X with the state where the voltage is lower than the VDD by about 0.3 V being kept. When the VDD reaches about 1 V, the SW turns on and the VNW reaches a voltage almost the same as the VDD in a brief time as represented by the broken line indicated by Y. The generation of the VDE by the CP  21  delays from the startup by a certain period of time, and therefore, the VDE remains 0 V. 
     As illustrated in  FIG. 2C , the PTr 1  is formed in an N well (N-well)  51  formed on the P-sub  11 . The PTr 1  has a drain electrode  52  and a source electrode  54  in the P+ region on the H well  51 , a gate electrode  53  formed right above the N well  51  between the drain electrode  52  and the source electrode  54 , and a back gate electrode  55  in the n+ region of the N well  51 . The source electrode  54  and the back gate electrode  55  are connected to the VDE wire  42  and the drain electrode  52  is connected to the VNW wire  43 . The P-sub  11  is connected to the VSS wire  41  via a region  56  and the voltage is the GND (0 V). 
     As described above, at the time of starting the power source, in the state where the VDE is 0 V, the VNW becomes the VDD (1 V). When such a state is brought about, as illustrated in  FIG. 2C , a diode whose forward direction is from the drain electrode  52  toward the back gate electrode  55  of the PTr 1  is formed and a current flows backward from the VNW wire  43  to the VDE wire  42 . 
       FIG. 3A  illustrates a first circuit example of the low drop out circuit (LDO) that prevents the backflow of a current.  FIG. 3B  illustrates a second circuit example of the low drop out circuit (LDO) that prevents the backflow of a current.  FIG. 3C  illustrates a sectional structure of a backflow preventing transistor that is added to the second circuit example. 
     The LDO in the first circuit, example illustrated in  FIG. 3A  is a circuit in which the P-channel PTr 1  has been replaced with the N-channel PTr 1  and the backflow from the VNW wire  43  to the VDE wire  42  is prevented by connecting the back gate to the VSS wire (GND). However, the LDO in  FIG. 3A  back-biases (about 1 V) the back gate of the output stage transistor, and therefore, the drive force of the output, stage transistor is reduced. Further, by changing the P channel to the N channel, the BSD (Electro-Static Discharge) resistance between the VDE wire  42  and the VNW wire  43  is reduced. 
     The LDO in the second circuit example illustrated in  FIG. 3B  is a circuit, in which a P-channel PTr 2  has been further connected between the output stage transistor PTr 1  and the VDE wire  42 . The PTr 2  is diode-connected and the back gate is connected to the drain (source of the PTr 1 ). The PTr 2  connected like this forms a diode whose forward direction is from the VDE wire  42  toward the source or the PTr 1 . Due to this, the backflow from the VNW wire  43  to the VDE wire  42  via the PTr 1  is prevented. The sectional structure of the PTr 2  illustrated in  FIG. 3C  is the same as that explained in  FIG. 2C , and therefore, explanation is omitted. 
     However, in the LDO in  FIG. 3B , the back gate of the PTr 2  is forward-biased during the normal operation and as illustrated in  FIG. 3C , there is a possibility that an overcurrent will flow through a diode that is formed so that the forward direction is from a source electrode  62  toward a back gate electrode  65  of the PTr 2 . 
     In the embodiment that is explained below, a low drop out circuit (LDO) is disclosed, which prevents the backflow of a current, and at the same time, through which an overcurrent does not flow during the normal operation. 
       FIG. 4A  illustrates a low drop out circuit (LDO) of a first embodiment.  FIG. 4B  illustrates an equivalent circuit of the LDO of the first embodiment when VNW&gt;VDE.  FIG. 4C  illustrates an equivalent circuit of the LDO of the first embodiment, when VNW&lt;VDE.  FIG. 4D  illustrates a sectional structure of a transistor that forms a back gate variable diode circuit. 
     It is possible to use the low drop out circuit (LDO) of the first embodiment as the LDO  22  of the integrated circuit in  FIG. 1 . 
     As illustrated in  FIG. 4A , the low drop out circuit (LDO) of the first embodiment has a back gate variable diode circuit that is connected between the output stage transistor PTr 1  and the VDE wire  42 . In other words, the LDO of the first embodiment differs from the LDO illustrated in  FIG. 3B  in that the back gate variable diode circuit is provided in place of the PTr 2 . 
     The LDO of the first embodiment has the output stage transistor PTr 1 , the AMP, the voltage-dividing circuit of the VDD including R 11  and R 12 , the voltage-dividing circuit of the VNW including R 21  and R 22 , the charging circuit between VNW and VDD including D 1  and SW, and the back gate variable diode circuit. The portions other than the back gate variable diode circuit are the same as the elements explained in  FIG. 2A  to  FIG. 3C , and therefore, explanation is omitted. 
     The back gate variable diode circuit has Pch transistors PTr 21 , PTr 22 , and PTr 23 . The PTr 21  is diode-connected between the output stage transistor PTr 1  and the VDE wire  42 . In other words, the gate of the PTr 21  is connected to the drain of the PTr 21  (source of PTr 1 ). The PTr 22  and PTr 23  are connected in series between the PTr 1  and the VDE wire  42 , and in parallel to the PTr 21 . The gate of the PTr 22  is connected to the VDE wire  42 , the gate of the PTr 23  is connected to the source of the PTr 1 , and the back gates of the PTr 22  and PTr 23  are connected to the connection node of the PTr 22  and PTr 23 . Further, the back gate of the PTr 21  is connected to the connection node of the PTr 22  and PTr 23 . Here, the potential of the source of the PTr 1  is denoted by Va. 
     When VNW&gt;VDE, the LDO in  FIG. 4A  becomes the equivalent circuit illustrated in  FIG. 4B . In other words, the back gate variable diode circuit is represented by the PTr 21  that is diode-connected and whose back gate is connected to the source of the PTr 1 . When VNW&gt;VDE, Va&gt;VDE (VNW&gt;Va&gt;VDE) holds, and the PTr 22  turns on and the PTr 23  turns off. Because of this, the back gate of the PTr 21  is connected to the source of the PTr 1  and a state where Va is applied is brought about. As explained in  FIG. 3B , the PTr 21  in  FIG. 4B  functions as a diode whose forward direction is from the VDE wire  42  toward the PTr 1 , and therefore, the backflow from the VNW wire  43  to the VDE wire  42 , which occurs when VNW&gt;VDE, is prevented. 
     When VNW&lt;VDE (during normal operation), the LDO in  FIG. 4A  becomes the equivalent circuit illustrated in  FIG. 4C . In other words, the back gate variable diode circuit is represented by the PTr 21  that is diode-connected and whose back gate is connected to the VDE wire  42 . When VNW&lt;VDE, Va&lt;VDE (VNW&lt;Va&lt;VDE) holds, and the PTr 22  turns off and the PTr 23  turns on. Because of this, the back gate of the PTr 21  is connected to the VDE wire  42  and a state where the VDE is applied is brought about. The PTr 21  in this state is in the conduction state and allows a current from the VDE wire  42  to the PTr 1  to pass. 
     The PTr 21  in the state in  FIG. 4C  is in the state where the VDE is applied to the source electrode  62 , Va is applied to a gate electrode and a drain electrode  64 , and the VDE is applied to the back gate electrode  65  as illustrated in  FIG. 4D . Consequently, no forward bias is applied to the back gate and a diode whose forward direction is from the source electrode  62  toward the back gate electrode  65  is not formed, and therefore, it is unlikely that an overcurrent flows. 
     As explained above, the low drop out circuit (LDO) of the first embodiment prevents the occurrence of an overcurrent when VNW&lt;VDE, as well as preventing a backflow when VNW&gt;VDE. 
       FIG. 5A  illustrates a low drop out circuit (LDO) of a second embodiment.  FIG. 5B  illustrates an equivalent circuit of the LDO of the second embodiment when VNW&gt;VDE.  FIG. 5C  illustrates an equivalent circuit of the LDO of the second embodiment when VNW&lt;VDE. 
     It is also possible to use the low drop out circuit (LDO) of the second embodiment as the LDO  22  of the integrated circuit in  FIG. 1 . 
     The LDO of the second embodiment differs from that of the first embodiment in that the gate of the PTr 21  of the back gate variable diode circuit is not connected to the source of the PTr 1  but is connected to the drain of the PTr 1 . 
     As in the first embodiment, the LDO of the second embodiment becomes the equivalent circuit illustrated in  FIG. 5B  when VNW&gt;VDE and prevents the backflow from the VNW wire  43  to the VDE wire  42 . Further, the LDO of the second embodiment becomes the equivalent circuit illustrated in  FIG. 5C  when VNW&lt;VDE (during normal operation) and prevents the forward bias of the back gate of the PTr 21 . 
     In the first embodiment, when VNW&lt;VDE (during normal operation), a gate-source voltage Vgs of the PTr 1  is reduced due to a drain-source voltage Vds of the PTr 21 , and therefore, the drive force of the LDO is reduced. In contrast to this, in the second embodiment, the gate potential of the PTr 21  is connected to the VNW wire  43 , which is lower than Va, and therefore, the gate-source voltage Vgs of the PTr 21  increases and it is possible to reduce the drain-source voltage Vds of the PTr 21 . Hereinafter, the principle that the Vds of the PTr 21  is reduced is explained. 
     A drain current Id in the saturation region of a MGS transistor is expressed as Id=1/2×W/L×μ×Co×(Vgs−Vth) 2 ×(1+λVds). Here, W is the channel width, L is the channel length, μ is the mobility. Co is a gate oxide film, Vgs is the gate-source voltage, Vth is a threshold value, λ is the channel length modulation coefficient, and Vds is the drain-source voltage. 
     In the LDO of the first embodiment, it is assumed that the drain current of the PTr 21  is denoted as Ids 1 , the gate-source voltage as Vgs 1 , and the drain-source voltage as Vds 1 . Similarly, in the LDO of the second embodiment, it is assumed that the drain current of the PTr 21  is denoted as Ids 2 , the gate-source voltage as Vgs 2 , and the drain-source voltage as Vds 2 . Then, if it is supposed that W, L, μ, Co, Vth, and λ are the same in the first and second embodiments, and Ids 1 =Ids 2 , then Vgs 1 &lt;Vgs 2 , and therefore, Vds 1 &gt;Vds 2  holds. 
     Consequently, the potential Va of the source of the PTr 1  increases, the Vgs of the PTr 1  increases, and the drive force of the LDO increases. 
     As explained above, the low drop out circuit (LDO) of the second embodiment prevents the occurrence of an overcurrent when VNW&lt;VDE, as well as preventing the backflow when VNW&gt;VDE, and the drive force of the output stage transistor PTr 1  when VNW&lt;VDE (during normal operation) is high compared to that of the first embodiment. 
     It is also possible to apply the back gate variable diode circuits explained in the first and second embodiments to a low-drop DC/DC converter. 
       FIG. 6A  illustrates a low-drop DC/DC converter of a third embodiment.  FIG. 6B  illustrates an equivalent circuit of the low-drop DC/DC converter of the third embodiment when Vout&gt;VDE.  FIG. 6C  illustrates an equivalent circuit of the low-drop DC/DC converter of the third embodiment when Vout&lt;VDE. The low-drop DC/DC converter is an example of voltage dropping circuits. 
     The low-drop DC/DC converter of the third embodiment generates an output voltage Vout by dropping the high voltage VDE. The low-drop DC/DC converter has the output stage transistor PTr 1 , a back gate variable diode circuit, an inductor (coil) L, a capacitive element G, a diode D 10 , a voltage-dividing circuit, a reference power source Vref, an AMP  10 , and a PWM control circuit  71 . 
     The source (first terminal) of the PTr 1  is connected to the VDE wire  42  via the back gate variable diode circuit. The back gate variable diode circuit is the same as that of the first embodiment. The gate of the PTr 1  is connected to the output of the PWM control circuit  71 . The drain (second terminal) of the PTr 1  is connected to the VSS wire (GND) via the diode D 10 . The diode  10  is connected so that the direction from the GND toward the second terminal of the PTr 1  is the forward direction. The inductor L is connected to the second terminal of the PTr 1  and the second node (VNW wire)  43 . The capacitive element C is connected between the second node and the GND. The voltage-dividing circuit has two resistors R 31  and R 32  connected in series between the second node and the GND. The resistors R 31  and R 32  output the Vout divided voltage, which is obtained by dividing the output voltage Vout that appears at the second node in a ratio between the resistances of R 31  and R 32 , from the connection node of R 31  and R 32 . The AMP compares the Vout divided voltage with the reference voltage Vref, generates a PWM signal in accordance with the results of the comparison, and applies the PWM signal to the gate of the PTr 1 . Specifically, in the case where the Vout divided voltage is lower than the reference voltage Vref, the ratio (duty) of the low level of the PWM signal is increased and in the case where the Vout divided voltage is higher than the reference voltage Vref, the ratio (duty) of the low level of the PWM signal is reduced. Due to this, the output voltage Vout is controlled to be a predetermined voltage. 
     Hereinafter, the operation of the back gate variable diode circuit in the third embodiment is explained. 
     When Vout&gt;VDE, the back gate variable diode circuit becomes the equivalent circuit illustrated in  FIG. 6B . In other words, the back gate variable diode circuit is represented by the PTr 21  that is diode-connected and whose back gate is connected to the source of the PTr 1 . When Vout&gt;VDE, Va&gt;VDE (and Va&lt;Vout) holds, and the PTr 22  turns on because the gate potential becomes the VDE and the source potential becomes Va. On the other hand, the PTr 23  turns off because the gate potential becomes Va and the source potential becomes the VDE. Because of this, the back gate potential becomes Va (Vout) and the PTr 21  turns off, and therefore, the backflow from the second node (VNW  43 ) to the VDE wire is prevented. 
     When Vout&lt;VDE (during normal operation), the back gate variable diode circuit becomes the equivalent circuit illustrated in  FIG. 6C . In other words, the back gate variable diode circuit is represented by the PTr 21  that is diode-connected and whose back gate is connected to the VDE wire  42 . When Vout&lt;VDE, Va&lt;VDE (and Va&gt;Vout) holds, and the PTr 23  turns on because the gate potential becomes Va and the source potential becomes the VDE. On the other hand, the PTr 22  turns off because the gate potential becomes the VDE and the source potential becomes the VDE. Because of this, the PTr 21  turns on because the back gate potential becomes the VDE, and at the same time, the forward bias is not applied to the back gate, and therefore, no overcurrent occurs. 
     All examples and conditional language provided herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a illustrating of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail. It should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.