Patent Document

RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 12/559,966 filed on Sep. 15, 2009, now U.S. Pat. No. 8,198,875 the entire content of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a voltage regulator having an output terminal connected to a backup battery. 
     2. Description of the Related Art 
     Such a circuit as illustrated in  FIG. 11  has been known as a conventional voltage regulator having an output terminal connected to a backup battery  112  (see, for example, Japanese Patent Application Laid-open No. 2001-51735). 
     Power supply voltage is applied between terminals, that is, a VDD terminal  121  and a VSS terminal  123 . An output terminal  122  is connected to the backup battery  112 , and even when the power supply voltage between the VDD terminal  121  and the VSS terminal  123  becomes zero, a load  113  (for example, RAM) connected to the output terminal  122  may be continued to be supplied with voltage. 
     While the power supply voltage is supplied between the VDD terminal  121  and the VSS terminal  123 , “VBAT 1 &gt;VBAT 2 ” is normally established, where VBAT 1  and VBAT 2  represent the voltage between the terminals and the voltage of the backup battery  112 , respectively. While the power supply voltage is supplied between the VDD terminal  121  and the VSS terminal  123 , a Vref circuit  101  outputs a given constant voltage (Vref), and an error amplifier  102  amplifies a differential voltage between the voltage Vref and a voltage (R 2 /(R 1 +R 2 )×VOUT) determined by dividing the voltage (VOUT) of the output terminal  122  by means of a resistor  107  (whose resistance is R 1 ) and a resistor  108  (whose resistance is R 2 ). Accordingly, a gate of a Pch transistor  103  is controlled so that a constant voltage is output to the output terminal  122 . 
     A comparator  1105  has a positive input terminal connected to a voltage determined by dividing the inter-terminal voltage between the VDD terminal  121  and the VSS terminal  123  by means of a resistor  1101  and a resistor  1102 , and has a negative input terminal connected to a voltage determined by dividing an inter-terminal voltage between the output terminal  122  and the VSS terminal  123  by means of a resistor  1103  and a resistor  1104 . Then, the comparator  1105  compares the terminal voltage of the VDD terminal  121  with the terminal voltage of the output terminal  122 . While the power supply voltage is supplied between the VDD terminal  121  and the VSS terminal  123 , the voltage determined by the voltage division by means of the resistor  1101  and the resistor  1102  is higher than the voltage determined by the voltage division by means of the resistor  1103  and the resistor  1104 . Therefore, an output of the comparator  1105  becomes “H”, and then a Pch transistor  105  is turned ON while a Pch transistor  106  is turned OFF. Accordingly, because of the Pch transistor  105 , a substrate (NWELL) potential of the Pch transistor  103  becomes a potential of the VDD terminal  121 . 
     On the other hand, when the inter-terminal voltage between the VDD terminal  121  and the VSS terminal  123  becomes lower than the inter-terminal voltage between the output terminal  122  and the VSS terminal  123 , the output of the comparator  1105  becomes “L”, and then the Pch transistor  106  is turned ON while the Pch transistor  105  is turned OFF. Accordingly, because of the Pch transistor  106 , the substrate (NWELL) potential of the Pch transistor  103  becomes a potential of the output terminal  122 . 
     In other words, by switching the substrate (NWELL) potential of the Pch transistor  103  to a higher one of the potentials on the VDD terminal  121  side and the output terminal  122  side, even if the voltage of the VDD terminal  121  becomes lower than the voltage of the output terminal  122 , a current is prevented from flowing from the output terminal  122  to the VDD terminal  121  via a parasitic diode formed with a substrate of the Pch transistor  103 . 
     However, in the conventional voltage regulator, when the potential on the VDD terminal  121  side becomes zero, a current flows thereinto from the backup battery  112  via the resistor  1103  and the resistor  1104 . As a result, there is a problem that a backup operation cannot be performed for a long time. 
     In addition, there is another problem that a reverse current flows thereinto because the Pch transistor  103  cannot be turned OFF when the potential on the VDD terminal  121  side becomes zero. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to solve the conventional problems described above, and to provide a voltage regulator that is capable of, when the potential on the VDD terminal  121  side becomes zero, achieving lower current consumption of the backup battery and securely preventing the reverse current by turning OFF the Pch transistor  103 . 
     The present invention solves the above-mentioned problems by adopting a circuit configuration in which voltage dividing resistors are not used for a comparator circuit for comparing the voltage of the VDD terminal  121  with the voltage of the output terminal  122  of the voltage regulator, to thereby eliminate a current flowing through the voltage dividing resistors. 
     According to the voltage regulator of the present invention, which has the configuration described above, irrespective of the magnitude of the voltage of the VDD terminal  121 , a reverse current may be prevented from flowing from the output terminal  122  to the VDD terminal  121  with low current consumption. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a circuit diagram illustrating a voltage regulator according to a first embodiment mode of the present invention; 
         FIG. 2  is a circuit diagram illustrating a first embodiment of a comparator circuit of the voltage regulator according to the present invention; 
         FIG. 3  is a circuit diagram illustrating a second embodiment of the comparator circuit of the voltage regulator according to the present invention; 
         FIG. 4  illustrates voltage waveforms of respective portions of the comparator circuit according to the second embodiment in the voltage regulator of the present invention; 
         FIG. 5  is a circuit diagram illustrating a third embodiment of the comparator circuit of the voltage regulator according to the present invention; 
         FIG. 6  illustrates voltage waveforms of respective portions of the comparator circuit according to the third embodiment in the voltage regulator of the present invention; 
         FIG. 7  is a circuit diagram of a general error amplifier of a voltage regulator; 
         FIG. 8  is a cross sectional view of a P-channel type MOS transistor; 
         FIG. 9  is a circuit diagram illustrating a second embodiment of an error amplifier of the voltage regulator according to the present invention; 
         FIG. 10  illustrates cross sectional views of P-channel type MOS transistors; 
         FIG. 11  is a circuit diagram illustrating a conventional voltage regulator; 
         FIG. 12  is a circuit diagram illustrating a voltage regulator according to a second embodiment mode of the present invention; 
         FIG. 13  is a circuit diagram illustrating a third embodiment of the error amplifier of the voltage regulator according to the present invention; and 
         FIG. 14  is a circuit diagram illustrating a fourth embodiment of the error amplifier of the voltage regulator according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the accompanying drawings, embodiment modes of the present invention are described. 
     First Embodiment 
       FIG. 1  is a circuit diagram illustrating a voltage regulator according to a first embodiment of the present invention. The voltage regulator according to the present invention includes a Vref circuit  101 , an error amplifier  102 , a comparator circuit  130 , a resistor  107 , a resistor  108 , a Pch transistor  103 , a Pch transistor  104 , a Pch transistor  105 , a Pch transistor  106 , an Nch transistor  109 , a VDD terminal  121 , a VSS terminal  123 , and an output terminal  122 . A difference from  FIG. 11  resides in that the comparator  1105  and the resistors  1101 ,  1102 ,  1103 , and  1104  are eliminated and the comparator circuit  130  controls the Pch transistors  105  and  106  and the added Pch transistor  104 . 
       FIG. 2  illustrates the comparator circuit  130  according to the present invention. 
     The comparator circuit  130  includes a constant current circuit  203 , a constant current circuit  204 , a Pch transistor  201 , a Pch transistor  202 , an inverter  205 , an inverter  206 , an inverter  208 , and a level shifter  207 . 
     A description is given of connections in the voltage regulator according to the present invention. An output of the Vref circuit  101  is connected to an inverting input terminal of the error amplifier  102 . A non-inverting input terminal of the error amplifier  102  is connected to a connection point between the resistor  107  and the resistor  108 , and an output thereof is connected to a gate of the Pch transistor  103  and a source of the Pch transistor  104 . A source of the Pch transistor  103  is connected to the VDD terminal  121  and a drain of the Pch transistor  105 . A drain of the Pch transistor  103  is connected to the output terminal  122  and a drain of the Pch transistor  106 . A back gate of the Pch transistor  103  is connected to a source of the Pch transistor  105  and a source of the Pch transistor  106 . A gate of the Pch transistor  105  is connected to a node  111 , and a back gate thereof is connected to the source of the Pch transistor  105 . A gate of the Pch transistor  106  is connected to a node  110 , and a back gate thereof is connected to the source of the Pch transistor  106 . A drain of the Pch transistor  104  is connected to the output terminal  122 . A gate of the Pch transistor  104  is connected to the node  110 , and a back gate thereof is connected to the output of the error amplifier  102 . One side of the resistor  107  is connected to the output terminal  122  while another side thereof is connected to the resistor  108 . A gate of the Nch transistor  109  is connected to the node  110 . A drain of the Nch transistor  109  is connected to the resistor  108 , and a source thereof is connected to the VSS terminal  123 . The comparator circuit  130  is connected to the output terminal  122 , the VDD terminal  121 , the VSS terminal  123 , the node  110 , and the node  111 . The output terminal  122  is connected to a backup battery  112  and a load  113  that are connected in parallel. 
     Next, a description is given of connections in the comparator circuit  130 . A gate of the Pch transistor  201  is connected to a gate of the Pch transistor  202 , a drain of the Pch transistor  201 , and the constant current circuit  203 . A source of the Pch transistor  201  is connected to the VDD terminal  121 , and a back gate thereof is connected to the VDD terminal  121 . A drain of the Pch transistor  202  is connected to the inverter  205  and the constant current circuit  204 . A source of the Pch transistor  202  is connected to the output terminal  122 , and a back gate thereof is connected to the output terminal  122 . An output of the inverter  205  is connected to the inverter  206 , and the inverter  205  is connected to the output terminal  122  for its power supply. An output of the inverter  206  is connected to the level shifter  207  and a CONT terminal  223 , and the inverter  206  is connected to the output terminal  122  for its power supply. An output of the level shifter  207  is connected to the inverter  208 , and the level shifter  207  is connected to the VDD terminal  121  for its power supply. An output of the inverter  208  is connected to a CONTX terminal  222 , and the inverter  208  is connected to the VDD terminal  121  for its power supply. The CONT terminal  223  is connected to the node  111  of  FIG. 1  while the CONTX terminal  222  is connected to the node  110  of  FIG. 1 . 
     Next, a description is given of operations of the voltage regulator according to the present invention. When a potential of the VDD terminal  121  is higher than a potential of the output terminal  122 , a gate-source voltage of the Pch transistor  201  is higher than a gate-source voltage of the Pch transistor  202 . Accordingly, a potential of the drain of the Pch transistor  202  becomes “L” level (potential of the VSS terminal  123 ). With the inverters  205  and  206  for waveform shaping, a voltage of the CONT terminal  223 , to which the output of the inverter  206  is connected, becomes “L” level. The level shifter  207  converts the potential level of the output terminal  122  to the potential level of the VDD terminal  121 . The inverter  208  inverts an output voltage of the level shifter  207 . When the voltage of the CONT terminal  223  is “L” level, the CONTX terminal  222 , which corresponds to the output of the inverter  208 , has the potential level of the VDD terminal  121 . On this occasion, a substrate (NWELL) potential of the Pch transistor  103  illustrated in  FIG. 1  becomes the potential of the VDD terminal  121  because the Pch transistor  105  is turned ON while the Pch transistor  106  is turned OFF. In other words, the substrate (NWELL) potential of the Pch transistor  103  becomes a higher one of the potential of the VDD terminal  121  and the potential of the output terminal  122 . On this occasion, the Pch transistor  104  is turned OFF. When the VDD terminal  121  is connected to a power source, the potential of the VDD terminal  121  normally becomes higher than the potential of the output terminal  122 . 
     On the other hand, when no power source is connected to the VDD terminal  121 , the potential of the VDD terminal  121  becomes lower than the potential of the output terminal  122  because the output terminal  122  is connected to the backup battery  112 . On this occasion, the gate-source voltage of the Pch transistor  201  is lower than the gate-source voltage of the Pch transistor  202 . Accordingly, the potential of the drain of the Pch transistor  202  becomes “H” level (potential of the output terminal  122 ). With the inverters  205  and  206  for waveform shaping, the voltage of the CONT terminal  223 , which corresponds to the output of the inverter  206 , becomes “H” level (potential of the output terminal  122 ). The level shifter  207  converts the potential level of the output terminal  122  to the potential level of the VDD terminal  121 . The inverter  208  inverts the output voltage of the level shifter  207 . When the voltage of the CONT terminal  223  is at “H” level (potential of the output terminal  122 ), the voltage of the CONTX terminal  222 , which corresponds to the output of the inverter  208 , is “L” level (potential level of the VSS terminal  123 ). On this occasion, the substrate (NWELL) potential of the Pch transistor  103  illustrated in  FIG. 1  becomes the potential of the output terminal  122  because the Pch transistor  106  is turned ON while the Pch transistor  105  is turned OFF. In other words, the substrate (NWELL) potential of the Pch transistor  103  becomes a higher one of the potential of the VDD terminal  121  and the potential of the output terminal  122 . On this occasion, the Pch transistor  104  is turned ON, and accordingly the gate of the Pch transistor  103  is allowed to have the same potential as the output terminal  122  so that the Pch transistor  103  is turned OFF. With this, even when the potential of the VDD terminal  121  becomes lower than the potential of the output terminal  122 , a current may be prevented by the Pch transistor  103  from flowing from the output terminal  122  to the VDD terminal  121 . 
     Next, a description is given of the error amplifier  102 , which is used in  FIG. 1 . A configuration of a general error amplifier is as illustrated in  FIG. 7 . The error amplifier includes a constant current circuit  705 , Nch transistors  701  and  702 , and Pch transistors  703  and  704 . The positive input terminal, the negative input terminal, and the output of the error amplifier are respectively represented by INP  721 , INM  722 , and EOUT  723 . Further,  FIG. 8  illustrates a cross sectional view of the Pch transistor  704 . Within an NWELL formed on a P-substrate, there exist P-type source and drain regions. The P-substrate is connected to the VSS terminal  123 , whose potential is lower. Further, the NWELL is connected to its source (VDD terminal  121 ). 
     In a case of using the general error amplifier illustrated in  FIG. 7 , when the potential of the output terminal  122  becomes higher than the potential of the VDD terminal  121 , and when the Pch transistor  104  is accordingly turned ON, the output  723  of the error amplifier  102  is connected to the output terminal  122 . At this time, in the case of the general error amplifier illustrated in  FIG. 7 , a PNP transistor whose emitter, base, and collector respectively correspond to the drain, the source, and the substrate of the transistor  704  is turned ON. As a result, the backup battery  112  is discharged via the Pch transistor  104 . To avoid this phenomenon, it is desirable to adopt such a configuration as illustrated in  FIG. 9  for the error amplifier. 
     In  FIG. 9  illustrating a second embodiment of the error amplifier  102 , a Pch transistor  801  is newly added between the output  723  of the error amplifier and the Pch transistor  704 . The Pch transistor  801  has a source and an NWELL that are connected to the output  723  of the error amplifier, a drain connected to the drain of the Pch transistor  704 , and a gate controlled by a signal (CONT signal) from the node  111  illustrated in  FIG. 1 .  FIG. 10  illustrates cross sectional views of the Pch transistors  704  and  801 . In this case, when the potential of the output terminal  122  becomes higher than the potential of the VDD terminal  121 , and when the Pch transistor  104  is accordingly turned ON, the output  723  of the error amplifier  102  is connected to the output terminal  122 . However, the signal from the node  111  becomes the same potential as the output terminal  122 , and accordingly the Pch transistor  801  is turned OFF. Therefore, a current is not allowed to flow from the drain of the Pch transistor  801  to the drain of the Pch transistor  704 . 
     Further, a difference from  FIG. 7  resides in that an Nch transistor  802  is inserted between the constant current circuit  705  and a source of a differential input circuit formed of the Nch transistors  701  and  702 . A drain of the Nch transistor  802  is connected to the sources of the Nch transistors  701  and  702 . A source of the Nch transistor  802  is connected to the constant current circuit  705 , and a gate thereof is connected to a signal (CONTX signal) from the node  110  illustrated in  FIG. 1  to be controlled. When the potential of the output terminal  122  becomes higher than the potential of the VDD terminal  121 , and when the Pch transistor  104  is accordingly turned ON, the output  723  of the error amplifier  102  is connected to the output terminal  122  so that the Nch transistor  702  enters an ON state. Then, the output terminal  122  and the sources of the Nch transistors  701  and  702  are brought into an electrically-connected state, but the Nch transistor  802  is turned OFF to interrupt a current path of the constant current circuit  705 . In this way, a current may be prevented from flowing from the output terminal  122  to the VSS terminal  123  via the Nch transistor  702 . 
     In the description of  FIG. 9 , the Nch transistor  802  is inserted between the sources of the Nch transistors  701  and  702  and the constant current circuit  705 . However, it should be understood that a similar effect can also be obtained when the Nch transistor  802  is inserted between the constant current circuit  705  and the VSS terminal  123 . Further, in the description of  FIG. 9 , the Pch transistor  801  is inserted between the output  723  of the error amplifier  102  and the Pch transistor  704 . However, it should be understood that a similar effect can also be obtained when the Pch transistor  801  is inserted between the VDD terminal  121  and the Pch transistor  704 . 
     In  FIG. 9 , the description has been given taking a one-stage amplifier circuit as an example of the error amplifier, but the error amplifier may be a multi-stage amplifier circuit with two or more stages. In this case, as in  FIG. 9 , the Pch transistor  801  having a function of interrupting a current path may be inserted between the output of the error amplifier and the VDD terminal, and the Nch transistor  802  having a function of interrupting a current path may be inserted between the output of the error amplifier and the VSS terminal. 
     As described above, compared to the conventional voltage regulator illustrated in  FIG. 11 , the resistor  1101 , the resistor  1102 , the resistor  1103 , and the resistor  1104  are not provided for comparing the potential of the VDD terminal  121  with the potential of the output terminal  122 . As a result, current consumption may be reduced correspondingly. For example, when it is assumed that the voltage of the backup battery  112  is 3 V and a total resistance of the resistor  1103  and the resistor  1104  is 3 Meg Ω, a current of 1 μA from the backup battery  112  is consumed by the resistor  1103  and the resistor  1104 . However, in the voltage regulator illustrated in  FIG. 1 , there is no element equivalent to those resistors, resulting in no consumption corresponding thereto. It is assumed that the comparator  1105  illustrated in  FIG. 11  and the comparator circuit  130  illustrated in  FIG. 2  have the same current consumption of 0.5 μA. On this occasion, the voltage regulator illustrated in  FIG. 11  consumes 1.5 μA from the backup battery  112 , whereas the voltage regulator illustrated in  FIG. 1  consumes only 0.5 μA therefrom, which is one-third of 1.5 μA. As a result, an operation time period with the backup battery  112  may be extended significantly. 
     Second Embodiment 
       FIG. 3  illustrates a second embodiment of the comparator circuit  130  of the voltage regulator according to the present invention illustrated in  FIG. 1 . The comparator circuit  130  according to the second embodiment includes a constant current circuit  303 , a constant current circuit  304 , the Pch transistor  201 , a Pch transistor  301 , a Pch transistor  302 , a Pch transistor  305 , the inverter  205 , the inverter  206 , the inverter  208 , and the level shifter  207 . Differences from  FIG. 2  reside in that an element equivalent to the Pch transistor  202  is formed of the two transistors, that is, the Pch transistor  301  and the Pch transistor  302 , and that the Pch transistor  305  is added for realizing a hysteresis function. Further, each of the constant current circuit  203  and the constant current circuit  204  is specifically illustrated as an N-channel depletion type MOS transistor whose gate and source are connected to the VSS terminal  123 . 
     Next, a description is given of connections in the comparator circuit  130 . The gate of the Pch transistor  201  is connected to a gate of the Pch transistor  301 , a gate of the Pch transistor  302 , a drain of the Pch transistor  201 , and the constant current circuit  303 . The source of the Pch transistor  201  is connected to the VDD terminal  121 , and the back gate thereof is connected to the VDD terminal  121 . A drain of the Pch transistor  302  is connected to the inverter  205  and the constant current circuit  304 . A source of the Pch transistor  302  is connected to a drain of the Pch transistor  301  and a drain of the Pch transistor  305 , and a back gate thereof is connected to the output terminal  122 . A source of the Pch transistor  301  is connected to the output terminal  122 , and a back gate thereof is connected to the output terminal  122 . A gate of the Pch transistor  305  is connected to the output of the inverter  205 . A source of the Pch transistor  305  is connected to the output terminal  122 , and a back gate thereof is connected to the output terminal  122 . The output of the inverter  205  is connected to the inverter  206 , and the inverter  205  is connected to the output terminal  122  for its power supply. The output of the inverter  206  is connected to the level shifter  207  and the CONT terminal  223 , and the inverter  206  is connected to the output terminal  122  for its power supply. The output of the level shifter  207  is connected to the inverter  208 , and the level shifter  207  is connected to the VDD terminal  121  for its power supply. The output of the inverter  208  is connected to the CONTX terminal  222 , and the inverter  208  is connected to the VDD terminal  121  for its power supply. The N-channel depletion type MOS transistors are used as the constant current circuit  303  and the constant current circuit  304 . Each of the N-channel depletion type MOS transistors has the gate and the source that are connected to the VSS terminal  123 , and a drain used as its output. The CONT terminal  223  is connected to the node  111  of  FIG. 1  while the CONTX terminal  222  is connected to the node  110  of  FIG. 1 . 
     Next, a description is given of operations of the voltage regulator, which uses the comparator circuit according to the second embodiment. When the potential of the VDD terminal  121  is sufficiently higher than the potential of the output terminal  122 , the gate-source voltage of the Pch transistor  201  is sufficiently higher than gate-source voltages of the Pch transistor  301  and the Pch transistor  302 . Accordingly, a potential of the drain of the Pch transistor  302  becomes “L” level (potential of the VSS terminal  123 ). With the inverters  205  and  206  for waveform shaping, the output of the inverter  205  becomes “H” (potential of the output terminal  122 ). Then, the Pch transistor  305  is turned OFF, and the voltage of the CONT terminal  223 , which corresponds to the output of the inverter  206 , becomes “L” level. The level shifter  207  converts the potential level of the output terminal  122  to the potential level of the VDD terminal  121 . The inverter  208  inverts the output voltage of the level shifter  207 . When the voltage of the CONT terminal  223  is “L” level, the CONTX terminal  222 , which corresponds to the output of the inverter  208 , has the potential level of the VDD terminal  121 . On this occasion, the substrate (NWELL) potential of the Pch transistor  103  becomes the potential of the VDD terminal  121  because the Pch transistor  105  is turned ON while the Pch transistor  106  is turned OFF. In other words, the substrate (NWELL) potential of the Pch transistor  103  becomes a higher one of the potential of the VDD terminal  121  and the potential of the output terminal  122 . On this occasion, the Pch transistor  104  is turned OFF. When the VDD terminal  121  is connected to a power source, the potential of the VDD terminal  121  normally becomes higher than the potential of the output terminal  122 . 
     Subsequently, when the potential of the VDD terminal  121  decreases, because the Pch transistor  305  is turned OFF, the voltage of the VDD terminal  121  is compared with the voltage of the output terminal  122  by means of the Pch transistor  201  and a compound transistor formed of the Pch transistor  301  and the Pch transistor  302 . When the potential of the VDD terminal  121  decreases to a potential lower by ΔV 1  than the potential of the output terminal  122 , the gate-source voltage of the Pch transistor  201  becomes lower by ΔV 1  than the gate-source voltages of the Pch transistor  301  and the Pch transistor  302 . Accordingly, the potential of the drain of the Pch transistor  302  becomes “H” level (potential of the output terminal  122 ). With the inverters  205  and  206  for waveform shaping, the output of the inverter  205  becomes “L” level. Then, the Pch transistor  305  is turned ON, and the voltage of the CONT terminal  223 , which corresponds to the output of the inverter  206 , becomes “H” level (potential of the output terminal  122 ). The level shifter  207  converts the potential level of the output terminal  122  to the potential level of the VDD terminal  121 . The inverter  208  inverts the output voltage of the level shifter  207 . When the voltage of the CONT terminal  223  is at “H” level, the CONTX terminal  222 , which corresponds to the output of the inverter  208 , is “L” level. On this occasion, the substrate (NWELL) potential of the Pch transistor  103  illustrated in  FIG. 1  becomes the potential of the output terminal  122  because the Pch transistor  106  is turned ON while the Pch transistor  105  is turned OFF. In other words, the substrate (NWELL) potential of the Pch transistor  103  becomes a higher one of the potential of the VDD terminal  121  and the potential of the output terminal  122 . On this occasion, the Pch transistor  104  is turned ON, and accordingly the gate of the Pch transistor  103  is allowed to have the same potential as the output terminal  122  so that the Pch transistor  103  is turned OFF. 
     The voltage of ΔV 1  is determined by Expression (1). 
     
       
         
           
             
               
                 
                   
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     In Expression (1), I represents a current value of the constant current circuits  303  and  304 ; μ, mobility of the Pch transistor  201 , the Pch transistor  301 , and the Pch transistor  302 ; L 6 , a total transistor L-length of the Pch transistor  301  and the Pch transistor  302 ; L 5 , a transistor L-length of the Pch transistor  201 ; W 6 , a total transistor W-length of the Pch transistor  301  and the Pch transistor  302 ; and W 5 , a transistor W-length of the Pch transistor  201 . 
     Subsequently, when the potential of the VDD terminal  121  increases, because the Pch transistor  305  is turned ON, the voltage of the VDD terminal  121  is compared with the voltage of the output terminal  122  by means of the transistors of the Pch transistor  201  and the Pch transistor  302 . In the cases where the constant current circuits  303  and  304  have the same current value, and where the Pch transistor  201  and the Pch transistor  302  have the same transistor types (VTH, mobility, etc.), the same L-length, and the same W-length, ΔV 1  in Expression (1) satisfies “ΔV 1 =0”. Therefore, when the voltage of the VDD terminal  121  and the voltage of the output terminal  122  are substantially equal to each other, the voltage of the CONT terminal  223  and the voltage of the CONTX terminal  222  are inverted. 
       FIG. 4  illustrates voltage waveforms of the CONT terminal  223  and the CONTX terminal  222  of when the voltage of the output terminal  122  is constant while the voltage of the VDD terminal  121  changes, in which the horizontal axis represents time and the vertical axis represents voltage. When the voltage of the VDD terminal  121  decreases to a voltage lower by ΔV 1  than the voltage of the output terminal  122 , the voltage of the CONT terminal  223  and the voltage of the CONTX terminal  222  are inverted. After that, the voltage of the VDD terminal  121  is raised, and when the voltage of the VDD terminal  121  becomes equal to the voltage of the output terminal  122 , the voltage of the CONT terminal  223  and the voltage of the CONTX terminal  222  are inverted. As described above, hysteresis is provided between the voltage of the VDD terminal  121  and the voltage of the output terminal  122 , between which the substrate (NWELL) potential of the Pch transistor  103  is switched over. This enables the switching-over of the substrate (NWELL) potential of the Pch transistor  103  to be securely performed without a malfunction even when the voltage of the VDD terminal  121  and the voltage of the output terminal  122  become approximate to each other. 
     Note that, in order to prevent a parasitic diode formed between the output terminal  122  and the substrate of the Pch transistor  103  from being turned ON when the voltage of the VDD terminal  121  decreases, the value of ΔV 1  needs to be set to a forward ON voltage (about 0.6 V) or lower of the parasitic diode. In general, the value of ΔV 1  is set to about 50 mV to 200 mV. 
     Further, the Pch transistor  305  is connected in parallel to the Pch transistor  301  in  FIG. 3 , but it should be understood that a similar effect can also be obtained when the Pch transistor  305  is connected in parallel to the Pch transistor  302 . Further, as has been described in the first embodiment, with regard to the error amplifier, it is desirable to adopt the configuration illustrated in  FIG. 9  similarly to the first embodiment. 
     Third Embodiment 
       FIG. 5  illustrates a third embodiment of the comparator circuit  130  of the voltage regulator according to the present invention illustrated in  FIG. 1 . The comparator circuit  130  according to the third embodiment includes the constant current circuit  303 , the constant current circuit  304 , the Pch transistor  202 , a Pch transistor  501 , a Pch transistor  502 , a Pch transistor  503 , the inverter  205 , the inverter  206 , the inverter  208 , and the level shifter  207 . Differences from  FIG. 2  reside in that an element equivalent to the Pch transistor  201  is formed of the two transistors, that is, the Pch transistor  501  and the Pch transistor  502 , and that the Pch transistor  503  is added for realizing a hysteresis function. Further, similarly to  FIG. 3 , each of the constant current circuits  203  and  204  is specifically illustrated as the N-channel depletion type MOS transistor whose gate and source are connected to the VSS terminal  123 . 
     Next, a description is given of connections in the comparator circuit  130 . A gate of the Pch transistor  501  is connected to the gate of the Pch transistor  202 , a gate of the Pch transistor  502 , a drain of the Pch transistor  502 , and the constant current circuit  303 . A source of the Pch transistor  501  is connected to the VDD terminal  121 . A drain of the Pch transistor  501  is connected to a source of the Pch transistor  502  and a drain of the Pch transistor  503 , and a back gate thereof is connected to the VDD terminal  121 . A gate of the Pch transistor  503  is connected to the output of the level shifter  207 . A source of the Pch transistor  503  is connected to the VDD terminal  121 , and a back gate thereof is connected to the VDD terminal  121 . The drain of the Pch transistor  202  is connected to the inverter  205  and the constant current circuit  304 . A source of the Pch transistor  202  is connected to the output terminal  122 , and a back gate thereof is connected to the output terminal  122 . The output of the inverter  205  is connected to the inverter  206 , and the inverter  205  is connected to the output terminal  122  for its power supply. The output of the inverter  206  is connected to the level shifter  207  and the CONT terminal  223 , and the inverter  206  is connected to the output terminal  122  for its power supply. The output of the level shifter  207  is connected to the inverter  208 , and the level shifter  207  is connected to the VDD terminal  121  for its power supply. The output of the inverter  208  is connected to the CONTX terminal  222 , and the inverter  208  is connected to the VDD terminal  121  for its power supply. The N-channel depletion type MOS transistors are used as the constant current circuit  303  and the constant current circuit  304 . Each of the N-channel depletion type MOS transistors has the gate and the source that are connected to the VSS terminal  123 , and a drain used as its output. The CONT terminal  223  is connected to the node  111  of  FIG. 1  while the CONTX terminal  222  is connected to the node  110  of  FIG. 1 . 
     Next, a description is given of operations of the voltage regulator, which uses the comparator circuit according to the third embodiment. When the potential of the VDD terminal  121  is sufficiently higher than the potential of the output terminal  122 , the Pch transistor  501  and the Pch transistor  502  are turned ON while the Pch transistor  202  is turned OFF. Accordingly, the potential of the drain of the Pch transistor  202  becomes “L” level (potential of the VSS terminal  123 ). With the inverters  205  and  206  for waveform shaping, the voltage of the CONT terminal  223 , which corresponds to the output of the inverter  206 , becomes “L” level. The level shifter  207  converts the potential level of the output terminal  122  to the potential level of the VDD terminal  121 . The inverter  208  inverts the output voltage of the level shifter  207 . When the voltage of the CONT terminal  223  is “L” level, the output of the level shifter  207  is “L” level. Accordingly, the Pch transistor  503  is turned ON, and the CONTX terminal  222 , which corresponds to the output of the inverter  208 , has the potential level of the VDD terminal  121 . On this occasion, the substrate (NWELL) potential of the Pch transistor  103  illustrated in  FIG. 1  becomes the potential of the VDD terminal  121  because the Pch transistor  105  is turned ON while the Pch transistor  106  is turned OFF. In other words, the substrate (NWELL) potential of the Pch transistor  103  becomes a higher one of the potential of the VDD terminal  121  and the potential of the output terminal  122 . On this occasion, the Pch transistor  104  is turned OFF. When the VDD terminal  121  is connected to a power source, the potential of the VDD terminal  121  normally becomes higher than the potential of the output terminal  122 . 
     Subsequently, when the potential of the VDD terminal  121  decreases, because the Pch transistor  503  is turned ON, the voltage of the VDD terminal  121  is compared with the voltage of the output terminal  122  by means of the Pch transistor  502  and the Pch transistor  202 . In the cases where the constant current circuits  303  and  304  have the same current value, and where the Pch transistor  502  and the Pch transistor  202  have the same transistor types (VTH, mobility, etc.), the same L-length, and the same W-length, when the potential of the VDD terminal  121  decreases to substantially the same value as the potential of the output terminal  122 , the Pch transistor  502  is turned OFF while the Pch transistor  202  is turned ON. Accordingly, the potential of the drain of the Pch transistor  202  becomes “H” level (potential of the output terminal  122 ). With the inverters  205  and  206  for waveform shaping, the voltage of the CONT terminal  223 , which corresponds to the output of the inverter  206 , becomes “H” level (potential of the output terminal  122 ). The level shifter  207  converts the potential level of the output terminal  122  to the potential level of the VDD terminal  121 . The inverter  208  inverts the output voltage of the level shifter  207 . When the voltage of the CONT terminal  223  is at “H” level, the output of the level shifter  207  corresponds to the voltage of the VDD terminal  121 . Accordingly, the Pch transistor  503  is turned OFF, and the voltage of the CONTX terminal  222 , which corresponds to the output of the inverter  208 , becomes “L” level. On this occasion, the substrate (NWELL) potential of the Pch transistor  103  becomes the potential of the output terminal  122  because the Pch transistor  106  is turned ON while the Pch transistor  105  is turned OFF. In other words, the substrate (NWELL) potential of the Pch transistor  103  becomes a higher one of the potential of the VDD terminal  121  and the potential of the output terminal  122 . On this occasion, the Pch transistor  104  is turned ON, and accordingly the gate of the Pch transistor  103  is allowed to have the same potential as the output terminal  122  so that the Pch transistor  103  is turned OFF. 
     Subsequently, when the potential of the VDD terminal  121  increases, because the Pch transistor  503  is turned OFF, the voltage of the VDD terminal  121  is compared with the voltage of the output terminal  122  by means of the Pch transistor  202  and a compound transistor formed of the Pch transistor  501  and the Pch transistor  502 . When the voltage of the VDD terminal  121  increases to a voltage higher by ΔV 2  than the voltage of the output terminal  122 , the voltage of the CONT terminal  223  and the voltage of the CONTX terminal  222  are inverted. 
     The voltage of ΔV 2  is determined by Expression (2). 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       Expression 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       2 
                     
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                     Δ 
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                     V 
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                     2 
                   
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                           2 
                           · 
                           I 
                         
                         
                           μ 
                           · 
                           Cox 
                         
                       
                     
                     × 
                     
                       ( 
                       
                         
                           
                             
                               L 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               5 
                             
                             
                               W 
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                               ⁢ 
                               5 
                             
                           
                         
                         - 
                         
                           
                             
                               L 
                               ⁢ 
                               
                                   
                               
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                               6 
                             
                             
                               W 
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                               6 
                             
                           
                         
                       
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     In Expression (2), I represents a current value of the constant current circuits  303  and  304 ; μ, mobility of the Pch transistor  202 , the Pch transistor  501 , and the Pch transistor  502 ; L 6 , a transistor L-length of the Pch transistor  202 ; L 5 , a total transistor L-length of the Pch transistor  501  and the Pch transistor  502 ; W 6 , a transistor W-length of the Pch transistor  202 ; and W 5 , a total transistor W-length of the Pch transistor  501  and the Pch transistor  502 . 
       FIG. 6  illustrates voltage waveforms of the CONT terminal  223  and the CONTX terminal  222  of when the voltage of the output terminal  122  is constant while the voltage of the VDD terminal  121  changes, in which the horizontal axis represents time and the vertical axis represents voltage. When the voltage of the VDD terminal  121  decreases to be equal to the voltage of the output terminal  122 , the voltage of the CONT terminal  223  and the voltage of the CONTX terminal  222  are inverted. After that, the voltage of the VDD terminal  121  is raised, and when the voltage of the VDD terminal  121  becomes higher by ΔV 2  than the voltage of the output terminal  122 , the voltage of the CONT terminal  223  and the voltage of the CONTX terminal  222  are inverted. As described above, hysteresis is provided between the voltage of the VDD terminal  121  and the voltage of the output terminal  122 , between which the substrate (NWELL) potential of the Pch transistor  103  is switched over. This enables the switching-over of the substrate (NWELL) potential of the Pch transistor  103  to be securely performed without a malfunction even when the voltage of the VDD terminal  121  and the voltage of the output terminal  122  become approximate to each other. 
     Note that, in order to prevent a parasitic diode formed between the VDD terminal  121  and the substrate of the Pch transistor  103  from being turned ON when the voltage of the VDD terminal  121  increases, the value of ΔV 2  needs to be set to a forward ON voltage (about 0.6 V) or lower of the parasitic diode. In general, the value of ΔV 2  is set to about 50 mV to 200 mV. 
     Further, the Pch transistor  503  is connected in parallel to the Pch transistor  501  in  FIG. 5 , but it should be understood that a similar effect can also be obtained when the Pch transistor  503  is connected in parallel to the Pch transistor  502 . Further, as has been described in the first embodiment, with regard to the error amplifier, it is desirable to adopt the configuration illustrated in  FIG. 9  similarly to the first embodiment. 
     Fourth Embodiment 
       FIG. 12  illustrates a circuit diagram of a voltage regulator according to a second embodiment mode of the present invention. Differences from  FIG. 1  reside in that the back gate of the Pch transistor  104  is connected to the back gate of the Pch transistor  103 , and that a delay circuit  1201  is added to the outputs of the comparator circuit  130 . Connections are made such that the outputs of the comparator circuit  130  are connected to the delay circuit  1201 , and the outputs of the delay circuit  1201  correspond to the outputs of the node  110  and the node  111 . 
     Next, a description is given of operations of the voltage regulator according to the second embodiment mode. When the voltage of the VDD terminal  121  is higher than the voltage of the output terminal  122 , the voltage of the node  111  is at “L” level while the voltage of the node  110  is at “H” level. Accordingly, the Pch transistor  105  is turned ON while the Pch transistor  106  is turned OFF. On this occasion, the substrate (NWELL) potential of the Pch transistor  104  becomes the voltage of the VDD terminal  121 , and hence the Pch transistor  104  can be turned OFF without fail. 
     The delay circuit  1201  uses a timer circuit to prevent the voltages of the node  110  and the node  111  from becoming “L” level simultaneously. This prevents the Pch transistors  105  and  106  from being turned ON simultaneously and a current from flowing from the VDD terminal  121  to the output terminal  122  or from the output terminal  122  to the VDD terminal  121 . 
     Note that, the voltage regulator according to the second embodiment mode may be allowed to operate without the delay circuit  1201 , though in this case there is a problem that the Pch transistors  105  and  106  may be turned ON simultaneously. 
     Fifth Embodiment 
       FIG. 13  illustrates a third embodiment of the error amplifier  102  of the voltage regulator according to the present invention illustrated in  FIG. 1 . A difference from  FIG. 9  resides in that a Pch transistor  803  is inserted downstream of the constant current circuit  705 , which has a gate connected to a CONT terminal  823 . 
     Next, a description is given of an operation. When the potential of the output terminal  122  becomes higher than the potential of the VDD terminal  121 , and when the Pch transistor  104  is accordingly turned ON, the output  723  of the error amplifier  102  is connected to the output terminal  122 . Because the Nch transistor  702  is in the ON state, the output terminal  122  and the sources of the Nch transistors  701  and  702  are brought into the electrically-connected state. Then, the Nch transistor  802  and the Pch transistor  803  are turned OFF, to thereby interrupt the current path of the constant current circuit  705  to prevent a current from flowing from the output terminal  122  to the VSS terminal  123  via the Nch transistor  702 . 
     In  FIG. 13 , the description has been given taking a one-stage amplifier circuit as an example of the error amplifier, but the error amplifier may be a multi-stage amplifier circuit with two or more stages. In this case, as in  FIG. 13 , the Pch transistor  801  having a function of interrupting a current path may be inserted between the output of the error amplifier and the VDD terminal, and the Nch transistor  802  and the Pch transistor  803  having a function of interrupting a current path may be inserted between the output of the error amplifier and the VSS terminal. 
     Sixth Embodiment 
       FIG. 14  illustrates a fourth embodiment of the error amplifier  102  of the voltage regulator according to the present invention illustrated in  FIG. 1 . Differences from  FIG. 13  reside in that the Nch transistor  802  and the Pch transistor  803  are eliminated, and that the CONT terminal  823  is connected to the constant current circuit  705 . 
     Next, a description is given of an operation. When the potential of the output terminal  122  becomes higher than the potential of the VDD terminal  121 , and when the Pch transistor  104  and the Pch transistor  801  are accordingly turned ON and OFF, respectively, the output  723  of the error amplifier  102  is connected to the output terminal  122 . Because the Nch transistor  702  is in the ON state, the output terminal  122  and the sources of the Nch transistors  701  and  702  are brought into the electrically-connected state. Then, by the signal from the CONT terminal  823 , the constant current circuit  705  is turned OFF to interrupt a current path so that a current is prevented from flowing from the output terminal  122  to the VSS terminal  123  via the Nch transistor  702 . 
     In  FIG. 14 , the description has been given taking a one-stage amplifier circuit as an example of the error amplifier, but the error amplifier may be a multi-stage amplifier circuit with two or more stages. In this case, the constant current circuit may be configured to be turned OFF by the signal from the CONT terminal  823 .

Technology Category: 3