Abstract:
A voltage detecting circuit for comparing a voltage to be detected with a reference voltage and outputting an output signal having a level depending on the comparison is disclosed. The voltage detecting circuit includes an inverting amplifier circuit configured to receive an intermediate signal having a level depending on the comparison and output the output signal. The inverting amplifier circuit includes an active element having a control terminal. A threshold voltage of the control terminal is as low as or lower than the reference voltage. The voltage to be detected is applied to the control terminal of the active element.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a voltage detecting circuit, and more specifically, to a voltage detecting circuit capable of accurately detecting a voltage even when a power source voltage is decreased. 
     2. Description of the Related Art 
     In portable information devices such as portable phones and mobile PCs, or systems such as car navigation systems, power is supplied by storage batteries. Such a power source device, which is charged to be used, is gradually discharged as a device having the power source device is used or in the course of time. As a result, the power source voltage is decreased. When the power source voltage is decreased to be lower than a predetermined level, the device cannot be operated normally. Conventionally, the power source voltage decreased to be lower than the predetermined level has been detected and a warning to charge the storage battery and the like has been given to a user. To realize such functions, a voltage detecting circuit capable of detecting a power source voltage has been used. By using the voltage detecting circuit, the power source voltage is detected as required and various processes can be performed based on the detected power source voltage. 
       FIG. 10  shows a general purpose voltage detecting circuit. In the circuit shown in  FIG. 10 , reference numeral  101  denotes a power source capable of supplying a voltage to be detected. That is, in this voltage detecting circuit, a voltage between terminals  102  and  103  is detected. Voltage dividing resistors  104  and  105  are connected between the terminals  102  and  103 . The voltage between the terminals  102  and  103  is divided by the voltage dividing resistors  104  and  105 , and inputted to a non-inverting input terminal of a comparator  106 . On the other hand, a reference voltage is supplied from a power source  107  to an inverting input terminal of the comparator  106 . An output terminal of the comparator  106  is connected to an inverter  108 . An output terminal of the inverter  108  is connected to a common gate terminal of a PMOS transistor  109  and an NMOS transistor  110 . The PMOS transistor  109  and the NMOS transistor  110  form an output circuit  111 . Drain terminals of the PMOS transistor  109  and the NMOS transistor  110 , which are connected function as an output terminal  112 . In this voltage detecting circuit, a voltage at a connection between the voltage dividing resistors  104  and  105  and the reference voltage of the power source  107  are compared by the comparator  106 . By detecting an inversion of an output voltage of the comparator  106 , it is detected that the power source voltage  101 , that is an input voltage, is decreased to be lower than a predetermined level. 
     In such a voltage detecting circuit, an output voltage of the output circuit  111  becomes unstable when an applied voltage is not higher than an operational voltage. Each of the PMOS transistor  109  and the NMOS transistor  110  forming the output circuit  111  is turned on when a voltage as high as or higher than its threshold voltage is applied between its gate and its source. When the power source voltage  101  is decreased, however, an operation of a differential amplifier circuit included in the comparator  106  becomes unstable. Therefore, operations of the PMOS transistor  109  and the NMOS transistor  110  become unstable, which makes it impossible to obtain a correct originally output voltage. 
     In view of this, there is a voltage detecting circuit disclosed in Patent Document 1, which operates correctly even when a power source voltage is decreased.  FIG. 11  shows a voltage detecting circuit disclosed in Patent Document 1. In  FIG. 11 , a second output circuit  115  formed of a depletion mode NMOS transistor  113  and a depletion mode PMOS transistor  114  is provided in a subsequent stage after the output circuit  111  of  FIG. 10 . The depletion mode PMOS transistor  114  has a gate terminal connected to a positive electrode (terminal  102 ) of a power source to be detected while the depletion mode NMOS transistor  113  has a gate terminal connected to a negative electrode (terminal  103 ) of the power source to be detected. The depletion mode PMOS transistor  114  and the depletion mode NMOS transistor  113  are connected in series between the output terminal  112  and the terminal  103 . As a result, even when the power source voltage  101  is decreased, the voltage level of the output terminal  112  can be held low or high and the output voltage can be prevented from becoming unstable. 
     [Patent Document 1] Japanese Patent Application Publication No. 2004-163315 
     In the voltage detecting circuit shown in  FIG. 11 , however, there is a problem in that a threshold value of each transistor cannot be easily controlled due to manufacturing reasons since the P-type depletion mode MOS transistor  114  and the N-type depletion mode MOS transistor  113  are used. 
     SUMMARY OF THE INVENTION 
     The present invention is made in view of solving the aforementioned problems and it is an object of at least one embodiment of the present invention to provide a voltage detecting circuit which is capable of correctly detecting a voltage even when a power source voltage is decreased and can be easily manufactured. 
     According to one aspect of the present invention, a voltage detecting circuit for comparing a voltage to be detected with a reference voltage and outputting an output signal having a level depending on the comparison is provided. The voltage detecting circuit includes an inverting amplifier circuit configured to receive an intermediate signal having a level depending on the comparison and output the output signal. The inverting amplifier circuit includes an active element having a control terminal. A threshold voltage of the control terminal is as low as or lower than the reference voltage. The voltage to be detected is applied to the control terminal of the active element. 
     According to another aspect of the present invention, a voltage detecting circuit is configured to compare a voltage to be detected with a reference voltage and output an output signal having a level depending on the comparison. The voltage detecting circuit includes a first power source terminal, a second power source terminal, a reference voltage generating circuit capable of generating the reference voltage, a differential amplifier circuit, and an inverting amplifier circuit. The inverting amplifier circuit includes a first MOS enhancement mode transistor having a source terminal connected to the first power source terminal, and a second MOS enhancement mode transistor having a source terminal connected to the second power source terminal through a current source. Drain terminals of the first and second MOS enhancement mode transistors being connected to each other. The voltage to be detected and the reference voltage are inputted to the differential amplifier circuit. An output signal of the differential amplifier circuit is inputted to a gate terminal of the first MOS enhancement mode transistor and the voltage to be detected is inputted to a gate terminal of the second MOS enhancement mode transistor. A connection between the drain terminals of the first and second MOS enhancement mode transistors functions as an output of the voltage detecting circuit. 
     According to another aspect of the present invention, a voltage detecting circuit is configured to compare a voltage to be detected with a reference voltage and output an output signal having a level depending on the comparison. The voltage detecting circuit includes a first power source terminal, a second power source terminal, and an output circuit through which the output signal is outputted. The output circuit includes an inverter and an active element connected in series between the first power source terminal and the second power source terminal. The level of the output signal is stabilized by turning off the active element when a voltage of the first power source terminal is decreased lower than a predetermined level. 
     According to another aspect of the present invention, a voltage detecting circuit includes a power source terminal, a differential amplifier circuit, and an inverting amplifier circuit. The differential amplifier circuit includes a first MOS enhancement mode transistor and a second MOS enhancement mode transistor having source terminals connected to the power source terminal, and a MOS depletion mode transistor and a third MOS enhancement mode transistor having grounded source terminals. Gate terminals of the first and second MOS enhancement mode transistors are connected together to a drain terminal of the second MOS enhancement mode transistor. The MOS depletion mode transistor has a gate terminal and a source terminal connected to each other and a drain terminal connected to a drain terminal of the first MOS enhancement mode transistor. The third MOS enhancement mode transistor has a drain terminal connected to a drain terminal of the second MOS enhancement mode transistor. The inverting amplifier circuit includes a fourth MOS enhancement mode transistor having a source terminal connected to the power source terminal and a fifth MOS enhancement mode transistor having a source terminal connected to ground potential through a current source. The fourth and fifth MOS enhancement mode transistors have drain terminals connected to each other. The drain terminal of the first MOS enhancement mode transistor is connected to a gate terminal of the fourth MOS enhancement mode transistor. The voltage to be detected is inputted to gate terminals of the third and fifth MOS enhancement mode transistors. A connection between the fourth and fifth MOS enhancement mode transistors functions as an output of the voltage detecting circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a voltage detecting circuit configuration of a first embodiment of the present invention; 
         FIGS. 2A and 2B  show other embodiments of output circuits of the present invention; 
         FIG. 3  is a voltage detecting circuit configuration of a second embodiment of the present invention; 
         FIG. 4  is a voltage detecting circuit configuration of a third embodiment of the present invention; 
         FIG. 5  is a voltage detecting circuit configuration of a fourth embodiment of the present invention; 
         FIG. 6  is a voltage detecting circuit configuration of a fifth embodiment of the present invention; 
         FIG. 7  is a voltage detecting circuit configuration of a sixth embodiment of the present invention; 
         FIG. 8  is a voltage detecting circuit configuration of a seventh embodiment of the present invention; 
         FIG. 9  is a voltage detecting circuit configuration of a eighth embodiment of the present invention; 
         FIG. 10  is a conventional general voltage detecting circuit; 
         FIG. 11  is a conventional voltage detecting circuit; and 
         FIG. 12  is a conventional voltage detecting circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention are described with reference to the drawings. 
       FIG. 1  is a voltage detecting circuit showing a first embodiment of the present invention. This voltage detecting circuit includes an NMOS depletion mode transistor M 1 , NMOS enhancement mode transistors M 2 , M 5 , and M 6 , PMOS enhancement mode transistors M 3 , M 4 , and M 7 , current sources I 1  and I 2 , inverters INV 1  through INV 3 , and resistors R 1  and R 2 . Further, an NMOS enhancement mode transistor M 8  is provided between a drain of the PMOS enhancement mode transistor M 7  and the current source I 2 . Moreover, a power source terminal Vdd, ground potential Vss, a voltage input terminal Vin, and an output terminal Vout are provided. 
     Here, a voltage to be monitored by the voltage detecting circuit is inputted to the input terminal Vin while a power source voltage by which this voltage detecting circuit is operated is inputted to the power source terminal Vdd. The voltage detecting circuit of the present invention can correctly detect a voltage Vin even when the power source voltage Vdd is decreased, however, the power source voltage  101  and the input terminal  102  may be short-circuited as in the conventional circuit shown in  FIG. 10  or  11 . That is, the power source terminal Vdd and the input terminal Vin in  FIG. 1  may be short-circuited as well. With such a configuration, a decrease of the input voltage Vin (that is, the power source voltage Vdd) is monitored. In the configuration of  FIG. 1  where these terminals are not short-circuited, on the other hand, a desired voltage Vin which is different than the power source voltage Vdd can be monitored. 
     In  FIG. 1 , a reference voltage generating circuit  1  is formed of the NMOS depletion mode transistor M 1  and the NMOS enhancement mode transistor M 2 , and generates a reference voltage Vref. 
     The NMOS depletion mode transistor M 1  has a drain connected to the power source terminal Vdd, and a source and a gate which are commonly connected to a drain of the NMOS enhancement mode transistor M 2 . The NMOS enhancement mode transistor M 2  has a gate connected to its drain, and a source connected to ground potential. 
     Since the gate and the source of the NMOS depletion mode transistor M 1  are connected, a zero bias voltage is applied as a gate voltage and a drain current has a predetermined constant current value. This constant current flows as a drain current of the NMOS enhancement mode transistor M 2 , therefore, a gate voltage of the NMOS enhancement mode transistor M 2  is a constant voltage determined by the drain current. Since the gate of the NMOS enhancement mode transistor M 2  is connected to its drain, a drain voltage of the NMOS enhancement mode transistor M 2  becomes equal to its gate voltage, which is a constant voltage. Then, this constant voltage is taken out as the reference voltage Vref. 
     A differential amplifier circuit  2  is formed of the NMOS enhancement mode transistors M 5  and M 6 , the PMOS enhancement mode transistors M 3  and M 4 , and the current source I 1 . 
     The NMOS enhancement mode transistors M 5  and M 6  serve as differential input transistors. Sources of the NMOS enhancement mode transistors MS and M 6  are commonly connected to ground potential through the current source I 1 . The reference voltage Vref is inputted to a gate of the NMOS enhancement mode transistor M 5 . Further, a voltage Vsen to be detected, which is obtained by dividing the voltage inputted to the voltage input terminal Vin by the resistors R 1  and R 2 , is inputted to a gate of the NMOS enhancement mode transistor M 6 . Here, a circuit  3  including the resistors R 1  and R 2  and capable of dividing the voltage inputted to the voltage input terminal Vin is called a voltage dividing circuit. Since the voltage inputted to the voltage input terminal Vin and the voltage Vsen are proportional to each other, it can be detected that the voltage inputted to the voltage input terminal Vin has become as low as or lower than the predetermined level by comparing the voltage Vsen and the reference voltage Vref. 
     Sources of the PMOS enhancement mode transistors M 3  and M 4  are connected to the power source terminal Vdd. Gates of the PMOS enhancement mode transistors M 3  and M 4  are commonly connected to a drain of the PMOS enhancement mode transistor M 4 . A drain of the PMOS enhancement mode transistor M 3  is connected to a drain of the NMOS enhancement mode transistor M 5 . The drain of the PMOS enhancement mode transistor M 4  is connected to a drain of the NMOS enhancement mode transistor M 6 . 
     An output voltage of the differential amplifier circuit  2  is taken out from the drain of the NMOS enhancement mode transistor M 5  and connected to a gate of the PMOS enhancement mode transistor M 7 . 
     An inverting amplifier circuit  4  is formed of the PMOS enhancement mode transistor M 7 , the NMOS enhancement mode transistor M 8 , and the current source I 2 . The PMOS enhancement mode transistor M 7  has a source connected to the power source terminal Vdd and a drain connected to a drain of the NMOS enhancement mode transistor M 8 . The detection voltage Vsen obtained by dividing the voltage inputted to the voltage input terminal Vin by the resistors R 1  and R 2  is inputted to a gate of the NMOS enhancement mode transistor M 8 . The current source I 2  is connected between the source of the NMOS enhancement mode transistor M 8  and the ground potential. An output voltage of the inverting amplifier circuit  4  is taken out from the drain of the PMOS enhancement mode transistor M 7  and inputted to the inverter INV 1 . 
     The inverters INV 1  through INV 3  are cascaded. An output voltage of the inverter INV 3  is used as the output Vout of the voltage detecting circuit. Here, the inverters INV 1  through INV 3  are used to realize a high speed response, however, plural inverters are not necessarily used. Effects of the present invention are not influenced at all by, for example, using only the inverter INV 1  and using its output voltage as an output voltage of the voltage detecting circuit. 
     Next, an operation of the voltage detecting circuit is described. 
     A voltage inputted to the voltage input terminal Vin is divided by the resistors R 1  and R 2  to be the detection voltage Vsen. The detection voltage Vsen is inputted to the gate of the NMOS enhancement mode transistor M 6 . Since the reference voltage Vref is inputted to the gate of the NMOS enhancement mode transistor M 5 , an output voltage of the differential amplifier circuit  2  is determined by comparing the voltage Vsen and the voltage Vref. 
     When the detection voltage Vsen is lower than the reference voltage Vref, the output voltage of the differential amplifier circuit  2 , that is a drain voltage of the NMOS enhancement mode transistor M 5  is decreased. Since the output voltage of the differential amplifier circuit  2  is inputted to the gate of the PMOS enhancement mode transistor M 7  of the inverting amplifier circuit  4 , a drain voltage of the PMOS enhancement mode transistor M 7 , that is an output voltage of the inverting amplifier circuit  4 , rises. When the drain voltage of the PMOS enhancement mode transistor M 7  becomes as high as or higher than a threshold voltage of the inverter INV 1 , the inverter INV 1  outputs an L-level signal. Thus, an output signal Vout of the voltage detecting circuit outputted through the INV 2  and INV 3  becomes L-level as well. As described above, when the power source voltage is decreased, the operation of the inverting amplifier circuit  4  becomes unstable. As a result, an unstable voltage is inputted to the inverter INV 1 . In view of this, in this embodiment, the NMOS enhancement mode transistor M 8  is provided between the drain of the PMOS enhancement mode transistor M 7  and the current source I 2 . A transistor having the same characteristics as the NMOS enhancement mode transistor M 2  is used as the NMOS enhancement mode transistor M 8 . A threshold voltage of the gate of the NMOS enhancement mode transistor M 8  is as low as or lower than the reference voltage Vref. The NMOS enhancement mode transistors M 8  and M 2  may be formed of transistors with different characteristics as long as the threshold voltage of the NMOS enhancement mode transistor M 8  is as low as or lower than the threshold voltage of the NMOS enhancement mode transistor M 2 . In this manner, when the detection voltage Vsen is lower than the reference voltage Vref, impedance of the NMOS enhancement mode transistor M 8  is increased. Therefore, the drain voltage of the PMOS enhancement mode transistor M 7  is not decreased even when the power source voltage Vdd of the voltage detecting circuit is decreased and impedance of the PMOS enhancement mode transistor M 7  is increased. Thus, the input voltage of the inverter INV 1  can be kept at an H-level and the level of an output voltage of the voltage detecting circuit can be stabilized. 
     When the detection voltage Vsen is higher than the reference voltage Vref, on the other hand, the output voltage of the differential amplifier circuit  2 , that is the drain voltage of the transistor M 5 , rises. Since the output voltage of the differential amplifier circuit  2  is inputted to the gate of the PMOS enhancement mode transistor M 7  as an input voltage of the inverting amplifier circuit  4 , the impedance of the PMOS enhancement mode transistor M 7  is increased. As a result, the drain voltage of the PMOS enhancement mode transistor M 7  as the output voltage of the inverting amplifier circuit  4  is decreased. When the drain voltage of the PMOS enhancement mode transistor M 7  is decreased to be as low as or lower than the threshold voltage of the inverter INV 1 , the inverter INV 1  outputs an H-level signal. Since this signal is inverted by the inverters INV 2  and INV 3 , an H-level signal is outputted as an output voltage of the voltage detecting circuit. At this time, the NMOS enhancement mode transistor M 8  is on, therefore, operations are similar to the case where the NMOS enhancement mode transistor M 8  is not provided. 
     In this embodiment, the inverting amplifier circuit  4  serving as an output circuit is formed of the PMOS enhancement mode transistor M 7  and the NMOS enhancement mode transistor M 8 . Therefore, threshold values can be easily controlled in manufacture. Further, since the threshold voltage of the NMOS enhancement mode transistor M 8  is set as low as or lower than the reference voltage Vref, load impedance of the inverting amplifier circuit  4  can be controlled only when the detection voltage Vsen is as low as or lower than the reference voltage Vref. Moreover, the reference voltage Vref is a gate voltage of the MOS transistor M 2 , of which drain current is the drain current of the depletion mode transistor M 1  to which a zero bias voltage is applied. Therefore, a MOS transistor having the same characteristics as the MOS transistor M 2  used for generating the reference voltage Vref can be used as the NMOS enhancement mode transistor M 8 . 
       FIGS. 2A and 2B  show another embodiment of an output circuit of the present invention.  FIG. 2A  shows an output circuit  21  and  FIG. 2B  shows an output circuit  31 . 
     The output circuit  21  shown in  FIG. 2A  includes an inverter  22  formed of a PMOS enhancement mode transistor M 21  and an NMOS enhancement mode transistor M 22 , and a PMOS enhancement mode transistor M 23  connected between the power source terminal Vdd and a source of the PMOS enhancement mode transistor M 21 . A source of the NMOS enhancement mode transistor M 22  and a gate of the PMOS enhancement mode transistor M 23  are connected to ground potential Vss. A source of the PMOS enhancement mode transistor M 23  is connected to the power source terminal Vdd. The output circuit  31  shown in  FIG. 2B  includes an inverter  32  formed of a PMOS enhancement mode transistor M 31  and an NMOS enhancement mode transistor M 32 , and a PMOS enhancement mode transistor M 33  connected between a drain of the PMOS enhancement mode transistor M 31  and a drain of the NMOS enhancement mode transistor M 32 . A source of the NMOS enhancement mode transistor M 32  and a gate of the PMOS enhancement mode transistor M 33  are connected to ground potential Vss. A source of the PMOS transistor M 31  is connected to the power source terminal Vdd. In  FIGS. 2A and 2B , IN and OUT denote an input terminal and an output terminal of the output circuit, respectively. 
     By using the circuit shown in  FIG. 2A  or  2 B as an output circuit of the voltage detecting circuit, the voltage detecting circuit is operated stably. That is, in the circuits shown in  FIGS. 2A and 2B , in a voltage area where the operation of the circuit becomes unstable due to a decrease of the power source voltage Vdd, a path between the power source voltage Vdd and the output terminal OUT is blocked by turning off the PMOS enhancement mode transistor M 23  or M 33 . Accordingly, the output voltage of the output terminal OUT can be stabilized. That is, by connecting the output circuit shown in  FIG. 2A  or  2 B as a subsequent stage after the differential amplifier circuit, the output voltage Vout of the voltage detecting circuit can be kept at a predetermined level even when the power source voltage Vdd is decreased. The level of a decrease in the power source voltage, by which the path between the power source voltage terminal Vdd and the output terminal OUT is determined to be blocked, can be set appropriately as required. The same applies to fourth through seventh embodiments that are described below, in which an NMOS transistor is used instead of the PMOS transistor so that a path between ground potential Vss and the output terminal Vout is blocked when the power source voltage Vdd is decreased. 
     In  FIGS. 2A and 2B , the PMOS transistor M 23  or M 33  corresponds to the NMOS enhancement mode transistor M 8  in  FIG. 1 . The detection voltage Vsen is inputted to the gate of the NMOS transistor M 8  while the gate of each of the PMOS transistor M 23  and M 33  is grounded so that a function equivalent to that of the NMOS enhancement mode transistor M 8  is realized. 
       FIG. 3  shows a voltage detecting circuit of a second embodiment of the present invention.  FIG. 12  shows a conventional voltage detecting circuit as a comparison example. In  FIG. 3 , an output circuit  21  is provided instead of an output circuit  116  in  FIG. 12 . That is, the output circuit  21  shown in  FIG. 2A  is used as an output circuit in  FIG. 3 . 
     In  FIGS. 3 and 12 , the reference voltage Vref generated by the reference voltage generating circuit  1  shown in  FIG. 1  is supplied from a power source V, and the comparison circuit formed of the differential amplifier circuit  2  shown in  FIG. 1  is expressed by COMP. However, these components as well as a function of the voltage dividing circuit  3  are equivalent to those shown in  FIG. 1  and detailed descriptions thereof are omitted. Hereinafter, the same applies to the third through seventh embodiments described with reference to  FIGS. 4 to 8 . 
     In such a configuration, a detection voltage Vsen obtained by dividing an input voltage Vin by resistors R 1  and R 2  is inputted to an inverting input terminal of the comparison circuit COMP. On the other hand, the reference voltage Vref supplied from the power source V is inputted to a non-inverting input terminal of the comparison circuit COMP. An output voltage of the comparison circuit COMP is inputted through inverters INV 4  and INV 5  to the inverter  22  of the output circuit  21 . An output voltage of the output circuit  21  is taken out as the output voltage Vout. 
     Next, an operation of the voltage detecting circuit shown in  FIG. 3  is described. 
     When the detection voltage Vsen is lower than the reference voltage Vref, the comparison circuit COMP outputs an H-level signal. Therefore, an input voltage A of the inverter  22  becomes the H-level through the inverters INV 4  and INV 5 . Thus, the output voltage Vout becomes an L-level. At this time, when the power source voltage Vdd is decreased, operations of the comparison circuit COMP, and the inverters INV 4  and INV 5  become unstable. As a result, the input voltage A of the inverter  22  cannot be kept at an H-level. In this embodiment, the PMOS enhancement mode transistor M 23  having the gate connected to the ground potential Vss is turned off at this time. Consequently, a path between the power source terminal Vdd and the output terminal Vout is blocked. Therefore, the output voltage Vout can be kept at an L-level even when the power source voltage Vdd is decreased. In this second embodiment, the circuit configuration is simpler and more effective than that of the first embodiment because Vout can be kept at an L-level until Vdd becomes almost 0 V. In addition, since the PMOS enhancement mode transistor M 23  capable of blocking the output path is provided in a stage after the inverter provided closest to the output terminal Vout among the plural stages of inverters, the output voltage can be accurately fixed at a required level. 
     Note that the present invention has been made to solve a defect caused when the detection voltage Vsen becomes lower than the reference voltage Vref and when the power source voltage Vdd becomes lower than the predetermined operational voltage. When Vdd is decreased in the case where Vsen is higher than Vref, Vout is kept at a level existing just before Vdd is decreased. Therefore, no defect is generated. The same applies to the third through seventh embodiments. 
     Next, a voltage detecting circuit of the third embodiment of the present invention is described with reference to  FIG. 4 . 
     In  FIG. 4 , the output circuit  31  is provided instead of the output circuit  116  in  FIG. 12 . That is, the output circuit  31  shown in  FIG. 2B  is used as an output circuit in  FIG. 4 . 
     In such a configuration, the detection voltage Vsen obtained by dividing the input voltage Vin is inputted to the inverting input terminal of the comparison circuit COMP. On the other hand, the reference voltage Vref supplied from the power source V is inputted to a non-inverting input terminal of the comparison circuit COMP. Then, an output voltage of the comparison circuit COMP is inputted through the inverters INV 4  and INV 5  to the inverter  32  formed of the PMOS enhancement mode transistor M 31  and the NMOS enhancement mode transistor M 32  of the output circuit  31 . Further, a drain voltage of the PMOS transistor M 33  is taken out as the output voltage Vout. 
     An operation of the voltage detection circuit shown in  FIG. 4  is similar to the second embodiment described with reference to  FIG. 3 . When the power source voltage Vdd is decreased in the case where the detection voltage Vsen is lower than the reference voltage Vref, the PMOS transistor M 33  having the gate connected to ground potential Vss is turned off. Therefore, a path between the power source terminal Vdd and the output terminal Vout is blocked. As a result, the output voltage Vout can be kept at an L-level. Similar to the second embodiment, the circuit configuration of this embodiment is simpler than the first embodiment. Moreover, Vout can be kept at an L-level until Vdd becomes almost 0 V. In addition, since the PMOS enhancement mode transistor M 33  capable of blocking the output path is provided in a stage of the inverter provided closest to the output terminal Vout among the plural stages of inverters, the output voltage can be accurately fixed at a required level. 
       FIGS. 5 and 6  show voltage detecting circuits of the fourth and fifth embodiments, respectively, of the present invention. 
     The fourth embodiment is different from the second embodiment described with reference to  FIG. 3  in that an NMOS transistor M 43  is provided instead of the PMOS enhancement mode transistor M 23 . Similarly, the fifth embodiment is different from the third embodiment described with reference to  FIG. 4  in that an NMOS transistor M 53  is provided instead of the PMOS enhancement mode transistor M 33 . 
     In  FIG. 5 , a gate of the NMOS transistor  43  capable of blocking the output path is connected to the detection voltage Vsen obtained by dividing the input voltage Vin by the resistors R 1  and R 2 . An output circuit  41  is connected between inverters INV 6  and INV 7 . In these embodiments, the input voltage Vin and the power source voltage Vdd are short-circuited (that is, Vin=Vdd). 
     In this configuration, the detection voltage Vsen obtained by dividing the input voltage Vin is inputted to the inverting input terminal of the comparison circuit COMP. On the other hand, the reference voltage Vref supplied from the power source V is inputted to the non-inverting input terminal of the comparison circuit COMP. An output voltage of the comparison circuit COMP is inputted through the inverter INV 6  to an inverter  42  formed of a PMOS transistor M 41  and an NMOS transistor M 42  of the output circuit  41 . Moreover, a drain voltage of the PMOS transistor M 41  is taken out as the output voltage Vout through the inverter INV 7 . 
     In such a configuration, when the detection voltage Vsen becomes lower than the reference voltage Vref, the comparison circuit COMP outputs an H-level voltage. The output voltage Vout becomes an L-level through the inverter INV 6 , the output circuit  41 , and the inverter INV 7 . When the power source voltage Vdd is decreased at this time, Vsen is decreased as well. Therefore, the NMOS transistor M 43  having a gate receiving Vsen is turned off and an input path between ground potential Vss and the inverter INV 7  is blocked. As a result, the input voltage of the inverter INV 7  is kept at an H-level. Therefore, the output voltage Vout can be kept at an L-level through the inverter INV 7 . 
     In the fifth embodiment shown in  FIG. 6 , the NMOS transistor M 53  is connected between the PMOS transistor M 51  and the NMOS transistor M 52 . Similarly to the fourth embodiment, when the power source voltage Vdd is decreased in the case where the detection voltage Vsen is lower than the reference voltage Vref, the NMOS transistor M 53  is turned off. As a result, since an input path between ground potential Vss and the inverter INV 7  is blocked, the input voltage of the inverter INV 7  can be kept at an H-level. Consequently, the output voltage Vout can be kept at an L-level through the inverter INV 7 . 
       FIGS. 7 and 8  show voltage detecting circuits of the sixth and seventh embodiments, respectively, of the present invention. 
     In the sixth embodiment, a gate of an NMOS transistor M 63  (that is, the gate of the NMOS transistor M 43  of the fourth embodiment described with reference to  FIG. 5 ) is connected to the power source voltage terminal Vdd instead of the detection voltage Vsen. Similarly, in the seventh embodiment, a gate of an NMOS transistor M 73  (that is, the gate of the NMOS transistor M 53  of the fifth embodiment described with reference to  FIG. 6 ) is connected to the power source voltage terminal Vdd instead of the detection voltage Vsen. These embodiments are different from the fourth or fifth embodiment in that the input voltage terminal Vin and the power source voltage terminal Vdd are not short-circuited and an input voltage independent of Vdd can be detected. 
     In the sixth or seventh embodiment, an operation is similar to the fourth or fifth embodiment. That is, when the detection voltage Vsen becomes lower than the reference voltage Vref, the comparison circuit COMP outputs an H-level voltage. The output voltage Vout becomes an L-level through the inverter INV 6 , an output circuit  61  or  71 , and the inverter INV 7 . When the power source voltage Vdd is decreased, the NMOS transistor M 63  or M 73  having a gate receiving the power source voltage Vdd is turned off. As a result, since an input path between ground potential Vss and the inverter INV 7  is blocked, the input voltage of the inverter INV 7  is kept at an H-level. Therefore, the output voltage through the inverter INV 7  can be kept at an L-level. 
     In the second and third embodiments, the output circuit  21  or  31  is provided as a subsequent stage of the inverters INV 4  and INV 5  in, while the output circuits  41  to  71  are provided between the inverters INV 6  and INV 7  in the fourth through seventh embodiments. This depends on whether the transistors M 23 , M 33 , M 43 , M 53 , M 63 , and M 73  for stabilizing the level of the output voltage Vout are PMOS transistors or NMOS transistors in the present invention. In the second and third embodiments employing the PMOS transistors M 23  and M 33 , the output voltage of the output circuit  21  or  31  is kept at an L-level due to circuit characteristics. On the other hand, in the fourth through seventh embodiments employing the NMOS transistors M 43 , M 53 , M 63 , and M 73 , the output voltage of the output circuits  41  to  71  is kept at an H-level. Further, the inverters INV 4  through INV 7  are used for improving response characteristics. With one stage of inverter, an output level is inverted from an L-level to an H-level, or from an H-level to an L-level. Therefore, plural units of two stages of inverters may be additionally provided since the properties of the circuit are not affected. Therefore, when a PMOS transistor is used for stabilizing the output voltage in these embodiments, an output circuit is to be provided as a subsequent stage after an even-numbered inverter when seen from the comparison circuit COMP. Further, when an NMOS transistor is used for stabilizing the output voltage, an output circuit is to be provided as a subsequent stage after an odd-numbered stage of inverter when seen from the comparison circuit COMP. Consequently, these circuits have equivalent characteristics. It is to be noted that the output circuit can be provided as a subsequent stage after an odd-numbered stage of inverter when a PMOS transistor is used to stabilize the output voltage, and the output circuit can be provided as a subsequent stage after an even-numbered stage of inverter when an NMOS transistor is used to stabilize the output voltage, depending on the required circuit characteristics. 
       FIG. 9  shows a voltage detection circuit of an eighth embodiment of the present invention. This voltage detecting circuit is different from  FIG. 1  in that a differential amplifier circuit  5  formed of an NMOS depletion mode transistor M 11 , an NMOS enhancement mode transistor M 12 , and PMOS enhancement mode transistors M 13  and M 14  is used instead of the reference voltage generating circuit  1  and the differential amplifier circuit  2  shown in  FIG. 1 . 
     A source and a gate of the NMOS depletion mode transistor M 11  are grounded, and a drain of the NMOS depletion mode transistor M 11  is connected to a drain of the PMOS enhancement mode transistor M 13 . 
     The PMOS enhancement mode transistor M 13  has a source connected to the power source terminal Vdd and a gate connected to a gate and a drain of the PMOS enhancement mode transistor M 14 . The PMOS enhancement mode transistor M 14  has a source connected to the power source terminal Vdd. Therefore, the PMOS enhancement mode transistor M 14  and the PMOS enhancement transistor M 13  form a current mirror circuit. Moreover, the drain of the PMOS enhancement mode transistor M 14  is connected to a drain of the NMOS enhancement mode transistor M 12 . The NMOS enhancement mode transistor M 12  has a grounded source and a gate receiving the detection voltage Vsen obtained by dividing the voltage inputted to the voltage input terminal Vin by the resistors R 1  and R 2 . Similar to the first embodiment, the voltage Vsen obtained by dividing the voltage inputted to the voltage input terminal Vin by the voltage dividing circuit  3  is the detection voltage in this embodiment. A voltage outputted to the inverting amplifier circuit  4  is taken out from the drain of the NMOS depletion mode transistor M 11  and inputted to the gate of the PMOS enhancement mode transistor M 7  included in the inverting amplifier circuit  4 . Note that a description of the circuit (inverting amplifier circuit  4 ) on the right side of a serial circuit of the resistors R 1  and R 2  (voltage dividing circuit  3 ) in  FIG. 9 , which is similar to  FIG. 1 , is not repeated. 
     Since the source and gate of the NMOS depletion mode transistor M 11  are grounded, a zero bias voltage is applied as a gate voltage. Thus, a drain current of the NMOS depletion mode transistor M 11  has a predetermined constant current value. This constant current flows as a drain current of the PMOS transistor M 13 . An output voltage (drain voltage of the NMOS depletion mode transistor M 11 ) of the differential amplifier circuit  5  formed of the transistors M 11  through M 14  is determined by the drain current of the NMOS depletion mode transistor M 11  and the drain current of the NMOS enhancement mode transistor M 12 . A gate voltage of the NMOS enhancement mode transistor M 12  at a time when an output voltage of the differential amplifier circuit  5  is inverted is determined by the drain current of the NMOS depletion mode transistor M 11 . The gate voltage of the NMOS enhancement mode transistor M 12  is used as the reference voltage Vref. In the following description, this voltage is called a deemed reference voltage Vref. 
     In this circuit, the same transistors are used as the NMOS transistors M 8  and M 12  so that the threshold voltage of the NMOS enhancement mode transistor M 8  is as low as or lower than the threshold voltage of the NMOS enhancement mode transistor M 12 . The NMOS enhancement mode transistors M 8  and M 12  may be formed of different transistors as long as the threshold voltage of the NMOS enhancement mode transistor M 8  is as low as or lower than the threshold voltage of the NMOS enhancement mode transistor M 12 . 
     When the voltage Vsen is higher than the deemed reference voltage Vref, the drain voltage of the NMOS depletion transistor M 11  rises. Then, impedance of the PMOS enhancement mode transistor M 7  is increased, thereby the input voltage of the inverter INV 1  is decreased to be as low as or lower than the threshold voltage of the inverter INV 1 . As a result, the output voltage Vout of the voltage detecting circuit becomes an H-level. Since the NMOS enhancement mode transistor M 8  is on, the NMOS enhancement mode transistor M 8  functions only to connect the constant current source I 2  to a load of the PMOS enhancement mode transistor M 7  and does not affect the operation of the voltage detecting circuit. 
     When the detection voltage Vsen is lower than the deemed reference voltage Vref, the NMOS enhancement transistor M 12  is turned off. As a result, the drain voltage of the NMOS depletion mode transistor M 11  is decreased. Then, since the impedance of the PMOS enhancement mode transistor M 7  is decreased, the input voltage of the inverter INV 1  is raised to be as high as or higher than the threshold voltage of the inverter INV 1 . Moreover, the impedance of the NMOS enhancement mode transistor M 8  is increased, therefore, the input voltage of the inverter INV 1  is further increased. As a result, the volatge detecting circuit outputs an L-level output voltage Vout. 
     When the power source voltage Vdd of the voltage detecting circuit is decreased in the case where the detection voltage Vsen is lower than the deemed reference voltage Vref, a gate-source voltage of the PMOS enhancement mode transistor M 7  falls so that the impedance of the PMOS enhancement transistor M 7  is increased. However, since the impedance of the NMOS enhancement mode transistor M 8  is high, the drain voltage of the PMOS enhancement mode transistor M 7  can be kept at an H-level. As a result, an operation of the voltage detecting circuit can be stabilized. 
     According to one embodiment, a voltage detecting circuit of the present invention can detect a voltage correctly even when a power source voltage is decreased, and can be manufactured easily. 
     This patent application is based on Japanese Priority Patent Application No. 2008-013449 filed on Jan. 24, 2008, and Japanese Priority Patent Application No. 2008-078182 filed on Mar. 25, 2008, the entire contents of which are hereby incorporated herein by reference.