Abstract:
In a power detector, a comparator for detection receives an input signal and a reference voltage, and compares the input signal to the reference voltage around the switching time of active and inactive states of the output of the comparator in accordance with an output of an input switching signal generator. Except for the switching time, an input voltage for non-use of the comparator is inputs to the comparator for detection, and the differential inputs are fixed to the same potential. Therefore, aging reduction in the accuracy of power detection caused by BT degradation is effectively mitigated.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This is a continuation of PCT International Application PCT/JP2010/007174 filed on Dec. 9, 2010, which claims priority to Japanese Patent Application No. 2010-132472 filed on Jun. 9, 2010. The disclosures of these applications including the specifications, the drawings, and the claims are hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates to power detectors detecting power and outputting signals. 
         [0003]    In general, a power detector detecting power based on a predetermined voltage is used as a reset signal generator for a calculator in a semiconductor integrated circuit. When a power supply voltage is lower than or equal to a predetermined voltage Vo, a power detection signal, which indicates that power is undetected, is generally output to stop the calculator. On the other hand, when the power supply voltage is higher than the predetermined voltage Vo, a power detection signal, which indicates that power is detected, is output to operate the calculator, thereby operating the semiconductor integrated circuit. 
         [0004]    The configuration of a power detector will be described. 
         [0005]    A power detector includes, as shown in  FIG. 8 , a power voltage divider  11  and a comparator  14 . The power voltage divider  11  linearly divides a power supply voltage using two resistors arranged in series between a power supply terminal and a ground terminal, and outputs a signal of an output voltage  12 , which is the voltage at the time of division. The comparator  14  compares the output voltage  12  of the power voltage divider  11  to a reference voltage  13 . 
         [0006]    The comparator  14  has, for example, a configuration shown in  FIG. 10 . Specifically, the comparator  14  includes two MOS transistors  20  and  21 , which have equal threshold voltages, a current supply  22 , and a current minor  23 , which is a semiconductor element. In the two MOS transistors  20  and  21 , gates are coupled to inputs of the comparator  14 , sources are commonly coupled to the current supply  22 , and drains are coupled to the current minor  23 . A drain  24  of one of the MOS transistors is coupled to an inverter  25 . Then, an output  26  of the inverter  25  is the output of the comparator  14 . The comparator  14  inverts the output of the inverter  25  when gate voltages of the two MOS transistors  20  and  21  are equal. 
         [0007]    The operation of the power detector shown in  FIG. 8  will be described. 
         [0008]    In  FIG. 8 , the output voltage  12  of the power voltage divider  11  is a voltage obtained by dividing the power supply voltage with the resistors, and thus changes in proportion to the power supply voltage. 
         [0009]    On the other hand, the reference voltage  13  is always fixed to the output voltage  12  of the power voltage divider  11  at the time when the power supply voltage is the predetermined voltage Vo, and is always constant regardless of the power supply voltage. 
         [0010]    The comparator  14  receives the output voltage  12  of the power voltage divider  11  and the reference voltage  13 , and inverts the power detection signal output from the comparator  14  when the two inputs are equal. 
         [0011]    In  FIG. 9 , based on the comparison result of the comparator  14 , when the output voltage  12  of the power voltage divider  11  is lower than or equal to the reference voltage  13 , the comparator  14  generally regards the power supply voltage as being lower than or equal to the predetermined voltage Vo, and outputs a power detection signal indicating that the power supply is undetected (a low level in  FIG. 9 ). When the output voltage  12  of the power voltage divider  11  is higher than the reference voltage  13 , the comparator  14  regards the power supply voltage as being higher than the predetermined voltage Vo, and outputs a power detection signal  15  indicating that the power supply is detected (a high level in  FIG. 9 ). 
         [0012]    In the power detector operating as described above, the comparator  14  receives the voltage  12  changing in proportional to the power supply voltage at one input, and the constant reference voltage  13  at the other input. This causes a difference in bias application between the inputs of the comparator  14 . 
         [0013]    An MOS transistor has the problem of bias temperature (BT) degradation. BT degradation is fluctuations in the threshold voltage of an MOS transistor caused by applying positive or negative bias to the gate of the MOS transistor. Moreover, it is known that the amount of fluctuations changes depending on the temperature or the time for applying bias. 
         [0014]    An MOS transistor, in which BT degradation occurs, influences the characteristics of an analog circuit. For example, the characteristics of the comparator  14  of inverting an output when input voltages are equal are on the assumption that the threshold voltages of the two MOS transistors  20  and  21  are equal. 
         [0015]    However, a difference in bias application arises between the inputs of the comparator  14  in the power detector, thereby causing a difference in the amount of fluctuations in the threshold voltages due to BT degradation in the two MOS transistors  20  and  21 , of which the gates are coupled to the inputs. This causes a difference in threshold voltage between the two MOS transistors  20  and  21 . Thus, the comparator  14  inverts the output when the input signals are not equal, i.e., the predetermined voltage Vo fluctuates, at which a power voltage is to be detected. 
         [0016]    Therefore, BT degradation reduces the power detection accuracy of the power detector. 
         [0017]    As a known measure against the problem, for example, Japanese Patent Publication No. 2004-172796 (e.g., page 1, FIG. 2, etc.) suggest clamping the gates of MOS transistors to be protected from BT degradation to the same potential as the sources to prevent BT degradation when a circuit is inactive.  FIG. 11  illustrates this configuration. In the drawing, MOS transistors  30  and  31  are to be protected from BT degradation. When the circuit is inactive, semiconductor elements  32  and  33  are powered on, and the gates and sources of the MOS transistors  30  and  31  are set to the same potential. 
       SUMMARY 
       [0018]    However, the measure against BT degradation suggested in Japanese Patent Publication No. 2004-172796 is on the assumption that the circuit is inactive, and is thus not applicable to a power detector which always needs to operate. 
         [0019]    The present disclosure was made in view of the problem. It is an objective of the present disclosure to provide a power detector, which detects power by comparing an output voltage of a power voltage divider to a reference voltage with a comparator, and outputs a signal, while preventing BT degradation and always enabling operation. 
         [0020]    In order to achieve the objective, in the present disclosure, the comparator is divided to the comparator for detection, which detects a predetermined voltage Vo, and an auxiliary comparator, which determines whether a power supply voltage is around the predetermined voltage Vo. Only when the power supply voltage is around the predetermined voltage Vo, the comparator for detection operates. When the power supply voltage is not around the predetermined voltage Vo, a same voltage is applied to two inputs of the comparator for detection. 
         [0021]    That is, when the power supply voltage is around the predetermined voltage Vo, the comparator for detection operates. However, the difference in bias application between the two inputs is small, and thus, the difference in fluctuations in the threshold voltages caused by BT degradation is small. Therefore, the accuracy of power detection is less reduced. 
         [0022]    On the other hand, when the power supply voltage is not around the predetermined voltage Vo, a same voltage is applied to the two inputs of the comparator for detection, and BT degradation in the two inputs progresses at the same speed. This prevents an increase in the difference in fluctuations in the threshold voltages, and thus the accuracy of power detection does not decrease. 
         [0023]    As a result, measures can be taken against BT degradation in a comparator for detection detecting a predetermined voltage Vo, and a power detector can always operate by combining the comparator for detection with an auxiliary comparator. 
         [0024]    In general, when input voltages of a comparator are equal, the output is inconstant. In a conventional power detector, however, this phenomenon is hardly problematic, since input voltages of a comparator are equal for a short time. In the power detector according to the present disclosure, the equal voltage is applied to the inputs of the comparator for detection when the power supply voltage is not around the predetermined voltage Vo. Thus, the output of the comparator for detection is inconstant for a long time period. If the indefinite state is transmitted to a power detection signal, an accurate power detection signal cannot be obtained. The power detector according to the present disclosure includes, at a subsequent stage of the comparator for detection, a fixing unit for fixing the output to a constant voltage when the power supply voltage is not around the predetermined voltage Vo. 
         [0025]    Specifically, a power detector according to a first aspect of the present disclosure includes a power voltage divider configured to divide a voltage of a power supply; a comparator for detection configured to compare a first output voltage of the power voltage divider to a reference voltage; a higher voltage side auxiliary comparator configured to compare a second output voltage of the power voltage divider, which is higher than the first output voltage, to the reference voltage; a lower voltage side auxiliary comparator configured to compare a third output voltage of the power voltage divider, which is lower than the first output voltage, to the reference voltage; an input switching signal generator configured to generate an input switching signal based on an output of the higher voltage side auxiliary comparator and an output of the lower voltage side auxiliary comparator; an input switch configured to switch inputs of the comparator for detection from the reference voltage and the first output voltage to an input voltage for non-use of the comparator in accordance with the input switching signal; a signal fixing unit configured to fix an output of the comparator for detection to a constant voltage when the input switch selects the input voltage for non-use of the comparator; and a power detection signal generator configured to generate a power detection signal based on the output of the lower voltage side auxiliary comparator and an output of the signal fixing unit. 
         [0026]    A power detector according to a second aspect of the present disclosure includes a power voltage divider configured to divide a voltage of a power supply; a comparator for detection configured to compare a first output voltage of the power voltage divider to a reference voltage; a higher voltage side auxiliary comparator configured to compare a second output voltage of the power voltage divider, which is higher than the first output voltage, to the reference voltage; an input switch configured to switch inputs of the comparator for detection from the reference voltage and the first output voltage to an input voltage for non-use of the comparator in accordance with an output of the higher voltage side auxiliary comparator; and a signal fixing unit configured to fix an output of the comparator for detection to a constant voltage when the input switch selects the input voltage for non-use of the comparator. 
         [0027]    According to a third aspect of the present disclosure, in the power detector according to the second aspect, the higher voltage side auxiliary comparator may have lower comparison accuracy than the comparator for detection. 
         [0028]    According to a fourth aspect of the present disclosure, in the power detector according to the second aspect, the signal fixing unit may be a selector. 
         [0029]    According to a fifth aspect of the present disclosure, the power detector according to the second aspect may further include a power breaker configured to break power of the comparator for detection when the input switch selects the input voltage for non-use of the comparator as the input of the comparator for detection. 
         [0030]    A power detector according to a sixth aspect of the present disclosure includes a power voltage divider configured to divide a voltage of a power supply; a comparator for detection configured to compare a first output voltage of the power voltage divider to a reference voltage; a lower voltage side auxiliary comparator configured to compare a second output voltage of the power voltage divider, which is lower than the first output voltage, to the reference voltage; an input switch configured to switch inputs of the comparator for detection from the reference voltage and the first output voltage to an input voltage for non-use of the comparator in accordance with an output of the lower voltage side auxiliary comparator; a signal fixing unit configured to fix an output of the comparator for detection to a constant voltage when the input switch selects the input voltage for non-use of the comparator; and a power detection signal generator configured to generate a power detection signal based on the output of the lower voltage side auxiliary comparator and an output of the signal fixing unit. 
         [0031]    In the first to sixth aspects of the present disclosure, an increase in the difference in fluctuations in the threshold voltages caused by BT gradation between differential input transistors of a comparator for detection can be mitigated, thereby mitigating reduction in the accuracy of power detection. 
         [0032]    In particular, in the third aspect, the area of the higher voltage side auxiliary comparator can be reduced, thereby reducing the area of a circuit. The advantage can be obtained because the accuracy of the auxiliary comparator is not required as much as the accuracy of comparator for detection, while the accuracy of semiconductor devices tends to decrease due to variations in a manufacturing process with reduction in the areas of the semiconductor devices in the circuit. 
         [0033]    In the fourth aspect, the signal fixing unit is a selector. An inconstant output of the comparator for detection can be prevented with a relatively small area when the input switch selects the input voltage for non-use of the comparator as the input of the comparator for detection. 
         [0034]    In addition, in the fifth aspect, no current flows to the comparator for detection when the input switch selects the input voltage for non-use of the comparator as the input of the comparator for detection, thereby reducing the power consumption. 
         [0035]    As described above, the power detector according to the present disclosure has a circuit configuration which fixes differential inputs of the comparator for detection to same potential except for the time around the switching time between the active and inactive states of the output of the comparator for detection. Therefore, the power detector is advantageous in mitigating reduction in the accuracy of power detection caused by BT degradation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]      FIG. 1  illustrates a configuration of a power detector according to a first embodiment of the present disclosure. 
           [0037]      FIG. 2A  illustrates a waveform of an output voltage of a power voltage divider to a comparator for detection, waveforms of inputs and an output of a higher voltage side auxiliary comparator, and waveforms of inputs and an output of a lower voltage side auxiliary comparator, in the first embodiment.  FIG. 2B  illustrates a waveform of an input switching signal to an input switch.  FIG. 2C  illustrates a waveform of an output signal of the comparator for detection.  FIG. 2D  illustrates a waveform of an output of a signal fixing circuit.  FIG. 2E  illustrates a waveform of a power detection signal. 
           [0038]      FIG. 3  illustrates a configuration of a power detector according to a second embodiment of the present disclosure. 
           [0039]      FIG. 4A  illustrates a waveform of an output voltage of a power voltage divider to a comparator for detection, and waveforms of inputs and an output of a higher voltage side auxiliary comparator, in the second embodiment.  FIG. 4B  illustrates a waveform of an output signal of the comparator for detection.  FIG. 4C  illustrates a waveform of a power detection signal. 
           [0040]      FIG. 5  illustrates a configuration of a power detector according to a third embodiment of the present disclosure. 
           [0041]      FIG. 6A  illustrates a waveform of an output voltage of a power voltage divider to a comparator for detection, and waveforms of inputs and an output of a lower voltage side auxiliary comparator, in the third embodiment.  FIG. 6B  illustrates a waveform of an output signal of the comparator for detection.  FIG. 6C  illustrates a waveform of an output of a signal fixing circuit.  FIG. 6D  illustrates a waveform of a power detection signal. 
           [0042]      FIG. 7  illustrates a configuration of a power detector according to a variation of the first embodiment. 
           [0043]      FIG. 8  illustrates an example configuration of a conventional power detector. 
           [0044]      FIG. 9  illustrates a waveform of a power detection signal of the conventional power detector. 
           [0045]      FIG. 10  illustrates a configuration of a comparator included in the conventional power detector. 
           [0046]      FIG. 11  illustrates a configuration of a circuit suggested in Japanese Patent Publication No. 2004-172796. 
       
    
    
     DETAILED DESCRIPTION 
       [0047]    Embodiments of the present disclosure will be described hereinafter with reference to the drawings. In the embodiments, the same reference characters are used to represent equivalent elements operating similarly, and the repetitive explanation thereof may be omitted. 
       First Embodiment 
       [0048]      FIG. 1  is a circuit diagram of a power detector according to this embodiment.  FIGS. 2A-2E  illustrate waveforms in the power detector. 
         [0049]    In  FIG. 1 , the power detector includes a power voltage divider  101  dividing the voltage of the power supply  100 , a comparator  104  for detection comparing a first output voltage  102  of the power voltage divider  101  to a reference voltage  103 , a higher voltage side auxiliary comparator  107  comparing a second output voltage  106  of the power voltage divider  101 , which is higher than the first output voltage  102 , to the reference voltage  103 , and a lower voltage side auxiliary comparator  109  comparing a third output voltage  108  of the power voltage divider  101 , which is lower than the first output voltage  102 , to the reference voltage  103 . 
         [0050]    The power detector of  FIG. 1  further includes an input switching signal generator (an input switching signal generating means)  112  generating an input switching signal  113  based on an output of the higher voltage side auxiliary comparator  107  and an output of the lower voltage side auxiliary comparator  109 , an input switch (an input switching means)  111  switching inputs of the comparator  104  for detection from the combination of the reference voltage  103  and the first output voltage  102  to an input voltage  110  for non-use of the comparator when the input switching signal  113  is inactive, a signal fixing circuit (a signal fixing means)  115  fixing an output of the comparator  104  for detection to a constant voltage when the input switching signal  113  is inactive, and a power detection signal generator (a power detection signal generating means)  117  generating a power detection signal  105  based on the output of the lower voltage side auxiliary comparator  109  and an output of the signal fixing circuit  115 . 
         [0051]    Then, the detailed configuration and operation of the power detector of  FIG. 1  will be described. 
         [0052]    The power supply  100  is the power supply to be detected by the power detector. The power voltage divider  101  divides the voltage of the power supply  100  using, for example, a resistor to generate the output voltage  102  to the comparator  104  for detection, the output voltage  106  to the higher voltage side auxiliary comparator  107 , and the output voltage  108  to the lower voltage side auxiliary comparator  109 . 
         [0053]    The comparator  104  for detection receives the output voltage  102  and the reference voltage  103  via the input switch  111 , and compares the output voltage  102  to the reference voltage  103  as shown in  FIG. 2A . As a result of the comparison, where the output voltage  102  is higher, the comparator  104  for detection activates its output  114 . 
         [0054]    The higher voltage side auxiliary comparator  107  receives the output voltage  106 , and compares the output voltage  106  to the reference voltage  103 . As a result of the comparison, where the output voltage  106  is higher, the higher voltage side auxiliary comparator  107  activates its output. 
         [0055]    The lower voltage side auxiliary comparator  109  receives the output voltage  108 , and compares the output voltage  108  to the reference voltage  103 . As a result of the comparison, where the output voltage  108  is higher, the lower voltage side auxiliary comparator  109  activates its output. 
         [0056]    The input switching signal generator  112  is, for example, a gate circuit, and performs AND operation of the inverted signal of the output of the lower voltage side auxiliary comparator  109  and the output of the higher voltage side auxiliary comparator  107 . Then, the input switching signal generator  112  generates the input switching signal  113  for controlling the input switch  111  as shown in  FIG. 2B . The input switch  111  allows the output voltage  102  and the reference voltage  103  to be applied to inputs of the comparator  104  for detection when the input switching signal  113  is active. The input switch  111  allows the input voltage  110  for non-use of the comparator to be applied to the inputs of the comparator  104  for detection when the input switching signal  113  is inactive. 
         [0057]    With this operation of the input switch  111 , different input voltages are applied to the two inputs of the comparator  104  for detection, only when the output of the higher voltage side auxiliary comparator  107  is active, and the output of the lower voltage side auxiliary comparator  109  is inactive. In this state, the comparator  104  for detection activates its output  114  when the output voltage  102  is higher than the reference voltage  103  as shown in  FIG. 2C . 
         [0058]    When the output of the higher voltage side auxiliary comparator  107  is inactive, or the output of the lower voltage side auxiliary comparator  109  is active, the input voltage  110  for non-use of the comparator is applied to both of the two inputs of the comparator  104  for detection. In this period, there is no difference in progress of BT degradation between the two inputs of the comparator  104  for detection. If there is no difference in progress of BT degradation between the two inputs, the difference in the amount of variations in the threshold voltage due to the BT degradation between the two inputs does not increase. This mitigates reduction in the accuracy of power detection caused by the BT degradation. 
         [0059]    The signal fixing circuit  115  is a selector, which receives the output  114  of the comparator  104  for detection at one input, and a fixed voltage (a ground voltage) at the other input. As shown in  FIG. 2D , when the input switch  111  selects the input voltage  110  for non-use of the comparator as the inputs of the comparator  104  for detection, the signal fixing circuit  115  fixes the output of the comparator  104  for detection to the constant voltage (the ground voltage). This prevents an indefinite state output from the comparator  104  for detection from being transmitted to the power detection signal  105  with a relatively small area. 
         [0060]    The power detection signal generator  117  performs OR operation of the output of the signal fixing circuit  115  and the output of the lower voltage side auxiliary comparator  109 . As shown in  FIG. 2E , when the output voltage  102  is higher than the reference voltage  103 , the power detection signal  105  is always set high. 
         [0061]    As described above, according to this embodiment, the time when different input voltages are applied to the two inputs of the comparator  104  for detection is limited. This mitigates reduction in the accuracy of power detection caused by BT degradation. 
         [0062]    While in this embodiment, the output voltage  102  is selected when the input switching signal  113  is active, the output voltage  102  may be selected when the input switching signal  113  is inactive. 
       Second Embodiment 
       [0063]      FIG. 3  is a circuit diagram of a power detector according to this embodiment.  FIGS. 4A-4C  illustrate waveforms in the power detector. 
         [0064]    In  FIG. 3 , the power detector includes a power voltage divider  101  dividing the voltage of the power supply  100 , a comparator  104  for detection comparing a first output voltage  102  of the power voltage divider  101  to a reference voltage  103 , and a higher voltage side auxiliary comparator  107  comparing a second output voltage  108  of the power voltage divider  101 , which is higher than the first output voltage  102 , to the reference voltage  103 . 
         [0065]    The power detector of  FIG. 3  further includes an input switch  111  switching inputs of the comparator  104  for detection from the combination of the reference voltage  103  and the first output voltage  102  to an input voltage  110  for non-use of the comparator when the output of the higher voltage side auxiliary comparator  107  is inactive, and a signal fixing circuit  115  fixing an output of the comparator  104  for detection to a constant voltage when the output of the higher voltage side auxiliary comparator  107  is inactive. 
         [0066]    Then, the detailed configuration and operation of the power detector of  FIG. 3  will be described. 
         [0067]    The power voltage divider  101  divides the voltage of the power supply  100  using, for example, a resistor to generate the output voltage  102  to the comparator  104  for detection, and the output voltage  106  to the higher voltage side auxiliary comparator  107 . 
         [0068]    The comparator  104  for detection receives the output voltage  102  and the reference voltage  103  via the input switch  111 , and compares the output voltage  102  to the reference voltage  103  as shown in  FIG. 4A . As a result of the comparison, where the output voltage  102  is higher, the comparator  104  for detection activates its output  114 . 
         [0069]    The input switch  111  allows the output voltage  102  and the reference voltage  103  to be applied to the inputs of the comparator  104  for detection when the output of the higher voltage side auxiliary comparator  107  is active. The input switch  111  allows the input voltage  110  for non-use of the comparator to be applied to the inputs of the comparator  104  for detection when the input switching signal  113  is inactive. 
         [0070]    With this operation of the input switch  111 , different input voltages are applied to the two inputs of the comparator  104  for detection, only when the output of the higher voltage side auxiliary comparator  107  is active. In this state, the comparator  104  for detection activates its output  114  when the output voltage  102  is higher than the reference voltage  103  as shown in  FIG. 4B . 
         [0071]    When the output of the higher voltage side auxiliary comparator  107  is inactive, the input voltage  110  for non-use of the comparator is applied to both of the two inputs of the comparator  104  for detection. In this period, there is no difference in progress of BT degradation between the two inputs of the comparator  104  for detection. If there is no difference in progress of BT degradation between the two inputs, the difference in the amount of variations in the threshold voltage due to the BT degradation between the two inputs does not increase. This mitigates reduction in the accuracy of power detection caused by the BT degradation. 
         [0072]    As shown in  FIG. 4C , when the input switch  111  selects the input voltage  110  for non-use of the comparator, the signal fixing circuit  115  fixes the output of the comparator  104  for detection to the constant voltage. This prevents an indefinite state output from the comparator  104  for detection from being transmitted to the power detection signal  105 . 
         [0073]    As described above, according to this embodiment, the time when different input voltages are applied to the two inputs of the comparator  104  for detection is limited. This reduces reduction in the accuracy of power detection caused by BT degradation. 
         [0074]    While in this embodiment, the output voltage  102  is selected when the output of the higher voltage side auxiliary comparator  107  is active, the output voltage  102  may be selected when the output of the higher voltage side auxiliary comparator  107  is inactive. 
       Third Embodiment 
       [0075]      FIG. 5  is a circuit diagram of a power detector according to this embodiment.  FIGS. 6A-6D  illustrate waveforms in the power detector. 
         [0076]    In  FIG. 5 , the power detector includes a power voltage divider  101  dividing the voltage of the power supply  100 , a comparator  104  for detection comparing a first output voltage  102  of the power voltage divider  101  to a reference voltage  103 , and a lower voltage side auxiliary comparator  109  comparing a second output voltage  108  of the power voltage divider  101 , which is lower than the first output voltage  102 , to the reference voltage  103 . 
         [0077]    The power detector of  FIG. 5  further includes an input switch  111  switching inputs of the comparator  104  for detection from the combination of the reference voltage  103  and the first output voltage  102  to an input voltage  110  for non-use of the comparator when an output of the lower voltage side auxiliary comparator  109  is active, a signal fixing circuit  115  fixing an output of the comparator  104  for detection to a constant voltage when the output of the lower voltage side auxiliary comparator  109  is active, and a power detection signal generator  117  generating a power detection signal  105  based on the output of the lower voltage side auxiliary comparator  109  and an output of the signal fixing circuit  115 . 
         [0078]    Then, the detailed configuration and operation of the power detector of  FIG. 5  will be described. 
         [0079]    The power voltage divider  101  divides the voltage of the power supply  100  using, for example, a resistor to generate the output voltage  102  to the comparator  104  for detection, and the output voltage  108  to the lower voltage side auxiliary comparator  109 . 
         [0080]    The comparator  104  for detection receives the output voltage  102  and the reference voltage  103  via the input switch  111 , and compares the output voltage  102  to the reference voltage  103  as shown in  FIG. 6A . As a result of the comparison, where the output voltage  102  is higher, the comparator  104  for detection activates the output  114 . 
         [0081]    The input switch  111  allows the output voltage  102  and the reference voltage  103  to be applied to the inputs of the comparator  104  for detection when the output of the lower voltage side auxiliary comparator  109  is inactive. The input switch  111  allows the input voltage  110  for non-use of the comparator to be applied to the inputs of the comparator  104  for detection when the output of the lower voltage side auxiliary comparator  109  is inactive. 
         [0082]    With this operation of the input switch  111 , different input voltages are applied to the two inputs of the comparator  104  for detection, only when the output of the lower voltage side auxiliary comparator  109  is inactive. In this state, the comparator  104  for detection activates its output  114  when the output voltage  102  is higher than the reference voltage  103  as shown in  FIG. 6B . 
         [0083]    When the output of the lower voltage side auxiliary comparator  109  is active, the input voltage  110  for non-use of the comparator is applied to both of the two inputs of the comparator  104  for detection. In this period, there is no difference in progress of BT degradation between the two inputs of the comparator  104  for detection. If there is no difference in progress of BT degradation between the two inputs, the difference in the amount of variations in the threshold voltage due to the BT degradation between the two inputs does not increase. This mitigates reduction in the accuracy of power detection caused by the BT degradation. 
         [0084]    As shown in  FIG. 6C , when the input switch  111  selects the input voltage  110  for non-use of the comparator, the signal fixing circuit  115  fixes the output of the comparator  104  for detection to the constant voltage (the ground voltage). This prevents an indefinite state output from the comparator  104  for detection from being transmitted to the power detection signal  105 . 
         [0085]    The power detection signal generator  117  performs OR operation of the output of the signal fixing circuit  115  and the output of the lower voltage side auxiliary comparator  109 . As shown in  FIG. 6D , when the output voltage  102  is higher than the reference voltage  103 , the power detection signal  105  is always set high. 
         [0086]    As described above, according to this embodiment, the time when different input voltages are applied to the two inputs of the comparator  104  for detection is limited. This reduces reduction in the accuracy of power detection caused by BT degradation. 
         [0087]    While in this embodiment, the output voltage  102  is selected when the output of the lower voltage side auxiliary comparator  109  is inactive, the output voltage  102  may be selected when the output of the lower voltage side auxiliary comparator  109  is active. 
       Fourth Embodiment 
       [0088]    With reduction in the areas of semiconductor devices in a single circuit, the accuracy of the semiconductor devices tends to decrease due to variations in a manufacturing process. However, in the power detectors according to the first to third embodiments, the accuracy of the power detection signal  105  depends on the comparison accuracy of the comparator  104  for detection. Thus, even if the comparison accuracy of the higher voltage side auxiliary comparator  107  or the lower voltage side auxiliary comparator  109  is lower than that of the comparator  104  for detection, this does not directly lead to reduction in the accuracy of the power detection signal  105 . 
         [0089]    Therefore, the circuit area can be reduced by setting the comparison accuracy of the higher voltage side auxiliary comparator  107  or the lower voltage side auxiliary comparator  109  to be lower than that of the comparator  104  for detection. 
       Variation 
       [0090]      FIG. 7  is a circuit diagram of a power detector according to a variation of the first embodiment. 
         [0091]    Different from the first embodiment, the power detector according to this variation includes a power breaker (a power breaking means)  122  breaking power of a comparator  104  for detection when an input switch  111  selects an input voltage  110  for non-use of the comparator as inputs of the comparator  104  for detection. With this configuration, no current flows to the comparator  104  for detection in this period. 
         [0092]    By employing this configuration, the power consumption of the comparator  104  for detection can be reduced, while the input switch  111  selects the input voltage  110  for non-use of the comparator as the inputs of the comparator  104  for detection. 
         [0093]    The present disclosure has been described by referring to the above-described embodiments and variation. However, such description of the embodiments should not be construed as limiting, and thus, various modifications can be made thereto, falling within the scope of the disclosure. 
         [0094]    As described above, the power detector according to the present disclosure mitigates reduction in the accuracy of power detection caused by BT degradation, and is thus useful as a measure against the reduction in the accuracy of power detection of an electronic device using a battery, etc.