Abstract:
Provided is a temperature sensor capable of performing more precise temperature measurement compared to conventional ones, even when manufacturing fluctuations are present in semiconductor elements forming a circuit for generating a temperature-dependent current. The temperature sensor includes: a temperature-dependent voltage generation circuit for generating a temperature-dependent potential that is dependent on temperature; a current generation circuit for allowing a temperature-dependent current to flow based on the temperature-dependent potential; a reference current generation circuit for generating a reference current that is independent of temperature; a capacitor that is charged alternately with the temperature-dependent current during a first charge period and the reference current during a second charge period; a pulse generation circuit for comparing a charged voltage of the capacitor with a reference voltage to generate a pulse; and a control circuit for alternately supplying the temperature-dependent current and the reference current to the capacitor. The temperature-dependent voltage generation circuit includes switches for switching connection relations between MOS transistors forming a current source circuit included in the temperature-dependent voltage generation circuit and bipolar transistors each serving as a load of the current source circuit.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2008-292512 filed on Nov. 14, 2008, the entire content of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a temperature sensor formed on a semiconductor device, and more particularly, to a temperature sensor for measuring ambient temperature of a semiconductor device. 
         [0004]    2. Description of the Related Art 
         [0005]    Conventionally, a thermometer utilizing a semiconductor device has been used for measuring an ambient temperature change or body temperature. 
         [0006]    For example, there is known a technology of adjusting a circuit operation by controlling an oscillation frequency in accordance with temperature changes (see, for example, JP 2006-101489 A), and there is also known a temperature sensor that is capable of indicating temperature using the controlled oscillation frequency itself (http://japan.maxim-ic.com/quick_view2.cfm/qv_pk/3625, accessed on Nov. 11, 2008 (hereinafter referred to as Non-patent Document)). 
         [0007]    In JP 2006-101489 A, as illustrated in  FIG. 11 , bipolar transistors Q 1  to Q 4  are used to generate a temperature-dependent current IPTAT, and then a mirror circuit formed of metal oxide semiconductor (MOS) transistors M 1  and M 2  is used to generate a current IIPTAT based on the temperature-dependent current IPTAT. 
         [0008]    Then, a capacitor C 1  is charged with the current IIPTAT, and a comparator compares a charged voltage of the capacitor C 1  with a reference voltage Vref. When the charged voltage becomes higher than the reference voltage, an AND circuit U 10 A outputs a pulse. The charged voltage of the capacitor C 1  is discharged in response to the pulse of the AND circuit U 10 A, and thereafter the charge to the capacitor C 1  is restarted. 
         [0009]    In other words, a time period necessary for the capacitor C 1  to be charged to a voltage higher than the reference voltage Vref varies in accordance with an amount of the current IIPTAT, resulting in a pulse width containing temperature information. 
         [0010]    By measuring the pulse width, the temperature may be measured indeed. However, due to manufacturing fluctuations in capacitor C 1 , the temperature is measured with low precision. 
         [0011]    In view of this, in Non-patent Document, as a current with which the capacitor C 1  is charged, a current Iref that is independent of temperature is additionally generated, and the capacitor C 1  is charged alternately with the current IIPTAT and the current Iref, to thereby alternately generate pulses with the current IIPTAT and the current Iref, which correspond to the above-mentioned pulse. The temperature is measured by using a ratio between the pulses, to thereby suppress an error that may occur in temperature measurement due to the manufacturing fluctuations in the capacitor C 1 . 
         [0012]    However, due to manufacturing fluctuations in bipolar transistors and MOS transistors, a value of the current IIPTAT deviates from its theoretical value. As a result, the charge to the capacitor C 1  varies, and hence a pulse width that is accurately dependent on temperature cannot be generated. Thus, measurement precision cannot be improved by merely using a ratio between the current IIPTAT and the current Iref. 
         [0013]    In particular, the current IIPTAT is generated based on a band gap of the bipolar transistor, whose potential difference is as small as several tens mV. 
         [0014]    Therefore, in a circuit for generating the current IIPTAT based on the minute potential difference, when the manufacturing fluctuations are present in semiconductor elements forming the circuit, a significantly large error value is contained in a finally measured temperature. As a result, precise temperature measurement cannot be performed. 
         [0015]    In particular, in a band gap reference circuit for generating a current IIPTAT, which is formed through a CMOS process, relative fluctuations in operational amplifier and MOS transistors forming a current source circuit become factors causing an error. 
       SUMMARY OF THE INVENTION 
       [0016]    The present invention has been made in view of such circumstances described above, and it is an object thereof to provide a temperature sensor that is capable of performing more precise temperature measurement compared to conventional ones, even when manufacturing fluctuations are present in semiconductor elements forming a circuit for generating a temperature-dependent current. 
         [0017]    A temperature sensor according to the present invention includes: a temperature-dependent voltage generation circuit (for example, circuit including metal oxide semiconductor (MOS) transistors M 1  and M 2 , bipolar transistors BT 1  and BT 2 , and an operational amplifier AP 1  according to each of embodiments) for generating a temperature-dependent potential that is dependent on temperature; a current generation transistor that allows a temperature-dependent current to flow through the current generation transistor based on the temperature-dependent potential; a reference current generation circuit for generating a reference current that is independent of temperature; a capacitor that is charged alternately with the temperature-dependent current during a first charge period and the reference current during a second charge period; a pulse generation circuit (for example, circuit including a reference voltage circuit BT and a comparator CMP 1  according to each of the embodiments) for comparing a charged voltage of the capacitor with a reference voltage to generate a pulse; and a control circuit (for example, circuit including flip-flops FF 2  and FF 3  or circuit including flip-flops FF 2 , FF 3 , and FF 5  according to each of the embodiments) for alternately supplying the temperature-dependent current and the reference current to the capacitor, the control circuit outputting a temperature-dependent pulse having a time width that is dependent on temperature during the first charge period, and outputting a reference pulse having a time width that is independent of temperature during the second charge period, the temperature-dependent voltage generation circuit including: a current source circuit including a first MOS transistor and a second MOS transistor; a first bipolar transistor that outputs a first potential, the first bipolar transistor being connected with one of the first MOS transistor and the second MOS transistor as a load of the first bipolar transistor; a second bipolar transistor that outputs a second potential, the second bipolar transistor being connected via a resistor with another one of the first MOS transistor and the second MOS transistor as a load of the second bipolar transistor, the second bipolar transistor being used as a temperature sensor utilizing a band gap of the second bipolar transistor; a first selection switch for switching a connection destination of the first bipolar transistor to one of the first MOS transistor and the second MOS transistor; a second selection switch for switching a connection destination of the second bipolar transistor to another one of the first MOS transistor and the second MOS transistor; and an operational amplifier for amplifying a potential difference between the first potential and the second potential, and outputting a voltage determined by amplifying the potential difference to a gate of the first MOS transistor and a gate of the second MOS transistor. 
         [0018]    In the temperature sensor according to the present invention, the control circuit is configured to: divide each of the first charge period and the second charge period into a first period and a second period; switch the first selection switch and the second selection switch between the first period and the second period of the each of the first charge period and the second charge period so that the connection destination of the first bipolar transistor is switched from the one of the first MOS transistor and the second MOS transistor to the another one of the first MOS transistor and the second MOS transistor and that the connection destination of the second bipolar transistor is switched from the another one of the first MOS transistor and the second MOS transistor to the one of the first MOS transistor and the second MOS transistor; and add together the first period and the second period of the each of the first charge period and the second charge period, to be output as the temperature-dependent pulse having the time width corresponding to the first charge period and the reference pulse having the time width corresponding to the second charge period, respectively. 
         [0019]    The temperature sensor according to the present invention further includes: a third selection switch for selecting one of the first potential and the second potential to be input to an inverting input terminal of the operational amplifier; a fourth selection switch for selecting another one of the first potential and the second potential to be input to a non-inverting input terminal of the operational amplifier; and a fifth selection switch for switching an output of the operational amplifier between an inverting output and a non-inverting output, and the control circuit switches the third selection switch, the fourth selection switch, and the fifth selection switch in synchronization with a timing of switching the first selection switch and the second selection switch. 
         [0020]    The temperature sensor according to the present invention further includes: a third selection switch for selecting one of the first potential and the second potential to be input to an inverting input terminal of the operational amplifier; a fourth selection switch for selecting another one of the first potential and the second potential to be input to a non-inverting input terminal of the operational amplifier; and a fifth selection switch for switching an output of the operational amplifier between an inverting output and a non-inverting output, and the control circuit is further configured to: divide the first charge period into a first sub charge period and a second sub charge period; switch the third selection switch, the fourth selection switch, and the fifth selection switch when a first pulse and a second pulse are output during the first sub charge period; and output, when the first pulse and the second pulse are output during the second sub charge period, as the temperature-dependent pulse, a pulse determined by dividing in frequency the first pulse and the second pulse output during the first sub charge period and the first pulse and the second pulse output during the second sub charge period. 
         [0021]    In the temperature sensor according to the present invention, the capacitor includes: a first capacitor; and a second capacitor, the first capacitor and the second capacitor being switched for use for each capacitor charge period. 
         [0022]    In the temperature sensor according to the present invention, the resistor is made of a material having such a temperature characteristic that a resistance of the resistor decreases as temperature increases. 
         [0023]    According to the present invention, even when the manufacturing fluctuations are present, by switching the connections between the MOS transistors included in a current source circuit and the bipolar transistors, variations in current values containing offsets between the above-mentioned MOS transistors due to the manufacturing fluctuations are averaged so as to eventually make uniform the charge time periods for the capacitor. Accordingly, the above-mentioned offsets may be canceled to prevent the offset due to the manufacturing fluctuations from affecting the width of the temperature-dependent pulse. As a result, more precise temperature measurement compared to conventional ones may be performed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    In the accompanying drawings: 
           [0025]      FIG. 1  is a block diagram illustrating a configuration example of a temperature sensor according to a first embodiment of the present invention; 
           [0026]      FIG. 2  is a block diagram illustrating a configuration example of a reference current generation circuit illustrated in  FIG. 1 ; 
           [0027]      FIG. 3  is a conceptual waveform diagram for illustrating a relation between a first charge period and a second charge period in a pulse width modulation (PWM) waveform that is output as temperature measurement results; 
           [0028]      FIG. 4  is a graph illustrating changes of the first charge period and the second charge period due to their temperature characteristics; 
           [0029]      FIG. 5  is a timing chart for illustrating an operation of the temperature sensor of  FIG. 1 ; 
           [0030]      FIG. 6  is a block diagram illustrating a configuration example of a temperature sensor according to a second embodiment of the present invention; 
           [0031]      FIG. 7  is a timing chart for illustrating an operation of the temperature sensor of  FIG. 6 ; 
           [0032]      FIG. 8  is a block diagram illustrating a configuration example of a temperature sensor according to a third embodiment of the present invention; 
           [0033]      FIG. 9  is a graph illustrating changes of voltages of bipolar transistors with respect to temperature; 
           [0034]      FIG. 10A  is a graph illustrating a change of an error between an output of the temperature sensor and a true value according to a voltage change in the bipolar transistor with respect to temperature; 
           [0035]      FIG. 10B  is a graph illustrating a change of the error between the output of the temperature sensor and the true value according to a resistance change in a resistor with respect to temperature; and 
           [0036]      FIG. 11  is a block diagram illustrating a conventional configuration for adjusting a circuit operation by controlling an oscillation frequency in accordance with temperature changes. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0037]    Hereinafter, referring to the accompanying drawings, a temperature sensor according to a first embodiment of the present invention is described.  FIG. 1  is a block diagram illustrating a configuration example of the temperature sensor according to the first embodiment of the present invention. 
         [0038]    In  FIG. 1 , P-channel MOS transistors M 1  and M 2  form a current source circuit. Each of the MOS transistors M 1  and M 2  has a source applied with a power supply voltage. Further, a P-channel MOS transistor M 3 , which is described later, has a gate applied with the same bias voltage as that applied to gates of the MOS transistors M 1  and M 2  forming the current source circuit described above, to thereby establish a current source connection. 
         [0039]    A bipolar transistor BT 1  is a PNP bipolar transistor connected as a load of one of the above-mentioned MOS transistors M 1  and M 2 . 
         [0040]    A bipolar transistor BT 2  is a PNP bipolar transistor connected as a load of another one of the MOS transistors M 1  and M 2  via a resistor R 1 . The resistor R 1  has one terminal connected to an emitter of the bipolar transistor BT 2 , and another terminal connected to a terminal of a selection switch SW 1 . The bipolar transistor BT 2  is formed so as to be larger in emitter area than the bipolar transistor BT 1 , and has a voltage Vbe (base-emitter voltage) varying in accordance with temperature. The bipolar transistor BT 2  is used as a temperature sensor utilizing a band gap for outputting the potential Vbe. The bipolar transistor BT 1  and the bipolar transistor BT 2  described above are of the same type, but have different transistor sizes, resulting in different temperature characteristics. In other words, compared to the bipolar transistor BT 1 , the bipolar transistor BT 2  has a more angled-inclination of decrease in voltage Vbe with respect to temperature changes. A difference ΔVbe between the potentials Vbe of the bipolar transistors BT 1  and BT 2  due to temperature changes is utilized to perform temperature measurement. Note that the difference ΔVbe increases monotonously with respect to an increase in temperature because of the reason as described above. 
         [0041]    Each of the bipolar transistors BT 1  and BT 2  has a base and a collector that are connected to a ground. 
         [0042]    The selection switch SW 1  switches, in response to a control signal S 1 , a connection destination of a drain of the MOS transistor M 1  to one of an emitter of the bipolar transistor BT 1  and the another terminal of the resistor R 1 . 
         [0043]    Further, the selection switch SW 1  switches, in response to the above-mentioned control signal S 1 , a connection destination of a drain of the MOS transistor M 2  to another one of the emitter of the bipolar transistor BT 1  and the another terminal of the resistor R 1 . 
         [0044]    For example, when the control signal S 1  becomes “H” level, the selection switch SW 1  connects the drain of the MOS transistor M 1  with the emitter of the bipolar transistor BT 1 , and connects the drain of the MOS transistor M 2  with the another terminal of the resistor R 1 . 
         [0045]    On the other hand, when the control signal S 1  becomes “L” level, the selection switch SW 1  connects the drain of the MOS transistor M 1  with the another terminal of the resistor R 1 , and connects the drain of the MOS transistor M 2  with the emitter of the bipolar transistor BT 1 . 
         [0046]    An operational amplifier AP 1  has a non-inverting input terminal connected to one of the emitter of the bipolar transistor BT 1  and the another terminal of the resistor R 1 , and an inverting input terminal connected to another one of the emitter of the bipolar transistor BT 1  and the another terminal of the resistor R 1 . The operational amplifier AP 1  performs control such that the difference ΔVbe between the voltage Vbe (first potential) of the bipolar transistor BT 1  and the voltage Vbe (second potential) of the bipolar transistor BT 2  may appear across the resistor R 1 , that is, a potential across the resistor R 1  may become ΔVbe. 
         [0047]    Besides, the operational amplifier AP 1  amplifies a differential voltage between a voltage input to the non-inverting input terminal and a voltage input to the inverting input terminal. The operational amplifier AP 1  outputs a voltage determined by amplifying the differential voltage from its non-inverting output terminal as its output voltage, and outputs a voltage determined by inverting a polarity of the amplified differential voltage from its inverting output terminal as its output voltage. 
         [0048]    The selection switch SW 2  switches, in response to a control signal S 1 , a connection destination of the non-inverting input terminal to one of the emitter of the bipolar transistor BT 1  and the another terminal of the resistor R 1 . 
         [0049]    Further, the selection switch SW 2  switches, in response to the above-mentioned control signal S 1 , a connection destination of the inverting input terminal to another one of the emitter of the bipolar transistor BT 1  and the another terminal of the resistor R 1 . 
         [0050]    For example, when the control signal S 1  becomes “H” level, the selection switch SW 2  connects the non-inverting input terminal with the emitter of the bipolar transistor BT 1 , and connects the inverting input terminal with the another terminal of the resistor R 1 . 
         [0051]    On the other hand, when the control signal S 1  becomes “L” level, the selection switch SW 2  connects the non-inverting input terminal with the another terminal of the resistor R 1 , and connects the inverting input terminal with the emitter of the bipolar transistor BT 1 . 
         [0052]    A selection switch SW 3  selects, in response to the control signal S 1 , which of the voltages output from the inverting output terminal and the non-inverting output terminal of the operational amplifier AP 1  is to be output as a bias voltage VB that is supplied to the gates of the MOS transistors M 1  and M 2  and the like. 
         [0053]    For example, when the control signal  51  becomes “H” level, the selection switch SW 3  selects the output voltage of the non-inverting output terminal to be output as the bias voltage VB. On the other hand, when the control signal S 1  becomes “L” level, the selection switch SW 3  selects the output voltage of the inverting output terminal to be output as the bias voltage VB. 
         [0054]    In other words, when each of the connection destinations of the non-inverting input terminal and the inverting input terminal of the operational amplifier AP 1  is switched over by the selection switch SW 2 , in order to match a polarity of the output voltage of the operational amplifier AP 1  with the polarity of the differential voltage between the non-inverting input terminal and the inverting input terminal, the selection switch SW 3  switches between the non-inverting output terminal and the inverting output terminal in synchronization with the switching of the connection destinations of the non-inverting input terminal and the inverting input terminal. 
         [0055]    The MOS transistor M 3  is a P-channel MOS transistor, and has a source applied with the power supply voltage and the gate applied with the bias voltage VB. Then, as a current corresponding to the bias voltage VB, a temperature-dependent current IPTAT is output from a drain of the MOS transistor M 3 . 
         [0056]    A reference current generation circuit  100  outputs a reference current Iref that is independent of temperature from its output terminal. 
         [0057]    A capacitor C 1  has one terminal connected to a terminal of a selection switch SW 4 , and another terminal connected to the ground. 
         [0058]    The selection switch SW 4  selects, in response to a control signal S 2 , which of the drain of the MOS transistor M 3  and the output terminal of the reference current generation circuit  100  is to be connected to the above-mentioned one terminal of the capacitor C 1 . 
         [0059]    For example, when the control signal S 2  becomes “L” level, the selection switch SW 4  connects the drain of the MOS transistor M 3  with the one terminal of the capacitor C 1 . On the other hand, when the control signal S 2  becomes “H” level, the selection switch SW 4  connects the output terminal of the reference current generation circuit  100  with the one terminal of the capacitor C 1 . 
         [0060]    A selection switch SW 5  controls, in response to a control signal S 3 , whether the one terminal of the capacitor C 1  is to be connected to the ground or not. For example, when the control signal S 3  becomes “H” level, the selection switch SW 5  connects the one terminal of the capacitor C 1  with the ground. 
         [0061]    A comparator CMP 1  compares a reference voltage Vref that is output from a constant voltage source BT with a charged voltage that is being charged into the capacitor C 1 . The comparator CMP 1  outputs the control signal S 3  of “H” level when the charged voltage exceeds the reference voltage Vref, and outputs the control signal S 3  of “L” level when the charged voltage is equal to or lower than the reference voltage Vref. 
         [0062]    On this occasion, when the control signal S 3  becomes “H” level, the selection switch SW 5  causes the capacitor C 1  to discharge its charged charges. When the charged voltage of the capacitor C 1  is discharged, the charged voltage becomes equal to or lower than the reference voltage Vref. Accordingly, the comparator CMP 1  changes the control signal S 3  from “H” level to “L” level to be output. Therefore, the control signal S 3  of “H” level is output as a one-shot pulse. 
         [0063]    In the following description, each of flip-flops FFs latches data input to its data terminal D in response to a rising edge of a clock, and the data is output from its output terminal Q. Further, an output terminal QB of each of the flip-flops FFs outputs inverted data of the data output from the output terminal Q, that is, for example, “H” level in the case where the output terminal Q outputs “L” level. In this embodiment, in each of the flip-flops FFs, at the start of its operation, the output terminal Q outputs “L” level while the output terminal QB outputs “H” level. 
         [0064]    The data terminal D and the output terminal QB of the flip-flop FF 2  are connected with each other so that the flip-flop FF 2  may function as a toggle flip-flop. The flip-flop FF 2  has a clock terminal CK connected to an output terminal of the comparator CMP 1 , and the control signal S 3  is input to the clock terminal CK. The output terminal Q is connected to a clock terminal CK of a subsequent flip-flop FF 3 , and outputs the control signal S 1  as its output signal. 
         [0065]    The data terminal D and the output terminal QB of the flip-flop FF 3  are connected with each other so that the flip-flop FF 3  may function as a toggle flip-flop. The flip-flop FF 3  has the clock terminal CK connected to the output terminal Q of the flip-flop FF 2 . The clock terminal CK receives the output signal of the output terminal Q of the flip-flop FF 2  as a clock signal. The output terminal Q of the flip-flop FF 3  outputs the control signal S 2 . 
         [0066]    A buffer BF 1  has an input terminal connected to the output terminal Q of the flip-flop FF 3 , and an output terminal connected to a pulse width modulation (PWM) waveform output terminal from which a PWM waveform representing temperature measurement results is output. 
         [0067]    Next, referring to  FIG. 2 , a configuration of the reference current generation circuit  100  illustrated in  FIG. 1  is described.  FIG. 2  is a block diagram illustrating a configuration example of the reference current generation circuit  100  illustrated in  FIG. 1 . In  FIG. 3 , a horizontal axis represents time while a vertical axis represents pulse level (“H” or “L” level). 
         [0068]    The reference current generation circuit  100  includes a reference voltage generation circuit  200 , a voltage-current conversion circuit  201 , and a P-channel MOS transistor M 4 . 
         [0069]    The MOS transistor M 4  has a source applied with the power supply voltage, and a gate connected to an output terminal of the above-mentioned voltage-current conversion circuit  201  to be applied with a bias voltage VBB. Then, the reference current Iref corresponding to the bias voltage VBB is output from a drain of the MOS transistor M 4 . 
         [0070]    The above-mentioned reference voltage generation circuit  200  includes a P-channel MOS transistor M 5 , a resistor R 2 , and a PNP bipolar transistor BT 2 . 
         [0071]    The MOS transistor M 5  has a source applied with the power supply voltage, a gate applied with the bias voltage VB, and a drain connected to one terminal of the above-mentioned resistor R 2  at a connection point A. 
         [0072]    The bipolar transistor BT 2  has an emitter connected to another terminal of the resistor R 2 , and a base and a collector that are connected to the ground. 
         [0073]    In other words, the bipolar transistor BT 2  is connected with the MOS transistor M 5  as its load via the resistor R 2 . 
         [0074]    Note that the resistor R 2  of  FIG. 2  is formed of a resistive element of the same type as that of the resistor R 1  of  FIG. 1 . A voltage generated across the resistor R 2  is a voltage determined by multiplying a voltage that is generated across the resistor R 1  and has a positive temperature characteristic by a resistance ratio of the resistor R 2  to the resistor R 1 . In other words, a current flowing through the resistor R 2  has a positive temperature characteristic, and thus the voltage having a positive temperature characteristic is generated across the resistor R 2 . On the other hand, the voltage Vbe of the bipolar transistor BT 2  decreases as temperature increases. Therefore, the change of the voltage Vbe due to the increase in temperature and the change of the voltage generated across the resistor R 2  due to its positive temperature characteristic cancel each other. As a result, a voltage of the connection point A becomes a constant voltage Vcnt that is independent of temperature. 
         [0075]    The above-mentioned voltage-current conversion circuit  201  includes an operational amplifier AP 3 , a P-channel MOS transistor M 6 , and a resistor R 3 . 
         [0076]    The resistor R 3  has one terminal connected to a drain of the above-mentioned MOS transistor M 6  at a connection point B, and another terminal connected to the ground. Note that the resistor R 3  is made of a material having a temperature-independent characteristic. 
         [0077]    The operational amplifier AP 3  has a non-inverting input terminal connected to the connection point A to be applied with the voltage Vcnt, and an inverting input terminal connected to the above-mentioned connection point B. Then, the bias voltage VBB is output from an output terminal of the operational amplifier AP 3 . 
         [0078]    The MOS transistor M 6  has a source applied with the power supply voltage, a gate connected to the output terminal of the above-mentioned operational amplifier AP 3  to be applied with the bias voltage VBB, and a drain connected to the ground via the resistor R 3 . 
         [0079]    With the configuration described above, the reference current generation circuit  100  outputs the reference current Iref that is independent of temperature. 
         [0080]    With the configuration described above, the measurement results of the temperature sensor according to this embodiment are output in the form of the PWM waveform illustrated in  FIG. 3 . 
         [0081]    In  FIG. 3 , a time width T 1  represents a reference time width that is independent of temperature, and a time width T 2  represents a temperature-dependent time width that is dependent on temperature as illustrated in a graph of  FIG. 4 . In  FIG. 3 , the vertical axis represents pulse level while the horizontal axis represents time. As illustrated in  FIG. 4 , the time width T 2  reduces as temperature increases. In  FIG. 4 , a horizontal axis represents temperature while a vertical axis represents a pulse width (time width). 
         [0082]    Specifically, the time width T 1  corresponds to a time period necessary for the capacitor C 1  to be charged to the reference voltage Vref with the reference current Iref. On the other hand, the time width T 2  corresponds to a time period necessary for the capacitor C 1  to be charged to the reference voltage Vref with the temperature-dependent current IPTAT. 
         [0083]    On this occasion, temperature T as the measurement results may be determined in the following expression: 
         [0000]        T =( T 1/ T 2)×α−β 
         [0000]    where α and β each represent a coefficient. 
         [0084]    Next, referring to  FIG. 5 , an operation of the temperature sensor illustrated in  FIG. 1  is described.  FIG. 5  is a timing chart for illustrating an operation example of the temperature sensor of  FIG. 1 . 
         [0085]    At a time point t 1 , a charged voltage that is being charged with the reference current Iref exceeds the reference voltage Vref, and accordingly the comparator CMP 1  sets the control signal S 3  to “H” level. 
         [0086]    Then, the selection switch SW 5  becomes a conduction state, and the charged voltage stored in the capacitor C 1  is discharged. 
         [0087]    After the discharge, the comparator CMP 1  sets the control signal S 3  to “L” level to be output. (Hereinafter, the operation described above is referred to as an operation of outputting a pulse of the control signal S 3  by the comparator CMP 1 .) 
         [0088]    Further, the flip-flop FF 2  changes the output signal S 1  from “L” level to “H” level in response to the rising edge to “H” level of the above-mentioned control signal S 3 . 
         [0089]    Similarly, the flip-flop FF 2  changes its output of the output terminal Q from “L” level to “H” level in response to the rising edge to “H” level of the above-mentioned control signal S 3 . 
         [0090]    In response to the rising edge to “H” level of the output of the output terminal Q of the flip-flop FF 2 , the flip-flop FF 3  changes the control signal S 2  from “H” level to “L” level to be output from its output terminal Q. 
         [0091]    Upon the change of the control signal S 1  to “H” level, the selection switch SW 1  connects the drain of the MOS transistor M 1  with the emitter of the bipolar transistor BT 1 , and connects the drain of the MOS transistor M 2  with the another terminal of the resistor R 1 . 
         [0092]    Similarly, upon the change of the control signal S 1  to “H” level, the selection switch SW 2  connects the non-inverting input terminal of the operational amplifier AP 1  with the emitter of the bipolar transistor BT 1 , and connects the inverting input terminal of the operational amplifier AP 1  with the another terminal of the resistor R 1 . 
         [0093]    In addition, upon the change of the control signal  51  to “H” level, the selection switch SW 3  causes the output voltage of the non-inverting output terminal of the operational amplifier AP 1  to be output as the bias voltage VB. 
         [0094]    Upon the change of the control signal S 2  to “L” level, the selection switch SW 4  connects the drain of the MOS transistor M 3  with the one terminal of the capacitor C 1 . 
         [0095]    Due to this connection, the charge to the capacitor C 1  is performed with the temperature-dependent current IPTAT. 
         [0096]    Next, at a time point t 2 , the charged voltage of the capacitor C 1  exceeds the reference voltage Vref, and accordingly the comparator CMP 1  outputs a pulse of the control signal S 3 . 
         [0097]    As a result, the flip-flop FF 2  changes the control signal S 1  from “H” level to “L” level to be output in response to a rising edge of the pulse of the control signal S 3 . 
         [0098]    Similarly, the flip-flop FF 2  changes its output of the output terminal Q from “H” level to “L” level in response to the rising edge of the pulse of the control signal S 3 . 
         [0099]    At this time, the flip-flop FF 3  continues to output the control signal S 2  of “L” level. 
         [0100]    Upon the change of the control signal S 1  to “L” level, the selection switch SW 1  connects the drain of the MOS transistor M 1  with the another terminal of the resistor R 1 , and connects the drain of the MOS transistor M 2  with the emitter of the bipolar transistor BT 1 . 
         [0101]    Similarly, upon the change of the control signal S 1  to “L” level, the selection switch SW 2  connects the non-inverting input terminal of the operational amplifier AP 1  with the another terminal of the resistor R 1 , and connects the inverting input terminal of the operational amplifier AP 1  with the emitter of the bipolar transistor BT 1 . 
         [0102]    In addition, upon the change of the control signal S 1  to “L” level, the selection switch SW 3  causes the output voltage of the non-inverting output terminal of the operational amplifier AP 1  to be output as the bias voltage VB. 
         [0103]    The control signal S 2  of “L” level is continued to be output, and hence the selection switch SW 4  continues the state in which the drain of the MOS transistor M 3  is connected with the one terminal of the capacitor C 1 . 
         [0104]    Due to this connection, the charge to the capacitor C 1  is performed with the temperature-dependent current IPTAT. 
         [0105]    Next, at a time point t 3 , the charged voltage of the capacitor C 1  exceeds the reference voltage Vref, and accordingly the comparator CMP 1  outputs a pulse of the control signal S 3 . 
         [0106]    As a result, the flip-flop FF 2  changes the control signal S 1  from “L” level to “H” level to be output in response to a rising edge of the pulse of the control signal S 3 . 
         [0107]    Similarly, the flip-flop FF 2  changes its output of the output terminal Q from “L” level to “H” level in response to the rising edge of the pulse of the control signal S 3 . 
         [0108]    In addition, the flip-flop FF 3  changes the control signal S 2  from “L” level to “H” level in response to a rising edge to “H” level of the output of the output terminal Q of the flip-flop FF 2 . 
         [0109]    Upon the change of the control signal S 1  to “H” level, the selection switch SW 1  connects the drain of the MOS transistor M 1  with the emitter of the bipolar transistor BT 1 , and connects the drain of the MOS transistor M 2  with the another terminal of the resistor R 1 . 
         [0110]    Similarly, upon the change of the control signal S 1  to “H” level, the selection switch SW 2  connects the non-inverting input terminal of the operational amplifier AP 1  with the emitter of the bipolar transistor BT 1 , and connects the inverting input terminal of the operational amplifier AP 1  with the another terminal of the resistor R 1 . 
         [0111]    In addition, upon the change of the control signal S 1  to “H” level, the selection switch SW 3  causes the output voltage of the non-inverting output terminal of the operational amplifier AP 1  to be output as the bias voltage VB. 
         [0112]    Upon the change of the control signal S 2  to “H” level, the selection switch SW 4  connects the output terminal of the reference current generation circuit  100  with the one terminal of the capacitor C 1 . 
         [0113]    Due to this connection, the charge to the capacitor C 1  is performed with the reference current Iref. 
         [0114]    Next, at a time point t 4 , the charged voltage of the capacitor C 1  exceeds the reference voltage Vref, and accordingly the comparator CMP 1  outputs a pulse of the control signal S 3 . 
         [0115]    As a result, the flip-flop FF 2  changes the control signal S 1  from “H” level to “L” level to be output in response to a rising edge of the pulse of the control signal S 3 . 
         [0116]    Similarly, the flip-flop FF 2  changes its output of the output terminal Q from “H” level to “L” level in response to the rising edge of the pulse of the control signal S 3 . 
         [0117]    At this time, the flip-flop FF 3  continues to output the control signal S 2  of “H” level. 
         [0118]    Upon the change of the control signal S 1  to “L” level, the selection switch SW 1  connects the drain of the MOS transistor M 1  with the another terminal of the resistor R 1 , and connects the drain of the MOS transistor M 2  with the emitter of the bipolar transistor BT 1 . 
         [0119]    Similarly, upon the change of the control signal S 1  to “L” level, the selection switch SW 2  connects the non-inverting input terminal of the operational amplifier AP 1  with the another terminal of the resistor R 1 , and connects the inverting input terminal of the operational amplifier AP 1  with the emitter of the bipolar transistor BT 1 . 
         [0120]    In addition, upon the change of the control signal S 1  to “L” level, the selection switch SW 3  causes the output voltage of the inverting output terminal of the operational amplifier AP 1  to be output as the bias voltage VB. 
         [0121]    The control signal S 2  of “H” level is continued to be output, and hence the selection switch SW 4  continues the state in which the output terminal of the reference current generation circuit  100  is connected with the one terminal of the capacitor C 1 . 
         [0122]    Due to this connection, the charge to the capacitor C 1  is performed with the reference current Iref. 
         [0123]    From a time point t 5 , the processing from the time point t 1  to the time point t 4  described above is repeatedly performed. 
         [0124]    As described above, in this embodiment, the first charge period T 2  during which the capacitor C 1  is charged with the temperature-dependent current IPTAT is divided into two periods of T 2 A (for example, period between the time point t 1  and the time point t 2 ) and T 2 B (for example, period between the time point t 2  and the time point t 3 ). Between the two periods of T 2 A and T 2 B, combinations of the connections between the MOS transistors M 1  and M 2  and the bipolar transistors BT 1  and BT 2  are switched over by the selection switch SW 1 , to thereby cancel an offset between the MOS transistor M 1  and the MOS transistor M 2  due to their manufacturing fluctuations. 
         [0125]    Similarly, between the above-mentioned two periods of T 2 A and T 2 B, combinations of the connections between the non-inverting input terminal and the inverting input terminal of the operational amplifier AP 1  and the bipolar transistors BT 1  and BT 2  are switched over (including the switching between the inverting output terminal and the non-inverting output terminal) by the selection switches SW 2  and SW 3 , to thereby cancel an offset between the non-inverting input terminal and the inverting input terminal of the operational amplifier AP 1 . 
         [0126]    Then, as a time period required for the charge to the capacitor C 1 , the two periods of T 2 A and T 2 B are added together, that is, the pulse of the control signal S 3  output from the comparator CMP 1  is divided in frequency by the flip-flop FF 3 , to thereby generate the first charge period T 2  having a time width determined by adding the two periods T 2 A and T 2 B together. Then, as a temperature-dependent pulse having the width of the first charge period T 2 , a PWM waveform (“H” level) is output via the buffer BF 1 . 
         [0127]    Due to the offset between the MOS transistors M 1  and M 2  and the offset between the non-inverting input terminal and the inverting input terminal of the operational amplifier AP 1  described above, a current value of the temperature-dependent current IPTAT varies depending on the products. Specifically, in  FIG. 5 , though a broken line illustrated between the time point t 1  and the time point t 3  indicates an ideal change of the charged voltage of the capacitor C 1 , an actual change thereof appears as a solid line to have such an error ΔT as described above. 
         [0128]    In view of this, in this embodiment, as described above, the error ΔT between the charge time period T 2 A and the charge time period T 2 B due to the variation in temperature-dependent current IPTAT is canceled by switching the combinations of the connections between the MOS transistors M 1  and M 2  and the bipolar transistors BT 1  and BT 2  as well as by switching the combinations of the connections between the non-inverting input terminal and the inverting input terminal of the operational amplifier AP 1  and the bipolar transistors BT 1  and BT 2 . 
         [0129]    As a result, according to this embodiment, an error that may occur in temperature measurement results due to the variations among the products may be suppressed, to thereby perform precise temperature measurement with a small error among the products. 
         [0130]    Further, in this embodiment, similarly to the first charge period T 2 , the second charge period T 1  during which the capacitor C 1  is charged with the reference current Iref is also divided into periods of T 1 A and T 1 B for the charge processing. The pulse of the control signal S 3  output from the comparator CMP 1  is divided in frequency by the flip-flop FF 3 , to thereby generate the second charge period T 1  having a time width determined by adding the two periods of T 1 A and T 1 B together. Then, as a temperature-dependent pulse having the width of the second charge period T 1 , a PWM waveform (“L” level) is output via the buffer BF 1 . 
         [0131]    Further, instead of providing the selection switch SW 2  and the selection switch SW 3 , the following configuration may be adopted. That is, the non-inverting input terminal of the operational amplifier AP 1  is connected with the emitter of the bipolar transistor BT 1 , the inverting input terminal thereof is connected with the another terminal of the resistor R 1 , and the bias voltage VB is output from the non-inverting output terminal thereof. In this configuration, only the combinations of the connections between the MOS transistors M 1  and M 2  and the bipolar transistors BT 1  and BT 2  may be switched over, to thereby cancel only the offset between the MOS transistors M 1  and M 2 . 
       Second Embodiment 
       [0132]    Next, referring to the accompanying drawings, a temperature sensor according to a second embodiment of the present invention is described.  FIG. 6  is a block diagram illustrating a configuration example of the temperature sensor according to the second embodiment of the present invention. 
         [0133]    The same component as that of  FIG. 1  illustrating the first embodiment is denoted by the same reference symbol, and description thereof is omitted. Hereinafter, only a configuration and an operation according to the second embodiment different from those of the first embodiment are described. 
         [0134]    A newly-added component is a flip-flop FF 5 . 
         [0135]    The flip-flop FF 5  is provided at a subsequent stage of the flip-flop FF 3 . A data terminal D and an output terminal QB of the flip-flop FF 5  are connected with each other so that the flip-flop FF 5  may function as a toggle flip-flop. The flip-flop FF 5  has a clock terminal CK that is connected to the output terminal Q of the flip-flop FF 3  and receives a control signal S 4 , which is the output signal of the output terminal Q of the flip-flop FF 3 . The flip-flop FF 5  outputs a control signal S 5  from its output terminal Q. 
         [0136]    Similarly to the first embodiment, the selection switch SW 1  operates to switch, in response to the control signal S 1 , the combinations of the connections between the MOS transistors M 1  and M 2  and the bipolar transistors BT 1  and BT 2 . 
         [0137]    On the other hand, the selection switches SW 2  and SW 3  operate to switch, in response to the control signal S 4 , the combinations of the connections between the non-inverting input terminal and the inverting input terminal of the operational amplifier AP 1  and the bipolar transistors BT 1  and BT 2 . 
         [0138]    For example, when the control signal S 4  becomes “H” level, the selection switch SW 2  connects the non-inverting input terminal of the operational amplifier AP 1  with the emitter of the bipolar transistor BT 1 , and connects the inverting input terminal of the operational amplifier AP 1  with the another terminal of the resistor R 1 . 
         [0139]    On the other hand, when the control signal S 4  becomes “L” level, the selection switch SW 2  connects the non-inverting input terminal of the operational amplifier AP 1  with the another terminal of the resistor R 1 , and connects the inverting input terminal of the operational amplifier AP 1  with the emitter of the bipolar transistor BT 1 . 
         [0140]    Further, a selection switch SW 3  selects, in response to the control signal S 4 , which of the voltages output from the non-inverting output terminal and the inverting output terminal of the operational amplifier AP 1  is to be output as a bias voltage VB that is supplied to the MOS transistors M 1  and M 2  and the like. 
         [0141]    For example, when the control signal S 4  becomes “H” level, the selection switch SW 3  selects the output voltage of the non-inverting output terminal to be output as the bias voltage VB. On the other hand, when the control signal S 4  becomes “L” level, the selection switch SW 3  selects the output voltage of the inverting output terminal to be output as the bias voltage VB. 
         [0142]    In other words, when each of the connection destinations of the non-inverting input terminal and the inverting input terminal of the operational amplifier AP 1  is switched over by the selection switch SW 2 , in order to match a polarity of the output voltage of the operational amplifier AP 1  with the polarity of the differential voltage between the non-inverting input terminal and the inverting input terminal, the selection switch SW 3  switches between the non-inverting output terminal and the inverting output terminal in synchronization with the switching of the connection destinations of the non-inverting input terminal and the inverting input terminal. 
         [0143]    As described above, in the second embodiment, as illustrated in a timing chart of  FIG. 7 , a first charge period T 2  during which a PWM waveform shows “L” level is divided into sub periods T 21  and T 22 . The sub period T 21  is further divided into a period T 21 A and a period T 21 B, and the sub period T 22  is further divided into a period T 22 A and a period T 22 B. 
         [0144]    Similarly, a second charge period T 1  is divided into sub periods T 11  and T 12 . The sub period T 11  is further divided into a period T 11 A and a period T 11 B, and the sub period T 12  is further divided into a period T 12 A and a period T 12 B. 
         [0145]    Operations of switching the combinations of the connections between the MOS transistors M 1  and M 2  and the bipolar transistors BT 1  and BT 2  performed by the selection switch SW 1  at time points t 1 , t 3 , t 5 , and t 7  in response to the control signal S 1  are identical with the operations performed at the time points t 1 , t 3 , t 5 , and t 7  according to the first embodiment. 
         [0146]    Further, operations of switching the combinations of the connections between the MOS transistors M 1  and M 2  and the bipolar transistors BT 1  and BT 2  performed by the selection switch SW 1  at time points t 2 , t 4 , t 6 , and t 8  in response to the control signal S 1  are identical with the operations performed at the time points t 2 , t 4 , t 6 , and t 8  according to the first embodiment. 
         [0147]    Further, operations of switching the combinations of the connections between the non-inverting input terminal and the inverting input terminal of the operational amplifier AP 1  and the bipolar transistors BT 1  and BT 2  (as well as of switching between the inverting output terminal and the non-inverting output terminal) performed by the selection switches SW 2  and SW 3  at the time points t 1  and t 5  in response to the control signal S 4  are identical with the operations performed at the time points t 2 , t 4 , t 6 , and t 8  according to the first embodiment. 
         [0148]    Similarly, operations of switching the combinations of the connections between the non-inverting input terminal and the inverting input terminal of the operational amplifier AP 1  and the bipolar transistors BT 1  and BT 2  (as well as of switching between the inverting output terminal and the non-inverting output terminal) performed by the selection switches SW 2  and SW 3  at the time points t 3  and t 7  in response to the control signal S 4  are identical with the operations performed at the time points t 1 , t 3 , t 5 , and t 7  according to the first embodiment. 
         [0149]    Further, an operation of selecting the combination of the connection between the another terminal of the capacitor C 1 , and the drain of the MOS transistor M 3  and the output terminal of the reference current generation circuit  100  performed by the selection switch SW 4  at the time point t 1  in response to the control signal S 5  is identical with the operations performed at the time points t 1  and t 5  according to the first embodiment. 
         [0150]    Similarly, an operation of selecting the combination of the connection between the another terminal of the capacitor C 1 , and the drain of the MOS transistor M 3  and the output terminal of the reference current generation circuit  100  performed by the selection switch SW 4  at the time point t 5  in response to the control signal S 5  is identical with the operations performed at the time points t 3  and t 7  according to the first embodiment. 
         [0151]    As described above, each of the first charge period T 2  and the second charge period T 1  is divided into the four periods. In the divided four periods, the combinations of the connections between the non-inverting input terminal and the inverting input terminal of the operational amplifier AP 1  and the bipolar transistors BT 1  and BT 2  are switched over every two periods, and the combinations of the connections between the MOS transistors M 1  and M 2  and the bipolar transistors BT 1  and BT 2  are switched over every period. Thus, compared to the first embodiment, the combinations of the connections among the respective elements are switched over more frequently. As a result, the degrees of canceling the offsets among the respective elements are improved, to thereby perform more precise measurement. 
         [0152]    In each period corresponding to the charge time period for the capacitor C 1  (period T 21 A, T 21 B, T 22 A, T 22 B, T 11 A, T 11 B, T 12 A, or T 12 B), the control signal S 3  output from the comparator CMP 1  is divided in frequency by 4, and accordingly a PWM waveform representing the first charge period T 2  and the second charge period T 1  is output via the buffer BF 1 . 
       Third Embodiment 
       [0153]    Next, referring to the accompanying drawings, a temperature sensor according to a third embodiment of the present invention is described.  FIG. 8  is a block diagram illustrating a configuration example of the temperature sensor according to the third embodiment of the present invention. 
         [0154]    The same component as that of  FIG. 6  illustrating the second embodiment is denoted by the same reference symbol, and description thereof is omitted. Hereinafter, only a configuration and an operation according to the third embodiment different from those of the second embodiment are described. 
         [0155]    In the third embodiment of the present invention, a newly-added component is a selection switch SW 6 . 
         [0156]    Further, in the third embodiment, a capacitor C 1 A and a capacitor C 1 B are provided instead of the capacitor C 1  according to the second embodiment. Similarly, a selection switch SW 4 A and a selection switch SW 4 B are provided instead of the selection switch SW 4  according to the second embodiment. A selection switch SW 5 A and a selection switch SW 5 B are provided instead of the selection switch SW 5  according to the second embodiment. A MOS transistor M 3 A and a MOS transistor M 3 B are provided instead of the MOS transistor M 3  according to the second embodiment. The selection switches SW 5 A and SW 5 B respectively perform discharge processing on the capacitors C 1 A and C 1 B in response to the control signal S 1 . Upon the turning on/off of the selection switches SW 5 A and SW 5 B in response to the control signal S 1 , one of the capacitors C 1 A and C 1 B becomes a charged state while another one becomes a discharged state. In other words, when the control signal S 1  becomes “L” level, the selection switch SW 5 A becomes an ON-state while the selection switch SW 5 B becomes an OFF-state. On the other hand, when the control signal S 1  becomes “H” level, the selection switch SW 5 A becomes the OFF-state while the selection switch SW 5 B becomes the ON-state. 
         [0157]    Further, a reference current generation circuit  100 A and a reference current generation circuit  100 B are provided instead of the reference current generation circuit  100  according to the second embodiment. 
         [0158]    Further, similarly to the selection switch SW 2  and the selection switch SW 3 , the selection switch SW 1  is controlled to be switched over in response to the control signal S 4 . 
         [0159]    For example, when the control signal S 4  becomes “H” level, the selection switch SW 1  connects the drain of the MOS transistor M 1  with the emitter of the bipolar transistor BT 1 , and connects the drain of the MOS transistor M 2  with the another terminal of the resistor R 1 . 
         [0160]    On the other hand, when the control signal S 4  becomes “L” level, the selection switch SW 1  connects the drain of the MOS transistor M 1  with the another terminal of the resistor R 1 , and connects the drain of the MOS transistor M 2  with the emitter of the bipolar transistor BT 1 . 
         [0161]    Each of the MOS transistors M 3 A and M 3 B is a P-channel MOS transistor, and has a source applied with the power supply voltage and a gate applied with the bias voltage VB. 
         [0162]    Each of the reference current generation circuits  100 A and  100 B has the same configuration as that of the reference current generation circuit  100 , and outputs the reference current Iref. 
         [0163]    The capacitor C 1 A has one terminal connected to a terminal of the selection switch SW 4 A, and another terminal connected to the ground. 
         [0164]    The capacitor C 1 B has one terminal connected to a terminal of the selection switch SW 4 B, and another terminal connected to the ground. 
         [0165]    The selection switch SW 4 A switches, in response to the control signal S 5 , a connection of the one terminal of the capacitor CIA to one of a drain of the MOS transistor M 3 A and the reference current generation circuit  100 A. 
         [0166]    For example, when the control signal S 5  becomes “L” level, the selection switch SW 4 A connects the drain of the MOS transistor M 3 A with the one terminal of the capacitor C 1 A. On the other hand, when the control signal S 5  becomes “H” level, the selection switch SW 4 A connects the reference current generation circuit  100 A with the one terminal of the capacitor CIA. 
         [0167]    The selection switch SW 4 B switches, in response to the control signal S 5 , a connection of the one terminal of the capacitor C 1 B to one of a drain of the MOS transistor M 3 B and the reference current generation circuit  100 B. 
         [0168]    For example, when the control signal S 5  becomes “L” level, the selection switch SW 4 B connects the drain of the MOS transistor M 3 B with the one terminal of the capacitor C 1 B. On the other hand, when the control signal S 5  becomes “H” level, the selection switch SW 4 B connects the reference current generation circuit  100 B with the one terminal of the capacitor C 1 B. 
         [0169]    The selection switch SW 6  switches, in response to the control signal S 1 , a connection of one of the one terminal of the capacitor C 1 A and the one terminal of the capacitor C 1 B to the non-inverting input terminal of the comparator CMP 1 . 
         [0170]    For example, when the control signal S 1  becomes “L” level, the selection switch SW 6  connects the one terminal of the capacitor CIA with the non-inverting input terminal of the comparator CMP 1 . On the other hand, when the control signal S 1  becomes “H” level, the selection switch SW 6  connects the one terminal of the capacitor C 1 B with the non-inverting input terminal of the comparator CMP 1 . 
         [0171]    With the configuration described above, in this embodiment, a charged voltage with which the reference voltage Vref is compared in the comparator CMP 1  is alternately selected between the charged voltages of the capacitors C 1 A and C 1 B. For example, in the timing chart of  FIG. 7 , the charged voltage with which the reference voltage Vref is compared in the comparator CMP 1  at the time point t 2  is the one being charged into the capacitor C 1 A. On the other hand, the charged voltage with which the reference voltage Vref is compared in the comparator CMP 1  at the time point t 3  is the one being charged into the capacitor C 1 B. In other words, for each period corresponding to the charge time period (period T 21 A, T 21 B, T 22 A, T 22 B, T 11 A, T 11 B, T 12 A, or T 12 B), a capacitor whose charged voltage is to be compared with the reference voltage Vref is switched alternately to one of the capacitors C 1 A and C 1 B by the selection switch SW 6 . 
         [0172]    Thus, in this embodiment, in order that a time period necessary for the capacitor C 1  to be discharged may be prevented from causing each time width of the charge periods T 1  and T 2  to be larger to result in a measurement error, the two capacitors C 1 A and C 1 B are provided so that one of the two capacitors starts to be charged at a time when another one starts to be discharged. Therefore, a measurement error caused by the discharge time period may be suppressed. 
         [0173]    Further, also in the first embodiment, it is possible to adopt the above-mentioned configuration, which uses the capacitors C 1 A and C 1 B. In the case of adopting this configuration in the first embodiment, the control signal S 2  is used as the control signal for controlling the selection switch SW 4 A and the selection switch SW 4 B. 
       Fourth Embodiment 
       [0174]    A fourth embodiment of the present invention has a configuration in which the resistor R 1  according to each of the first to third embodiments is replaced with one made of a material having a temperature-dependent characteristic. 
         [0175]      FIG. 9  is a graph illustrating temperature changes of the voltages Vbe of the bipolar transistor BT 1  and the bipolar transistor BT 2 . 
         [0176]    The temperature change of the voltage Vbe of the bipolar transistor BT 1  is represented by P while the temperature change of the voltage Vbe of the bipolar transistor BT 2  is represented by Q. 
         [0177]    A straight line P 1  and a straight line Q 1  respectively indicate ideal changes of the voltages Vbe of the bipolar transistor BT 1  and the bipolar transistor BT 2 , but actual changes thereof respectively appear as a curved line P 3  and a curved line Q 3  each containing quadratic components. 
         [0178]    In other words, in the following Equation (1) expressing the temperature-dependent characteristic of the voltage Vbe of the bipolar transistor, the term of G contains a square-law component of a temperature ratio between a temperature difference ΔTr (that is, “T−Tr”) and a reference temperature Tr (25° C. in this embodiment) as expressed in Equation (2), which results in linearity distortion. As a result, as indicated as the line P 3  and the line Q 3 , the bipolar transistors BT 1  and BT 2  have the characteristics deviating from the ideal lines P 1  and Q 1  on both sides with respect to the reference temperature Tr. The operational amplifier AP 1  operates so that the difference ΔVbe may accurately appear across the resistor R 1  so as to equalize with each other the voltage Vbe of the bipolar transistor BT 1  and the voltage of the another terminal of the resistor R 1 . 
         [0179]    On this occasion, though an ideal difference AVbe increases linearly with respect to temperature, an actual difference ΔVbe does not increase linearly because of the reason described above, resulting in an error. 
         [0000]    
       
         
           
             
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   VBe 
                   = 
                   
                     
                       Vg 
                        
                       
                         ( 
                         
                           1 
                           - 
                           
                             T 
                             Tr 
                           
                         
                         ) 
                       
                     
                     + 
                     
                       
                         T 
                         Tr 
                       
                        
                       
                         Vbe 
                          
                         
                           ( 
                           Tr 
                           ) 
                         
                       
                     
                     - 
                     
                       
                         η 
                          
                         
                             
                         
                          
                         
                           kT 
                           q 
                         
                          
                         In 
                          
                         
                             
                         
                          
                         
                           T 
                           Tr 
                         
                       
                       
                          
                         G 
                       
                     
                     + 
                     
                       
                         kT 
                         q 
                       
                        
                       In 
                        
                       
                           
                       
                        
                       
                         
                           I 
                           
                             ( 
                             T 
                             ) 
                           
                         
                         
                           I 
                           
                             ( 
                             Tr 
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   η 
                    
                   
                       
                   
                    
                   
                     
                       kTr 
                       q 
                     
                     · 
                     
                       
                         ( 
                         
                           
                             Δ 
                              
                             
                                 
                             
                              
                             Tr 
                           
                           Tr 
                         
                         ) 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    in the case of “T−Tr&lt;&lt;Tr” 
         [0180]    To make the respective actual characteristic lines approximate to the ideal lines P 1  and Q 1 , the characteristic of the resistor expressed in Equation (3) is utilized. 
         [0000]    
       
         
           
             
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     3 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         
                           R 
                            
                           
                             ( 
                             T 
                             ) 
                           
                         
                         = 
                           
                          
                         
                           
                             
                               R 
                                
                               
                                 ( 
                                 Tr 
                                 ) 
                               
                             
                             · 
                             
                               { 
                               
                                 1 
                                 + 
                                 
                                   TC 
                                    
                                   
                                       
                                   
                                    
                                   1 
                                    
                                   
                                     ( 
                                     
                                       T 
                                       - 
                                       Tr 
                                     
                                     ) 
                                   
                                 
                               
                               } 
                             
                           
                            
                           
                             { 
                             
                               1 
                               + 
                               
                                 TC 
                                  
                                 
                                     
                                 
                                  
                                 2 
                                  
                                 
                                   
                                     ( 
                                     
                                       T 
                                       - 
                                       Tr 
                                     
                                     ) 
                                   
                                   2 
                                 
                               
                             
                             } 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             R 
                              
                             
                               ( 
                               Tr 
                               ) 
                             
                           
                            
                           
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   Δ 
                                    
                                   
                                       
                                   
                                    
                                   
                                     T 
                                     · 
                                     TC 
                                   
                                    
                                   
                                       
                                   
                                    
                                   1 
                                 
                               
                               ) 
                             
                             · 
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   Δ 
                                    
                                   
                                       
                                   
                                    
                                   
                                     
                                       T 
                                       2 
                                     
                                     · 
                                     TC 
                                   
                                    
                                   
                                       
                                   
                                    
                                   2 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0181]    In Equation (3), TC 1  represents a first-order temperature coefficient, and TC 2  represents a second-order temperature coefficient. 
         [0182]    In other words, as understood from the term of “(1+ΔT 2 ·TC 2 )” in Equation (3), the resistor has a square-law characteristic with respect to the temperature difference. In this case, ΔT corresponds to “T−Tr”. 
         [0183]    Therefore, when a material whose resistance decreases with respect to temperature is used for the resistor, a current flowing through the bipolar transistor increases in accordance with temperature changes. As a result, an error Terr between an output of the temperature sensor and a true value has such a temperature characteristic that the error Terr changes as illustrated in  FIG. 10B , owing to the resistor. 
         [0184]    On the other hand, because of the influence of the term of G in Equation (1) described above regarding the bipolar transistor, the error Ten between the output of the temperature sensor and the true value has such a temperature characteristic that the error Terr changes as illustrated in  FIG. 10A , owing to the bipolar transistor. 
         [0185]    The change illustrated in  FIG. 10A  and the change illustrated in  FIG. 10B  described above cancel each other. Therefore, the temperature characteristics of the voltages Vbe of  FIG. 9  may be made approximate from the line P 3  to a line P 2  and from the line Q 3  to a line Q 2  so as to approximate to the ideal characteristic lines P 1  and Q 1 , respectively. Accordingly, the temperature characteristic of the difference ΔVbe may be made to approximate to such a characteristic that the difference ΔVbe increases linearly with respect to temperature, to thereby improve precision of temperature measurement.