Patent Publication Number: US-2022214228-A1

Title: Temperature measurement circuit and temperature measurement device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2021-1307, filed Jan. 7, 2021, the contents of which are incorporated herein by reference in their entirety. 
     BACKGROUND 
     1. Field of the Invention 
     The present disclosure relates to a temperature measurement circuit and a temperature measurement device. 
     2. Description of the Related Art 
     Temperature detecting devices are known in which a thermistor and a voltage divider are coupled in series and a temperature is detected based on an output voltage at a connection point of the thermistor and the voltage divider. In such a temperature detecting device, when the thermistor operates in a low resistance range, increases in the current flowing through the thermistor may be mitigated by reducing a supply voltage to the thermistor (see, for example, Patent Document 1). 
     CITATION LIST 
     [Patent Document] 
     
         
         Patent Document 1: Japanese Unexamined Patent Application Publication No. 2015-145823 
       
    
     SUMMARY 
     In the technique described in Patent Document 1, when the thermistor operates in the low resistance range, a resistance value of the voltage divider coupled in series with the thermistor is reduced, and thus the current flowing through the thermistor is increased. For example, the increased current to flow through the thermistor may result in increased power consumption. Also, the temperature detecting device described in Patent Document 1 employs a system that detects changes in the voltage that is divided through the thermistor and the voltage divider, and thus a voltage divider constant needs to be changed in accordance with a constant of a used temperature sensitive element, such as the thermistor. Therefore, unless the voltage divider constant is changed, it may be difficult to sufficiently deal with a wide range of temperature sensitive element constants (nominal values of resistance). 
     The present disclosure provides a temperature measurement circuit and a temperature measurement device that is applicable to a wide range of temperature sensitive element constants. 
     According to a first aspect of the present disclosure, a temperature measurement circuit for measuring a temperature using a temperature sensitive element is provided. The temperature measurement circuit includes a voltage control circuit configured to apply a control voltage to the temperature sensitive element. The temperature measurement circuit includes a first switching circuit configured to switch levels of the control voltage based on a current flowing through the temperature sensitive element. The temperature measurement circuit includes a conversion circuit configured to convert the current flowing through the temperature sensitive element into a voltage level corresponding to the measured temperature, by using predetermined conversion gain. The temperature measurement circuit includes a second switching circuit configured to switch values of the conversion gain based on the voltage level. 
     According to a second aspect of the present disclosure, a temperature measurement circuit for measuring a temperature using a temperature sensitive element is provided. The temperature measurement circuit includes a transistor for driving the temperature sensitive element. The temperature measurement circuit includes a differential circuit configured to compare an output voltage of the transistor against a reference voltage. The temperature measurement circuit includes a voltage control circuit configured to apply a control voltage to the temperature sensitive element by controlling a gate of the transistor based on an output of the differential circuit. The temperature measurement circuit includes a conversion circuit configured to convert an output current of the transistor into a voltage level corresponding to the measured temperature. The output voltage of the transistor is a voltage into which the output current of the transistor is converted based on a resistance value of the temperature sensitive element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an example of the configuration of a temperature measurement device including a temperature measurement circuit according to one embodiment; 
         FIG. 2  is a diagram illustrating an example of the relationship among first information BIT 1 , second information BIT 2 , and a coefficient n; 
         FIG. 3  is a diagram illustrating an example of a change characteristic of a voltage level VADC with respect to a resistance value RNTC of a temperature sensitive element; 
         FIG. 4  is a diagram illustrating the configuration of the temperature measurement circuit in a comparative example; 
         FIG. 5  is a diagram illustrating an example of the change characteristic of a voltage level VAD measured by the temperature measurement circuit that uses the temperature sensitive element of which the resistance value RNTC is 10 KΩ at an atmospheric temperature of 25° C. in the comparative example; 
         FIG. 6  is a diagram illustrating an example of the change characteristic of the voltage level VAD measured by the temperature measurement circuit that uses the temperature sensitive element of which the resistance value RNTC is 100 KΩ at the atmospheric temperature of 25° C. in the comparative example; 
         FIG. 7  is a diagram illustrating an example of the change characteristic of the voltage level VADC measured by the temperature measurement circuit that uses the temperature sensitive element of which the resistance value RNTC is 10 KΩ at the atmospheric temperature of 25° C. according to one embodiment; and 
         FIG. 8  is a diagram illustrating an example of the change characteristic of the voltage level VADC measured by the temperature measurement circuit that uses the temperature sensitive element of which the resistance value RNTC is 100 kΩ at the atmospheric temperature of 25° C. according to one embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     One or more embodiments of the present disclosure will be described with reference to the drawings. 
       FIG. 1  is a diagram illustrating an example of the configuration of a temperature measurement device  301  including a temperature measurement circuit  101  according to one embodiment. The temperature measurement device  301  illustrated in  FIG. 1  detects a temperature of a target object to be measured, by using a temperature sensitive element  60 . The target object may include a solid, a liquid, or gas, and is not particularly limited to these examples. A specific example of the target object may include a secondary battery or the like. The temperature measurement device  301  includes the temperature sensitive element  60  and the temperature measurement circuit  101 . 
     The resistance value of the temperature sensitive element  60  changes with changes in the temperature of the target object. For example, the temperature sensitive element  60  is a negative temperature coefficient (NTC) thermistor. The NTC thermistor is a resistance temperature detector of which a resistance value is changed based on a negative temperature characteristic. 
     The temperature measurement circuit  101  is a semiconductor integrated circuit that measures the temperature of the target object by using the temperature sensitive element  60 . In this example, the temperature measurement circuit  101  includes terminals  11  to  16  used for an external connection with the temperature measurement circuit  101 . The temperature measurement circuit  101  may be a single semiconductor integrated circuit that has a function of measuring temperatures, or may be a circuit that is a portion of a semiconductor integrated circuit that has a different function (for example, a function or the like of protecting a secondary battery) different from the function of measuring the temperatures. 
     A terminal  11  is a power supply terminal, and a terminal  12  is a ground terminal. A positive electrode of a power supply  201 , such as a secondary battery, is electrically coupled to the terminal  11 , and a negative electrode of the power supply  201  is electrically coupled to the terminal  12 . The temperature measurement circuit  101  operates with a supply voltage that the power supply  201  applies between the terminal  11  and the terminal  12 . For example, the temperature measurement circuit  101  operates with a voltage VB that a constant voltage source  29  such as a regulator generates. The constant voltage source  29  may be disposed within the temperature measurement circuit  101 , or may disposed outside the temperature measurement circuit  101 . 
     A terminal  13  is a temperature measurement terminal to which one end of the temperature sensitive element  60  is coupled. The one end of the temperature sensitive element  60  is coupled to the terminal  13 , and another end of the temperature sensitive element  60  is coupled to the terminal  12 . 
     A terminal  14  is a first output terminal for outputting first information BIT 1  to outside of the temperature measurement circuit  101 . The first information BIT 1  changes in accordance with the level of a control voltage VTH that is applied to the temperature sensitive element  60 . A terminal  15  is a second output terminal for outputting second information BIT 2  to the outside of the temperature measurement circuit  101 . The second information BIT 2  changes in accordance with a value indicating conversion gain β described below. A terminal  16  is an output terminal for outputting a measurement value to the outside of the temperature measurement circuit  101 . The measurement value is obtained by the temperature measurement circuit  101  that uses the temperature sensitive element  60 . 
     The temperature measurement circuit  101  includes a voltage control circuit  20 , a first switching circuit  30 , a conversion circuit  40 , and a second switching circuit  50 . 
     The voltage control circuit  20  is a circuit that applies the control voltage VTH to the temperature sensitive element  60 . In this example, with use of a gate of a transistor  21  that drives the temperature sensitive element  60 , the voltage control circuit  20  applies the control voltage VTH to the temperature sensitive element  60 . The voltage control circuit  20  applies the control voltage VTH across the temperature sensitive element  60 , based on the output of a differential circuit  23  that compares the control voltage VTH against a reference voltage Vr. By applying the control voltage VTH between the terminal  13  and the terminal  12 , the voltage control circuit  20  applies the control voltage VTH to the temperature sensitive element  60  of which both ends are coupled to the terminal  13  and the terminal  12 . In this example, an output voltage of the transistor  21  is a voltage determined based on an output current of the transistor  21  and a resistance value RNTC of the temperature sensitive element  60 . The output voltage corresponds to the control voltage VTH. 
     In the example in  FIG. 1 , the voltage control circuit  20  includes a reference voltage circuit  22 , a differential circuit  23 , and the transistor  21 . 
     The reference voltage circuit  22  generates multiple reference voltages Vr each having a different voltage level. In this example, by dividing the supply voltage VB through a voltage divider that includes resistance of each of multiple resistors, the reference voltage circuit  22  generates two different reference voltages, i.e., a voltage level Vref and a voltage level Vref×a. The voltage level Vref is an example of a first voltage level, and the voltage level Vref×a is an example of a second voltage level that is greater than the first voltage level. 
     The “a” is a positive coefficient greater than 1. In this example, the coefficient “a” is determined by a ratio between resistance values of multiple resistors that are included in a voltage divider circuit of the reference voltage circuit  22 . The voltage divider circuit in the reference voltage circuit  22  may include a trimmable resistor in order to adjust a value of the coefficient “a.” 
     The differential circuit  23  controls a gate voltage for the transistor  21  such that a difference between the control voltage VTH and the reference voltage Vr is zero. The differential circuit  23  is an operational amplifier that includes, for example, (i) an inverting input to which the reference voltage Vr is applied, (ii) a non-inverting input to which the control voltage VTH is applied, and (iii) an output coupled to a gate of the transistor  21 . 
     The transistor  21  is a semiconductor element that causes a current ITH to flow through the temperature sensitive element  60 . In this example, the transistor  21  is a P-channel metal oxide semiconductor field effect transistor (MOSFET) that has an output terminal to be coupled to one end of the temperature sensitive element  60 . The transistor  21  is, for example, a P-channel MOSFET that has (i) a gate coupled to an output of the differential circuit  23 , (ii) a source coupled to the terminal  11 , and (iii) a drain coupled to both of one end of the temperature sensitive element  60  and the non-inverting input of the differential circuit  23 . In this case, the drain of the P-channel MOSFET corresponds to the output terminal of the transistor  21 . 
     The first switching circuit  30  switches levels of the reference voltage VTH, based on the current ITH flowing through the temperature sensitive element  60 . With this arrangement, the voltage control circuit  20  can selectively switch the levels of the control voltage VTH to be applied to the temperature sensitive element  60 , based on the magnitude of the current ITH. In the example in  FIG. 1 , the first switching circuit  30  switches the levels of the control voltage VTH, by switching the levels of the reference voltage Vr based on the magnitude of the current ITH flowing through the transistor  21 . 
     If the magnitude of the current ITH flowing through the transistor  21  is greater than a first current threshold I 1 , the first switching circuit  30  switches the levels of the reference voltage Vr such that the level of the control voltage VTH is reduced from the voltage level Vref×a to the voltage level Vref. With this arrangement, even if the magnitude of the current ITH is increased due to reductions in the resistance value RNTC of the temperature sensitive element  60 , the level of the control voltage VTH applied to the temperature sensitive element  60  is reduced, and thus the magnitude of the current ITH can be prevented from increasing excessively. In the example in  FIG. 1 , the first switching circuit  30  changes the level of the reference voltage Vr, from the voltage level Vref×a to the voltage level Vref, through a switch  31 , to thereby reduce the level of the control voltage VTH from the voltage level Vref×a to the voltage level Vref. 
     In contrast, if the magnitude of the current ITH flowing through the transistor  21  is less than a second current threshold  12  less than the first current threshold I 1 , the first switching circuit  30  switches the levels of the reference voltage Vr such that the level of the control voltage VTH is increased from the voltage level Vref to the voltage level Vref×a. With this arrangement, even if the magnitude of the current ITH is reduced due to increases in the resistance value RNTC of the temperature sensitive element  60 , the level of the control voltage VTH to be applied to the temperature sensitive element  60  is increased, and thus the magnitude of the current ITH can be prevented from being reduced excessively. In the example in  FIG. 1 , the first switching circuit  30  changes the level of the reference voltage Vr from the voltage level Vref×a to the voltage level Vref, through the switch  31 , to thereby increase the level of the control voltage VTH from the voltage level Vref to the voltage level Vref×a. 
     In the example in  FIG. 1 , the first switching circuit  30  includes the switch  31 , the transistor  32 , and a current detecting circuit  33 . 
     In this example, in response to the current ITH flowing through the transistor  21 , the first switching circuit  30  switches the levels of the reference voltage Vr based on a first mirror current Im output from a first current mirror circuit  71 . Thus, the first switching circuit  30  switches the levels of the control voltage VTH output from the voltage control circuit  20 . The first current mirror circuit  71  is a circuit that is constituted by the transistor  21  and the transistor  32 . The first mirror current Im is output from the transistor  32 , in response to the magnitude of the current ITH flowing through the transistor  21 . In this example, a ratio (a mirror ratio for the first current mirror circuit  71 ) between the magnitude of the current ITH and the magnitude of the first mirror current Im is 1:1. Such a ratio is not limited to this example. 
     As described above, in response to the current ITH flowing through the transistor  21 , the first switching circuit  30  switches the levels of the control voltage VTH based on the first current Im output from the first current mirror circuit  71 . With use of a second current mirror circuit  72 , the first switching circuit  30  can switch the levels of the control voltage VTH, without directly controlling the current ITH flowing through the temperature sensitive element  60 . Thus, accuracy of the current ITH flowing through the temperature sensitive element  60  to which the control voltage VTH is applied can be ensured. Therefore, accuracy in measuring a given temperature by using the temperature sensitive element  60  is improved. 
     In the example in  FIG. 1 , the first switching circuit  30  shares the transistor  21  with the voltage control circuit  20 . The transistor  21  is used as a drive transistor for passing the current ITH into the temperature sensitive element  60 . The transistor  21  is also used as a detection transistor for detecting the magnitude of the current ITH. The transistor  21  functions as both the drive transistor and the detection transistor, and thus the temperature measurement circuit  101  can be made compact. 
     In this example, the transistor  32  is a p-channel MOSFET that has (i) a gate coupled to the gate of the transistor  21 , (ii) a source coupled to the terminal  11 , and (iii) a drain coupled to the current detecting circuit  33 . 
     By monitoring the first mirror current Im, the current detecting circuit  33  detects the magnitude of the current ITH. Upon detecting that the magnitude of the current ITH flowing through the transistor  21  is greater than the first current threshold I 1  by monitoring the first mirror current Im, the current detecting circuit  33  uses the switch  31  to change the level of the reference voltage Vr from the voltage level Vref×a to the voltage level Vref. In contrast, upon detecting that the magnitude of the current ITH flowing through the transistor  21  is less than the second current threshold  12  by monitoring the first mirror current Im, the current detecting circuit  33  uses the switch  31  to change the level of the reference voltage Vr, from the voltage level Vref to the voltage level Vref×a. 
     The current detecting circuit  33  includes a Schmitt trigger inverter  34 , a first constant current source  35 , a second constant current source  36 , and a switch  37 . The first current threshold I 1  is set based on a total sum of the magnitude of a constant current delivered from the first constant current source  35 , and the magnitude of a constant current delivered from the second constant current source  36 . 
     A mirror ratio for the first current mirror circuit  71  is 1:1. If the magnitude of the current ITH (first mirror current Im) increases, an input voltage level of the Schmitt trigger inverter  34  increases. If the magnitude of the current ITH (first mirror current Im) exceeds the first current threshold I 1 , a logic level at the output of the Schmitt trigger inverter  34  is changed from a high level to a low level. With this arrangement, the switch  37  is changed from an on-state to an off-state, and the level of the reference voltage Vr is changed from the voltage level Vref×a to the voltage level Vref, through the switch  31 . As a result, a current threshold for the current detecting circuit  33  is changed from the first current threshold I 1  to the second current threshold  12 , and the level of the control voltage VTH is changed from the voltage level Vref×a to the voltage level Vref. The current detecting circuit  33  outputs first information BIT 1  at a low level to the outside of the temperature measurement circuit  101 , through the terminal  14 , and the first information BIT 1  indicates that the level of the control voltage VTH is the voltage level Vref. 
     In contrast, if the magnitude of the current ITH (first mirror current Im) decreases, the input voltage level of the Schmitt trigger inverter  34  is reduced. If the magnitude of the current ITH (first mirror current Im) is less than the second current threshold  12 , the logic level at the output of the Schmitt trigger inverter  34  is changed from the low level to the high level. With this arrangement, the switch  37  is changed from the off-state to the on-state, and the level of the reference voltage Vr is changed from the voltage level Vref to the voltage level Vref×a, through the switch  31 . As a result, the current threshold for the current detecting circuit  33  is changed from the second current threshold  12  to the first current threshold I 1 , and the level of the control voltage VTH is changed from the voltage level Vref to the voltage level Vref×a. The current detecting circuit  33  outputs first information BIT 1  at a high level to the outside of the temperature measurement circuit  101 , through the terminal  14 , and the first information BIT 1  indicates that the level of the control voltage VTH is the voltage level Vref×a. 
     With use of predetermined conversion gain β, the conversion circuit  40  converts the current ITH flowing through the temperature sensitive element  60 , into a voltage level VADC corresponding to a measured temperature. In this example, in the conversion circuit  40 , the second current mirror circuit  72  generates the voltage level VADC by performing conversion of the current ITH flowing through the transistor  21 . The second current mirror circuit  72  is a circuit that is constituted by the transistor  21 , the transistor  41 , and the transistor  42 . 
     In the conversion circuit  40 , the second current mirror circuit  72  converts the current ITH flowing through the transistor  21 , into a second mirror current Iref. Then, the conversion circuit  40  outputs an analog voltage indicative of the voltage level VADC, where the analog voltage is generated by passing the second mirror current Iref into a resistor  43 . The conversion circuit  40  outputs a measurement value (voltage level VADC or a value corresponding to the voltage level VADC) to the outside of the temperature measurement circuit  101 , through the terminal  16 . With this arrangement, the temperature measurement circuit  101  can provide an external device of the temperature measurement circuit  101  with the measurement value corresponding to the temperature that is obtained using the temperature sensitive element  60 . The second mirror current Iref is a reference current that flows through the resistor  43 . 
     The conversion circuit  40  may include an AD converter  44  that performs an analog-to-digital (AD) conversion of an analog voltage level VADC into a digital measurement value, where the analog voltage level VADC is generated by passing the second mirror current Iref into the resistor  43 . In this case, the conversion circuit  40  outputs the digital measurement value to the outside of the temperature measurement circuit  101 , through the terminal  16 . The digital measurement value is an example of a value corresponding to the voltage level VADC. 
     Instead of the AD converter  44 , the conversion circuit  40  may include a comparator that compares the analog voltage level VADC against a predetermined determination value. In addition to outputting the first information BIT 1  and the second information BIT 2  through the terminals  14  and  15 , respectively, the conversion circuit  40  can output the output of the comparator to the outside of the temperature measurement circuit  101 , through the terminal  16 , thereby providing the external device with an indication of whether a target voltage level exceeds the predetermined determination value (temperature). The output of the comparator is an example of a value corresponding to the voltage level VADC. 
     The external device uses the measurement value provided by the temperature measurement circuit  101 , in order to enable a predetermined control. The predetermined control is not particularly limited. 
     For example, the external device may correct a detected value indicative of a residual capacity the secondary battery, based on the measurement value, or may use the measurement value in order to protect the secondary battery against the temperature. 
     In the example illustrated in  FIG. 1 , the conversion circuit  40  includes the transistor  41 , the transistor  42 , the resistor  43 , and the AD converter  44 . 
     The conversion circuit  40  shares the transistor  21  with the voltage control circuit  20 . The transistor  21  is used as a drive transistor for passing the current ITH into the temperature sensitive element  60 . The transistor  21  is also used as a conversion transistor for converting the current ITH into the voltage level VADC. The transistor  21  functions as both the drive transistor and the conversion transistor, and thus the temperature measurement circuit  101  can be made compact. 
     In this example, the transistor  41  is a P-channel MOSFET that has (i) a gate coupled to the gate of the transistor  21 , (ii) a source coupled to the terminal  11 , and (iii) a drain coupled to the end of the resistor  43 . In this example, the transistor  42  is a P-channel MOSFET that has (i) a gate coupled to a gate of the transistor  21 , (ii) a source coupled to the terminal  11 , and (iii) a drain coupled to an end of the resistor  43  via a switch  54 . 
     The second switching circuit  50  switches values of conversion gain β for the conversion circuit  40 , based on the voltage level VADC. With this arrangement, the conversion circuit  40  can selectively change the conversion gain β, based on the voltage level VADC. The second switching circuit  50  switches the values of the conversion gain β based on the voltage level VADC, such that the voltage level VADC is set within a predetermined range. Thus, the voltage level VADC can be changed within the predetermined range. 
     If the voltage level VADC exceeds a first threshold Vd 1 , the second switching circuit  50  changes the value of the conversion gain β to a first conversion gain value β 1  (in this example, “1”). In contrast, if the voltage level VADC is less than a second threshold Vd 2  less than the first threshold Vd 1 , the second switching circuit  50  changes the value of the conversion gain β to a second conversion gain value β 2  (in this example, “b”). The value “b” is a positive coefficient greater than 1. In this example, the conversion gain β and coefficient b are each determined based on a mirror ratio for the second current mirror circuit  72 . 
     In this example, the second switching circuit  50  switches the values of the conversion gain β, based on the voltage level VADC generated by the second current mirror circuit  72  that performs conversion of the current ITH flowing through the transistor  21 . With use of the second current mirror circuit  72 , the second switching circuit  50  can switch the values of the conversion gain β, without directly controlling the current ITH flowing through the temperature sensitive element  60 . With this arrangement, accuracy of the current ITH flowing through the temperature sensitive element  60  that is applied to the control voltage VTH can be ensured. Therefore, accuracy in measuring the temperature by using the temperature sensitive element  60  is improved. 
     In the example illustrated in  FIG. 1 , the second switching circuit  50  includes a threshold generating circuit  51 , a switch  52 , a comparator  53 , and the switch  54 . 
     The threshold generating circuit  51  generates multiple thresholds Vd each indicating a different voltage level. In this example, the threshold generating circuit  51  generates two thresholds Vd, i.e., the first threshold Vd 1  and the second threshold Vd 2 , by dividing the supply voltage VB through a voltage divider circuit that includes resistance of each of multiple resistors. A voltage level of the second threshold Vd 2  is less than that of the first threshold Vd 1 . 
     The comparator  53  compares a threshold Vd against the voltage level VADC. Based on the result of the comparison, the comparator  53  turns the switch  54  on or off, and performs switching of the switch  52 . 
     When detecting that the voltage level VADC exceeds the first threshold Vd 1 , the comparator  53  changes the value of the conversion gain β, from β 2  (in this example, “b”) to β 1  (in this example, “1”), by turning the switch  54  off, and further changes the threshold Vd from Vd 1  to Vd 2 , through the switch  52 . The comparator  53  outputs second information BIT 2  at a low level to the outside of the temperature measurement circuit  101 , via the terminal  15 . The second information BIT 2  indicates that the value of the conversion gain β is the gain β 1 . 
     In contrast, when detecting that the voltage level VADC is less than the second threshold Vd 2 , the comparator  53  changes the value of the conversion gain β, from β 1  (in this example, “1”) to β 2  (in this example, “b”), by turning the switch  54  on, and further changes the threshold Vd from Vd 2  to Vd 1 , through the switch  52 . The comparator  53  outputs second information BIT 2  at a high level to the outside of the temperature measurement circuit  101 . The second information BIT 2  indicates that the value of the conversion gain β is the gain β 2 . 
     In this description, the following equations are given. 
     
       
         
           
             
               
                 
                   RNTC 
                   = 
                   
                     VTH 
                     ⁢ 
                     
                       / 
                     
                     ⁢ 
                     ITH 
                   
                 
               
             
             
               
                 
                   = 
                   
                     
                       ( 
                       
                         Vref 
                         × 
                         α 
                       
                       ) 
                     
                     ⁢ 
                     
                       / 
                     
                     ⁢ 
                     ITH 
                   
                 
               
             
             
               
                 
                   = 
                   
                     
                       ( 
                       
                         Vref 
                         × 
                         α 
                       
                       ) 
                     
                     × 
                     
                       ( 
                       
                         β 
                         ⁢ 
                         
                           / 
                         
                         ⁢ 
                         Iref 
                       
                       ) 
                     
                   
                 
               
             
             
               
                 
                   = 
                   
                     
                       ( 
                       
                         Vref 
                         × 
                         α 
                       
                       ) 
                     
                     × 
                     
                       ( 
                       
                         β 
                         × 
                         Rref 
                         ⁢ 
                         
                           / 
                         
                         ⁢ 
                         VADC 
                       
                       ) 
                     
                   
                 
               
             
             
               
                 
                   = 
                   
                     Rref 
                     × 
                     Vref 
                     × 
                     
                       ( 
                       
                         n 
                         ⁢ 
                         
                           / 
                         
                         ⁢ 
                         VADC 
                       
                       ) 
                     
                   
                 
               
             
           
         
       
     
     Where, RNTC is the resistance value of the temperature sensitive element  60 , Vref×α is the level of the control voltage VTH applied to the temperature sensitive element  60 , and ITH is the current flowing through the temperature sensitive element  60 . Also, β×ITH is the magnitude of the second mirror current Iref flowing through the resistor  43 , and Rref is the resistance value of the resistor  43 . The coefficient n is a value obtained by α×β, α is 1 or a, and β is 1 or b. 
       FIG. 2  is a diagram illustrating an example of the relationship among the first information BIT 1 , the second information BIT 2 , and the coefficient n. Where, L indicates a low level, and H indicates a high level. The external device compares, against a corresponding logic level in the relationship illustrated in  FIG. 2 , a logic level of each of the first information BIT 1  and the second information BIT 2 , which is obtained from the temperature measurement circuit  101 . Then, the external device determines the coefficient n in a given equation described above. The temperature measurement circuit  101  may provide the external device with information indicating the coefficient n. 
     The external device acquires a measurement value (voltage level VADC or a value corresponding to the voltage level VADC) from the temperature measurement circuit  101 , and calculates the resistance value RNTC of the temperature sensitive element  60  by substituting the measurement value and the coefficient n into the above-described equation. The external device calculates a given temperature of the target object based on a calculated value of the resistance value RNTC. For example, the external device calculates the temperature (ambient temperature T) of the target object by substituting the calculated value of the resistance value RNTC into the equations below. 
         R=R   0 ×exp( B ×(1/ T− 1/ T   0 ))  (1a)
 
       1/ T= 1/ B ×ln( R/R   0 )+1/ T   0   (1b)
 
     Where, T is ambient temperature (K), T 0  is a reference temperature (K), R is a resistance value (=RNTC) of the temperature sensitive element  60  at the ambient temperature T, R 0  is a resistance value of the temperature sensitive element  60  at the reference temperature T 0 , and B is a constant.
 
Equation (1b) is an equation obtained by modifying Equation (1a). Also, T 0 , R 0 , and B are parameters for the temperature sensitive element  60 .
 
       FIG. 3  is a diagram illustrating an example of the change characteristic of the voltage level VADC with respect to the resistance value RNTC of the temperature sensitive element  60 . In the temperature measurement circuit  101 , as illustrated in  FIG. 3 , the voltage level VADC exhibits the change characteristic with changes in the resistance value RNTC, for the logic level of each of the first information BIT 1  and the second information BIT 2 . The temperature measurement circuit  101  employs a system that detects changes in the current that flows through the temperature sensitive element  60 . With this arrangement, the circuit constant of the voltage control circuit  20  or the like of the temperature measurement circuit  101  can be set without depending on a constant of the temperature sensitive element  60 . With this arrangement, a wide range of constants of the temperature sensitive element can be used without changing the circuit constant of the voltage control circuit  20  or the like. That is, the temperature measurement circuit  101  can measure the wide range of resistance values RNTC, with high accuracy. Therefore, for example, the temperature measurement circuit  101  may use multiple types of temperature sensitive elements  60  each having a different temperature characteristic for a resistance value. The external device can calculate the temperature with high accuracy, based on (i) a given measurement value (voltage level VADC or a value corresponding to the voltage level VADC), (ii) the first information BIT 1 , and (iii) the second information BIT 2 . Further, in contrast to a circuit configuration described in Patent Document 1, the temperature measurement circuit  101  does not have the circuit configuration in which when the temperature sensitive element  60  operates in a low resistance range, the resistance value of a given resistor coupled in series with the temperature sensitive element  60  is reduced. Thus, in the temperature measurement circuit  101 , increases in the current flowing into the temperature sensitive element  60  can be reduced. 
       FIG. 4  is a diagram illustrating an example of the configuration of the temperature measurement circuit in a comparative example. The temperature measurement circuit  100  illustrated in  FIG. 4  measures the temperature of a measurement target by using the temperature sensitive element  60 . The temperature measurement circuit  100  differs from the temperature measurement circuit  101  according to the first embodiment in that the temperature measurement circuit  100  includes a resistor  124  coupled in series with the temperature sensitive element  60  and outputs a voltage level VAD from a connection point of the resistor  124  and the temperature sensitive element  60 . 
     A terminal  111  is a power supply terminal, and a terminal  112  is a ground terminal. A positive electrode of a power supply  201 , such as a secondary battery, is electrically coupled to the terminal  111 . A negative electrode of the power supply  201  is electrically coupled to the terminal  112 . The temperature measurement circuit  100  operates with a supply voltage between the terminals  111  and  112 , where the supply voltage is applied by the power supply  201 . For example, the temperature measurement circuit  100  operates with the voltage VB that is generated by a constant voltage source  129  such as a regulator. A terminal  113  is a temperature measurement terminal to be coupled to one end of the temperature sensitive element  60 . The one end of the temperature sensitive element  60  is coupled to the terminal  113 , and another end is coupled to the terminal  112 . 
     The temperature measurement circuit  100  includes a reference voltage circuit  122 , a differential circuit  123 , a transistor  121 , the resistor  124 , and an AD converter  144 . 
     The reference voltage circuit  122  generates a reference voltage Vr (in this example, 3.0 V). The differential circuit  123  controls a gate voltage for the transistor  121  such that a difference between a constant voltage, which is applied across a series circuit including the temperature sensitive element  60  and the resistor  124 , and the reference voltage Vr, which is generated by the reference voltage circuit  122 , is zero. 
       FIG. 5  is a diagram illustrating an example of the change characteristic of the voltage level VAD measured by the temperature measurement circuit  100  that uses the temperature sensitive element of which the resistance value RNTC is 10 kΩ at an atmospheric temperature of 25° C. in the comparative example.  FIG. 6  is a diagram illustrating an example of the change characteristic of a voltage level VAD measured by the temperature measurement circuit  100  that uses the temperature sensitive element of which the resistance value RNTC is 100 kΩ at the atmospheric temperature of 25° C. in the comparative example. 
     In  FIGS. 5 and 6 , small changes in the voltage level VAD in accordance with changes in the temperature (resistance value RNTC) indicate less sensitivity (accuracy) in measuring temperatures. In  FIG. 5 , although measurement accuracy at around 25° C. is relatively good, measurement accuracy in a high temperature range and a low temperature range is reduced. In  FIG. 6 , in a low temperature range, changes in the voltage level VAD are negligible, and thus measurement accuracy is reduced. Because the temperature measurement circuit  100  in the comparative example employs a system that detects changes in the voltage that is divided through the temperature sensitive element  60  and the resistor  124 , the constant of the temperature sensitive element  60  and the constant of the resistor  124  need to correspond to each other in one-to-one relation, and thus the constant of the temperature sensitive element  60  cannot be changed as needed. With this arrangement, in the temperature measurement circuit  100  in the comparative example, the resistance value of the resistor  124  in the comparative example needs to be changed depending on the characteristic of the temperature sensitive element  60  or a temperature range. 
       FIG. 7  is a diagram illustrating an example of the change characteristic of the voltage level VADC measured by the temperature measurement circuit  101  that uses the temperature sensitive element of which the resistance value RNTC is 10 kΩ at the atmospheric temperature of 25° C. according to one embodiment.  FIG. 8  is a diagram illustrating an example of the change characteristic of the voltage level VADC measured by the temperature measurement circuit  101  that uses the temperature sensitive element of which the resistance value RNTC is 100 kΩ at the atmospheric temperature of 25° C. according to one embodiment. 
     As illustrated in  FIGS. 7 and 8 , in the temperature measurement circuit  101 , the voltage level VADC changes greatly in accordance with changes in the temperature (resistance value RNTC), and thus sensitivity (accuracy) in measuring the temperature is improved. Because the temperature measurement circuit  101  according to one embodiment employs a system that detects changes in the current that flows through the temperature sensitive element  60 , the circuit constant of the voltage control circuit  20  or the like of the temperature measurement circuit  101  can be set without depending on the constant of the temperature sensitive element  60 . With this arrangement, a wide range of constants of the temperature sensitive element can be used without changing the circuit constant of the voltage control circuit  20  or the like. Further, in the temperature measurement circuit  101  according to one embodiment, a wide range of changes in the resistance value RNTC can be measured, and thus multiple types of temperature sensitive elements each having a different temperature characteristic for the resistance value can be used. 
     Although one or more embodiments have been described, the present disclosure is not limited to the embodiments. Various modifications and changes, such as combinations and substitutions with some or all of other embodiments, can be made. 
     For example, the temperature measurement circuit is not limited to an integrated circuit, and may be a discrete circuit that is constituted by multiple discrete components. An object of which the temperature is to be measured includes a solid, a liquid, or gas. The temperature sensitive element may include an element other than the NTC thermistor.