Abstract:
Provided is a temperature sensor device operable at a lower voltage. The temperature sensor device detects temperature based on an output voltage of a forward voltage generator for generating a forward voltage of a PN junction. The forward voltage generator includes a level shift voltage generation circuit, and an output voltage of the temperature sensor device is given based on the forward voltage of the PN junction and a voltage of the level shift voltage generation circuit.

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2011-006101 filed on Jan. 14, 2011, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a temperature sensor device for detecting temperature. 
     2. Description of the Related Art 
     Conventionally, a temperature sensor device for detecting temperature, which utilizes a forward voltage of the PN junction, is well known (see, for example, Japanese Patent Application Laid-open No. Hei 5-248962). 
       FIG. 7  is a circuit diagram illustrating a conventional temperature sensor device. The conventional temperature sensor device includes a constant current source circuit  701  and Darlington-connected bipolar transistors  702 ,  703 , and  704  supplied with a constant current from the constant current source circuit  701 . 
     An emitter of the bipolar transistor  704  is connected to an output terminal. An output voltage of the temperature sensor device is the sum of forward voltages of the PN junctions of the bipolar transistors. The forward voltage of the PN junction changes in accordance with temperature, and hence the output voltage is a voltage that changes in accordance with temperature. 
     In the temperature sensor device configured as described above, temperature detection accuracy of the temperature sensor device is enhanced as the sensitivity with which the output voltage changes in accordance with temperature becomes higher. Therefore, the temperature detection accuracy of the temperature sensor device can be enhanced by increasing the sum of the forward voltages of the PN junctions. In general, it is known that the temperature sensitivity of the forward voltage of the PN junction is approximately 2.5 mV/° C. 
     In the case of the temperature sensor device of  FIG. 7 , the number of effective stages of PN junctions is three, and hence the sum of the forward voltages is three times the forward voltage of one PN junction. Therefore, the sensitivity with which the output voltage changes in accordance with temperature is approximately 7.5 mV/° C., which is three times the temperature sensitivity of the forward voltage of one PN junction. 
     In the conventional temperature sensor device, however, if the number of effective stages of PN junctions is increased for enhancing the sensitivity with which the output voltage changes in accordance with temperature, the sum of the forward voltages of the PN junctions becomes larger. Therefore, the conventional temperature sensor device has a problem that an operating voltage cannot be suppressed to be lower. This problem results in inefficiency in the sense that a low voltage range of a power supply voltage supplied from a battery or the like cannot be used. 
     SUMMARY OF THE INVENTION 
     The present invention has been devised in order to solve the above-mentioned problem, and realizes a temperature sensor device operable at a lower voltage. 
     The present invention provides a temperature sensor device including a forward voltage generator for generating a forward voltage of a PN junction, for detecting temperature based on an output voltage of the forward voltage generator, in which the forward voltage generator includes a level shift voltage generation circuit, and an output voltage of the temperature sensor device is given based on the forward voltage of the PN junction and a voltage of the level shift voltage generation circuit. 
     According to the present invention, the temperature sensor device operable at a lower voltage can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a circuit diagram illustrating a temperature sensor device according to an embodiment of the present invention; 
         FIG. 2  is a circuit diagram illustrating a specific example of a voltage source included in the temperature sensor device according to the embodiment of the present invention; 
         FIG. 3  is a circuit diagram illustrating another specific example of the voltage source included in the temperature sensor device according to the embodiment of the present invention; 
         FIG. 4  is a circuit diagram illustrating still another specific example of the voltage source included in the temperature sensor device according to the embodiment of the present invention; 
         FIG. 5  is a circuit diagram illustrating an example of a current source of  FIG. 4 ; 
         FIG. 6  is a circuit diagram illustrating another example of the temperature sensor device according to the embodiment of the present invention; and 
         FIG. 7  is a circuit diagram illustrating a conventional temperature sensor device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  is a circuit diagram illustrating a temperature sensor device according to an embodiment of the present invention. The temperature sensor device of this embodiment includes a constant current source circuit  701 , Darlington-connected bipolar transistors  702 ,  703 , and  704  supplied with a constant current from the constant current source circuit  701 , and voltage sources  101 ,  102 , and  103 . 
     The voltage source  101  is connected between an emitter of the bipolar transistor  702  and a base of the bipolar transistor  703 . The voltage source  102  is connected between an emitter of the bipolar transistor  703  and a base of the bipolar transistor  704 . The voltage source  103  is connected between an emitter of the bipolar transistor  704  and an output terminal. Here, voltages V 1 , V 2 , and V 3  of the voltage sources  101 ,  102 , and  103  do not have temperature characteristics. 
     Next, an operation of the temperature sensor device of this embodiment is described. 
     A forward voltage of the PN junction is represented by Vpn. In this case, a voltage V(A) at the point A, a voltage V(B) at the point B, and a voltage V(C) at the point C are calculated as follows, respectively.
 
 V ( A )=1× Vpn−V 1  (1)
 
 V ( B )=2× Vpn −( V 1 +V 2)  (2)
 
 V ( C )=3× Vpn −( V 1+ V 2+ V 3)  (3)
 
     The voltage V(C) is a voltage at the output terminal, and accordingly an output voltage of the temperature sensor device is given as 3×Vpn−(V 1 +V 2 +V 3 ). That is, the output voltage of the temperature sensor device of this embodiment is lower than the output voltage of the conventional temperature sensor device by the voltage (V 1 +V 2 +V 3 ). It follows that the temperature sensor device of this embodiment is capable of suppressing an operating voltage to be low even when the number of effective stages of PN junctions is increased. The voltages V 1 , V 2 , and V 3  do not have temperature characteristics, and hence the sensitivity with which the output voltage changes in accordance with temperature is comparable to that in the conventional temperature sensor device. 
     In other words, it can be said that the operating voltage can be suppressed to be lower, though the sensitivity with which the output voltage changes in accordance with temperature is comparable to that in the conventional temperature sensor device. 
     According to the temperature sensor device of this embodiment, by employing the configuration described above, it is possible to provide a temperature sensor device operable at a lower voltage. 
     Note that, in the temperature sensor device of this embodiment, the voltage source is provided to the emitter of each bipolar transistor, but not all the voltage sources need to be provided. For example, even with only the voltage source  101 , the effect to achieve low voltage operation of the temperature sensor device can be expected. 
     Further, in the temperature sensor device of this embodiment described above, the number of effective stages of PN junctions is three. However, it is apparent that the same effect can be obtained irrespective of the number of effective stages of PN junctions. 
       FIG. 2  is a circuit diagram illustrating a specific example of the voltage source included in the temperature sensor device of this embodiment. For simplification, a specific example of only the voltage source  101  is illustrated. 
     The voltage source  101  includes a current source  201  and a MOS transistor  202 . 
     The voltage V 1  of the voltage source  101  is applied based on a gate-source voltage of the MOS transistor  202  biased by the current source  201 . 
     As another example, the voltage source may be a circuit as illustrated in  FIG. 3 . The voltage source  101  includes current sources  201  and  301  and MOS transistors  202  and  302 . That is, the voltage source may be realized by Darlington-connected MOS transistors. 
     As still another example, the voltage source may be a circuit as illustrated in  FIG. 4 . The voltage source  101  includes a current source  401  and a resistor  402 . The voltage V 1  of the voltage source  101  is applied based on a resistance of the resistor  402  biased by the current source  401 . 
     In this case, if there are fluctuations in resistance of the resistor  402 , the influence of the fluctuations in resistance can be reduced by configuring the current source  401  by a circuit as illustrated in  FIG. 5 , for example. The current source  401  performs impedance conversion on a voltage supplied by a voltage source  501 , and generates a current obtained through division by a resistance of a resistor  502 . That is, the current source  401  uses the resistor  502  of the same type as the resistor  402  to output a current I for canceling the fluctuations in resistance of the resistor  402 . Note that, the circuit of the current source  401  illustrated in  FIG. 5  is an example, and the current source  401  is not limited to this circuit. 
     Note that, in the temperature sensor device of this embodiment described above, the voltage of the voltage source does not have temperature characteristics, but may have temperature characteristics. Particularly when the voltage source has controlled positive temperature characteristics, a higher-sensitive temperature sensor device can be provided. 
     For example, such a temperature sensor device may be configured by using, as the current source  401  of  FIG. 4 , the current source of  FIG. 5  in which a resistor having negative temperature characteristics is used as the resistor  502 , and using the resistor  402  having positive temperature characteristics. 
     Further, the temperature sensor device of this embodiment described above is a temperature sensor using a forward voltage of the PN junction of a bipolar transistor, but a diode element may be used instead. As an example, a temperature sensor device includes a constant current source  601 , three-stage diodes, and a voltage source  602  as illustrated in  FIG. 6 . An output voltage is determined by subtracting a voltage of the voltage source  602  from a voltage generated in the three-stage diodes. The operating voltage can be suppressed to be lower, though the sensitivity is substantially comparable to that in the case where no voltage source  602  is provided.