Abstract:
Provided is an overheat detection circuit capable of easily adjusting a detection temperature of the overheat detection circuit. The overheat detection circuit includes: a first resistor; a second resistor, which has the same temperature characteristics with the first resistor, and has an adjustable resistance value; and a heat sensitive element connected to one end of the second resistor, in which a first current, which is based on a first voltage, is supplied to the first resistor, a current, which is proportional to the first current, is supplied to the second resistor so that a second voltage is generated at another end of the second resistor, and when the first voltage and the second voltage are compared, a result of the comparison is output as an overheat detection signal.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2015-219614 filed on Nov. 9, 2015, 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 an overheat detection circuit, an overheat protection circuit including the overheat detection circuit, and a semiconductor device including the overheat protection circuit. 
         [0004]    2. Description of the Related Art 
         [0005]      FIG. 6  is a circuit diagram for illustrating a related-art overheat detection circuit  600 . 
         [0006]    The related-art overheat detection circuit  600  is configured in the following manner. 
         [0007]    A constant current source  608  is connected between a power supply terminal  60  and an anode of a diode  601 , to thereby output a constant current JO. 
         [0008]    A cathode of the diode  601  is connected to one end of an adjustable resistor  613 , and another end of the adjustable resistor  613  is connected to a ground terminal  61 . The anode of the diode  601  is connected to a gate of a P-channel transistor  611 . A source of the P-channel transistor  611  is connected to the power supply terminal  60  and a drain thereof is connected to an output terminal  62 . 
         [0009]    Resistors  609  and  610  are connected in series between the power supply terminal  60  and the ground terminal  61 . A gate of an N-channel transistor  612  is connected to a node between the resistor  609  and the resistor  610 . A drain of the N-channel transistor  612  and a source thereof are connected to the output terminal  62  and the ground terminal  61 , respectively. 
         [0010]    When temperature increases, an absolute value of a threshold voltage (Vth) of the P-channel transistor  611  decreases. Meanwhile, a voltage at an anode terminal of the diode  601  decreases. That is, a difference between a voltage at a gate terminal of the P-channel transistor  611  and a voltage at the power supply terminal  60  increases. 
         [0011]    Therefore, when temperature increases, the threshold voltage (Vth) of the P-channel transistor  611  and the difference between the voltage at the gate terminal of the P-channel transistor  611  and the voltage at the power supply terminal  60  reverse, to thereby turn on the P-channel transistor  611  and invert a voltage at the output terminal  62  from an electric potential at the ground terminal  61  to an electric potential at the power supply terminal  60 . 
         [0012]    In this way, the related-art overheat detection circuit  600  is configured to detect a state of overheating based on change in voltage at the output terminal  62  (for example, see Japanese Patent No. 4920305). 
         [0013]    However, in order to adjust a detection temperature of the overheat detection circuit with the related art, a resistance value of the adjustable resistor  613  needs to be adjusted in consideration of a threshold voltage of the P-channel transistor  611 , temperature characteristics of the threshold voltage of the P-channel transistor  611 , a value of an output current IO of the constant current source  608 , temperature characteristics of the constant current source  608 , and temperature characteristics of the adjustable resistor  613 . The threshold value and the resistance value of the transistor, and the constant current each vary due to process variation, and thus, all of those variations need to be taken into consideration. This has made such adjustment extremely difficult. 
       SUMMARY OF THE INVENTION 
       [0014]    The present invention has been made in order to solve the problems described above, and realizes an overheat detection circuit having satisfactory detection precision with which adjustment can be easily performed. 
         [0015]    According to one embodiment of the present invention, there is provided an overheat detection circuit including: a first resistor; a second resistor, which has the same temperature characteristics with the first resistor, and has an adjustable resistance value; and a heat sensitive element connected to one end of the second resistor, in which a first current, which is based on a first voltage, is supplied to the first resistor, a current, which is proportional to the first current, is supplied to the second resistor so that a second voltage is generated at another end of the second resistor, and when the first voltage and the second voltage are compared, a result of the comparison is output as an overheat detection signal. 
         [0016]    The overheat detection circuit according to the present invention can easily adjust manufacturing variation because the temperature characteristics of the first resistor and the second resistor are the same. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a circuit diagram for illustrating an overheat detection circuit according to a first embodiment of the present invention. 
           [0018]      FIG. 2  is a circuit diagram for illustrating a first example of an overheat detection circuit according to a second embodiment of the present invention. 
           [0019]      FIG. 3  is a circuit diagram for illustrating a second example of the overheat detection circuit according to the second embodiment. 
           [0020]      FIG. 4  is a circuit diagram for illustrating a third example of the overheat detection circuit according to the second embodiment. 
           [0021]      FIG. 5  is a circuit diagram for illustrating a fourth example of the overheat detection circuit according to the second embodiment. 
           [0022]      FIG. 6  is a diagram for illustrating a related-art overheat detection circuit. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]      FIG. 1  is an illustration of an overheat detection circuit  100  according to a first embodiment of the present invention. 
         [0024]    The overheat detection circuit  100  is configured in the following manner. 
         [0025]    A voltage (hereinafter referred to as Vref) output from a reference voltage source  105  is input to an inverting input terminal of a differential amplifier  106 . One end of a resistor  13  and a drain of a P-channel transistor  5  are connected to a non-inverting input terminal of the differential amplifier  106 . An output terminal of the differential amplifier  106  is connected to a gate of the P-channel transistor  5  and a gate of a P-channel transistor  6 . A source of the P-channel transistor  5  is connected to a power supply terminal  10 . Another end of the resistor  13  is connected to a ground terminal  11 . A source of the P-channel transistor  6  is connected to the power supply terminal  10 , and a drain thereof is connected to one end of an adjustable resistor  113  that has an adjustable resistance value. An anode and a cathode of a diode  101  are connected to another end of the adjustable resistor  113  and the ground terminal  11 , respectively. 
         [0026]    A voltage comparator  107  compares a value of Vref and an electric potential at the one end of the adjustable resistor  113 , to thereby output a signal of a result of the comparison to an output terminal  12  as an overheat detection signal. 
         [0027]    The differential amplifier  106  and the P-channel transistor  5  are controlled such that an electric potential at the one end of the resistor  13  becomes equal to Vref. Therefore, assuming that a resistance value of the resistor  13  is R 1 , a current flowing through the resistor  13  is: 
         [0000]        I 1=Vref/ R 1  Expression 1.
 
         [0028]    The P-channel transistor  5  and the P-channel transistor  6  share a gate and a source, and hence current I 2  output from the P-channel transistors can be made proportional to the current I 1 . 
         [0000]        I 1 =αI 2  Expression 2,
 
         [0000]    where α is an arbitrary proportionality factor. Assuming that a forward voltage of the diode  101  is Vf and the resistance value of the adjustable resistor  113  is R 2 , a signal output from the voltage comparator  107  is inverted with Expression 3 as a threshold value. 
         [0000]        V ref= I 2* R 2 +Vf   Expression 3.
 
         [0029]    By configuring each of the resistor  13  and the adjustable resistor  113  such that the temperature characteristics thereof are equal to each other, a proportional relationship is established between the resistance values R 1  and R 2 . 
         [0000]        R 1 =βR 2  Expression 4,
 
         [0000]    where β is an arbitrary proportionality factor. The following expression holds true from Expressions 1 to 4: 
         [0000]      (1−1/(α·β)) V ref= Vf   Expression 5.
 
         [0030]    α and β are proportionality factors, and therefore a change in temperature of Vf and a change in temperature of Vref cause the signal output by the voltage comparator  107  to invert at a certain temperature. In this way, it is possible to detect a change in temperature. 
         [0031]    In particular, when the change in temperature of Vref is sufficiently small, only β needs to be adjusted by the adjustable resistor  113  by only taking into consideration the change in temperature of Vf and not relying on the value of Vref, such that the output signal inverts at an arbitrary temperature. That is, in this embodiment, in adjustment of detection temperature of the overheat detection circuit, it is not necessary to take into consideration all variations in threshold values and resistance values of the transistors as in a related-art overheat detection circuit, thereby providing the effect of being able to easily adjust the detection temperature. 
         [0032]      FIG. 2  is a circuit diagram for illustrating an overheat detection circuit  200   a  as a first example of an overheat detection circuit according to a second embodiment of the present invention. 
         [0033]    The overheat detection circuit  200   a  is configured in the following manner. 
         [0034]    A drain of an N-channel depletion transistor  1  is connected to a drain and a gate of a P-channel transistor  8 , a gate of a P-channel transistor  9 , a gate of a P-channel transistor  15 , and a gate of a P-channel transistor  16 . A gate and a source of the N-channel depletion transistor  1  are connected to the ground terminal  11 . 
         [0035]    A source of the P-channel transistor  8  is connected to the power supply terminal  10 . 
         [0036]    A drain of the P-channel transistor  9  is connected to a drain of an N-channel transistor  4  and a gate of an N-channel transistor  7 . A source of the P-channel transistor  9  is connected to the power supply terminal  10 . 
         [0037]    A source of the N-channel transistor  4  is connected to the ground terminal  11 , and a gate thereof is connected to the one end of the resistor  13  and the drain of the P-channel transistor  5 . 
         [0038]    A source of the N-channel transistor  7  is connected to the ground terminal  11 , and a drain thereof is connected to the gate of the P-channel transistor  5 , a drain of the P-channel transistor  15 , and the gate of the P-channel transistor  6 . 
         [0039]    A source of the P-channel transistor  15  and the source of the P-channel transistor  5  are connected to the power supply terminal  10 . 
         [0040]    The other end of the resistor  13  is connected to the ground terminal  11 . 
         [0041]    The source and the drain of the P-channel transistor  6  are connected to the power supply terminal  10 , and the one end of the adjustable resistor  113  and a gate of an N-channel transistor  3 , respectively. 
         [0042]    The anode and the cathode of the diode  101  are connected to the another end of the adjustable resistor  113  and the ground terminal  11 , respectively. 
         [0043]    A drain of the P-channel transistor  16  is connected to the output terminal  12  and a drain of the N-channel transistor  3 , and a source thereof is connected to the power supply terminal  10 . 
         [0044]    A source of the N-channel transistor  3  is connected to the ground terminal  11 . 
         [0045]    Next, operation of the overheat detection circuit  200   a  is described. 
         [0046]    The N-channel depletion transistor  1  is configured to supply a bias current to the N-channel transistor  4  via a current mirror circuit constructed with the P-channel transistors  8  and  9 . The N-channel transistors  4  and  7  and the P-channel transistors  15  and  5  construct a negative feedback circuit, and are controlled in the same way as in the first embodiment such that a gate voltage of the N-channel transistor  4  becomes equal to the constant voltage Vref determined by a threshold voltage of the N-channel transistor  4  and the bias current supplied by the N-channel depletion transistor  1 . 
         [0047]    The N-channel depletion transistor  1  is further configured to supply the bias current to the N-channel transistor  3  via a current mirror circuit constructed with the P-channel transistors  8  and  16 . 
         [0048]    Through adjustment of a current mirror ratio and a size ratio between the N-channel transistors  4  and  3 , the threshold voltage at which a signal output from the output terminal  12  inverts can be synchronized with Vref when the gate voltage of the N-channel transistor  3  changes. 
         [0049]    Therefore, the signal output from the output terminal  12  inverts at a certain temperature. This temperature is set similarly to that in the first embodiment. 
         [0050]    As a result, the second embodiment provides similar effects to those of the first embodiment. 
         [0051]      FIG. 3  is a circuit diagram for illustrating an overheat detection circuit  200   b  as a second example of the overheat detection circuit according to the second embodiment. 
         [0052]    Compared to the overheat detection circuit  200   a  illustrated in  FIG. 2  which employs the N-channel transistor  7  and the P-channel transistors  15  and  5 , a gate of an N-channel transistor  17  is connected to the drain of the N-channel transistor  4  and the drain of the P-channel transistor  9 . Further, a drain of the N-channel transistor  17  is connected to a source and a gate of a P-channel transistor  18  and the gate of the P-channel transistor  6 , and a source of the N-channel transistor  17  is connected to a gate of the N-channel transistor  4  and the one end of the resistor  13 . 
         [0053]    Further, the source of the P-channel transistor  18  is connected to the power supply terminal  10 . 
         [0054]    By connecting in this way, the N-channel transistor  17  can be controlled such that the source and the gate thereof have the same electric potential. 
         [0055]    Further, the P-channel transistor  18  and the P-channel transistor  6  construct a current mirror circuit, and therefore the current I 1  flowing through the resistor  13  and the current I 2  flowing through the adjustable resistor  113  can be made to have a proportional relationship. 
         [0056]    Therefore, operation equivalent to that of the overheat detection circuit  200   a  of  FIG. 2  can also be achieved with the configuration described above. 
         [0057]      FIG. 4  is a circuit diagram for illustrating an overheat detection circuit  200   c  as a third example of the overheat detection circuit according to the second embodiment. 
         [0058]    Compared to the overheat detection circuit  200   b  illustrated in  FIG. 3  which employs the P-channel transistor  9 , the gate and the source of the N-channel depletion transistor  1  are connected to the drain of the N-channel transistor  4  and the gate of the N-channel transistor  17 . 
         [0059]    By connecting in this way, the bias current can be supplied to the N-channel transistor  4  without the intermediation of the current mirror circuit. Therefore, operation equivalent to that of the overheat detection circuit  200   b  of  FIG. 3  can be achieved. 
         [0060]      FIG. 5  is a circuit diagram for illustrating an overheat detection circuit  200   d  as a fourth example of the overheat detection circuit according to the second embodiment. 
         [0061]    Compared to the overheat detection circuit  200   c  illustrated in  FIG. 4  which employs the P-channel transistor  16 , a gate and a source of an N-channel depletion transistor  2  are connected to the drain of the N-channel transistor  3  and the output terminal  12 . 
         [0062]    Through combination of the characteristics of the N-channel depletion transistor  2  and the characteristics of the N-channel depletion transistor  1 , a current proportional to the bias current flowing through the N-channel transistor  4  can be supplied to the N-channel transistor  3  without intermediation of the current mirror circuit, and thus operation equivalent to that of the overheat detection circuit  200   a  of  FIG. 2  can be achieved. 
         [0063]    As described above, according to the present invention, it is possible to easily adjust the detection temperature in an overheat detection circuit. 
         [0064]    The present invention can also provide an overheat protection circuit, which includes the overheat detection circuit described above in the respective embodiments and which is configured to protect a device from overheating through use of a detection signal detected by the overheat detection circuit, and a semiconductor device including the overheat protection circuit.