Abstract:
Provided is an overheat detection circuit that is capable of quickly outputting an overheated state detection signal in an overheated state without outputting an unintended erroneous output caused by disturbance noise, such as momentary voltage fluctuations in the power supply. The overheat detection circuit includes: a temperature sensor; a comparison section; and a disturbance noise removal section configured to output an overheated state detection signal to an output section after a predetermined delay time has elapsed. The delay time is reduced in proportion to temperature.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2014-260737 filed on Dec. 24, 2014, 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 configured to detect an abnormal temperature of a semiconductor device. 
         [0004]    2. Description of the Related Art 
         [0005]    A related-art overheat detection circuit is illustrated in  FIG. 2 . The related-art overheat detection circuit includes a reference voltage section  210 , a temperature sensor  211 , and a comparison section  212 . The temperature sensor  211  includes a current source  202  and a PN junction element  203  for sensing temperature. The comparison section  212  includes a comparator  204 . An output of the comparator  204  is connected to an output terminal Vout of the overheat detection circuit. 
         [0006]    In the related-art overheat detection circuit, the comparator  204  compares and determines a voltage generated at the PN junction element  203 , and a reference voltage Vref output from a reference voltage circuit  210 , to thereby output an overheated state detection signal. 
         [0007]    In general, the voltage generated at the PN junction element  203  exhibits negative temperature characteristics, and hence when an ambient temperature increases and the voltage generated at the PN junction element  203  falls below the reference voltage Vref, the comparator  204  outputs the overheated state detection signal to the output terminal Vout of the overheat detection circuit. 
         [0008]    However, the above-mentioned overheat detection circuit suffers from a problem in that when disturbance noise, such as momentary fluctuations in the power supply, occurs, the comparator  204  may erroneously output the overheated state detection signal. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention has been made in order to solve the above-mentioned problem, and provides an overheat detection circuit configured to avoid an erroneous output caused by disturbance noise. 
         [0010]    In order to solve the related-art problem, an overheat detection circuit according to one embodiment of the present invention is configured as follows. 
         [0011]    The overheat detection circuit includes: a temperature sensor; a comparison section; and a disturbance noise removal section configured to output an overheated state detection signal to an output section after a predetermined delay time has elapsed, the delay time being reduced in proportion to temperature. 
         [0012]    According to the overheat detection circuit of the one embodiment of the present invention, the overheat detection circuit may be provided that is capable of quickly outputting the overheated state detection signal in an overheated state without outputting an unintended erroneous output caused by disturbance noise, such as momentary fluctuations in the power supply. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a circuit diagram of an overheat detection circuit according to one embodiment of the present invention. 
           [0014]      FIG. 2  is a circuit diagram relating to a related-art overheat detection circuit. 
           [0015]      FIG. 3A  and  FIG. 3B  are circuit diagrams for illustrating a current source of the overheat detection circuit according to the embodiment the present invention. 
           [0016]      FIG. 4  is a circuit diagram for illustrating another example of a current source of the overheat detection circuit according to the embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]      FIG. 1  is a circuit diagram of an overheat detection circuit according to one embodiment of the present invention. 
         [0018]    The overheat detection circuit of this embodiment includes a reference voltage section  210 , a temperature sensor  211 , a comparison section  212 , and a disturbance noise removal section  110 . The temperature sensor  211  includes a current source  202  and a PN junction element  203  for sensing temperature. The comparison section  212  includes a comparator  204 . The disturbance noise removal section  110  includes an N-channel transistor  101 , a current source  102 , a capacitor  103 , and an inverter  104 . 
         [0019]    The current source  202  and the PN junction element  203  are connected in series between a power supply terminal and a ground terminal. The comparator  204  has an inverting input terminal connected to an output terminal of the reference voltage section  210 , and a non-inverting input terminal connected to a node between the current source  202  and the PN junction element  203 . The N-channel transistor  101  has a control terminal connected to an output terminal of the comparator  204 , and a source connected to the ground terminal. The current source  102  and the capacitor  103  are connected between the power supply terminal and the ground terminal. The inverter  104  has an input terminal connected to a node between the current source  102  and the capacitor  103 , and an output terminal connected to an output terminal Vout of the overheat detection circuit. 
         [0020]    Next, the operation of the overheat detection circuit of this embodiment is described. 
         [0021]    In general, the voltage generated at the PN junction element  203  exhibits negative temperature characteristics. When the ambient temperature increases and the voltage generated at the PN junction element  203  falls below a reference voltage Vref, the comparator  204  outputs an overheated state detection signal (Low level) to the control terminal of the N-channel transistor  101 . The N-channel transistor  101  is off, and hence the capacitor  103  is charged by current from the current source  102 . When the voltage generated at the capacitor  103  increases and reaches a threshold voltage of the inverter  104 , the overheated state detection signal (Low level) is output to the output terminal Vout of the overheat detection circuit. 
         [0022]    In this case, for example, due to disturbance noise, such as momentary voltage fluctuations in the power supply, there are cases in which the comparator  204  erroneously outputs the overheated state detection signal. In such a case, the capacitor  103  is charged as a result of the overheated state detection signal. However, unless the overheated state detection signal continues for a period of time longer than that required in order for the voltage of the capacitor  103  to reach the threshold voltage of the inverter  104 , the overheated state detection signal is not output to the output terminal Vout of the overheat detection circuit. In other words, an overheat detection circuit that avoids an erroneous output caused by disturbance noise can be provided by giving the period of time required in order to charge the capacitor  103  as a period of time within which the effects of disturbance noise can be removed. 
         [0023]    On the other hand, in an overheated state, the signal indicating the overheated state needs to be output quickly. Consequently, the overheat detection circuit according to this embodiment is configured to shorten, in proportion to temperature, the time within which the effects of disturbance noise can be removed. 
         [0024]    More specifically, the overheat detection circuit is configured to increase the current of the current source  102  in proportion to temperature. As a result, the capacitor  103  is charged more quickly as the temperature increases, which allows the overheated state detection signal to be output quickly in an overheated state. 
         [0025]      FIG. 3A  and  FIG. 3B  are circuit diagrams of the current source  102 , in which output current increases in proportion to temperature based on resistance, the resistance value decreasing in proportion to temperature. 
         [0026]    In  FIG. 3A , a P-channel transistor  303  and a resistor  301  are connected in series. A difference between a reference voltage Vref 2  and the voltage of a node between the P-channel transistor  303  and the resistor  301  is amplified by an amplifier  302 , and the amplified output is input to a gate of the P-channel transistor  303 . A P-channel transistor  304  is connected by a current mirror to the P-channel transistor  303 . The current flowing through the P-channel transistor  304  is the output current of the current source  102 . 
         [0027]    In this case, a current inversely proportional to the resistance value of the resistor  301  flows through the P-channel transistor  303 . As a result, the current flowing through the P-channel transistor  304 , which is in a current mirror relationship with the P-channel transistor  303 , namely, the current of the current source  102 , increases in proportion to temperature. 
         [0028]    In contrast to  FIG. 3A , an N-channel transistor  305  is connected between the P-channel transistor  303  and the resistor  301  in  FIG. 3B . A difference between the reference voltage Vref 2  and the voltage of a node between the N-channel transistor  305  and the resistor  301  is amplified by the amplifier  302 , and the amplified output is input to a gate of the N-channel transistor  305 . A drain and the gate of the P-channel transistor  303  are connected to each other. 
         [0029]    In this case, a current inversely proportional to the resistance value of the resistor  301  flows through the P-channel transistor  303 . As a result, the current flowing through the P-channel transistor  304 , which is in a current mirror relationship with the P-channel transistor  303 , namely, the current of the current source  102 , increases in proportion to temperature. 
         [0030]    Note that,  FIG. 3A  and  FIG. 3B  are illustrations of examples in which the current of the current source  102  increases in proportion to temperature based on resistance, the resistance value decreasing in proportion to temperature. However, the present invention is not necessarily limited to this mode. 
         [0031]      FIG. 4  is a circuit diagram of the current source  102 , in which output current increases in proportion to temperature based on thermal voltage. 
         [0032]    A P-channel transistor  402  and a PN junction element  407  are connected in series between a power supply terminal and a ground terminal. A P-channel transistor  403 , a resistor  405 , and a PN junction element  408  are connected in series between a power supply terminal and a ground terminal. An amplifier  401  is configured to amplify a difference between a voltage VA of a node between the P-channel transistor  402  and the PN junction element  407  and a voltage VB of a node between the P-channel transistor  403  and the resistor  405 , and input the amplified output voltage to the gates of the P-channel transistors  402 ,  403 , and  404 . 
         [0033]    Through the P-channel transistor  402  and the P-channel transistor  403 , a current proportional to the thermal voltage flows. The thermal voltage is proportional to temperature, and hence the current of the P-channel transistor  402  and the P-channel transistor  403  exhibits positive temperature characteristics. The current of the current source  102  can be made to exhibit positive temperature characteristics by using the P-channel transistor  404 , which is in a current mirror relationship with the P-channel transistor  402  and the P-channel transistor  403 , as the current source  102 . 
         [0034]    Note that,  FIG. 4  is an illustration of an example in which the current of the current source  102  can be made to exhibit positive temperature characteristics based on thermal voltage. However, the present invention is not necessarily limited to this mode. 
         [0035]    As described above, according to the overheat detection circuit of this embodiment, an overheat detection circuit can be provided that is capable of avoiding an erroneous output caused by disturbance noise, without problems occurring in a function for quickly outputting an overheated state detection signal. 
         [0036]    In the overheat detection circuit according to this embodiment as described above, the current of the current source  102  increases in proportion to temperature. However, the overheat detection circuit may also be configured such that the threshold voltage of the inverter  104  decreases in proportion to temperature. For example, the threshold voltage of the inverter  104  may be determined to be about the threshold voltage of the N-channel transistor by increasing the aspect ratio of the N-channel transistor forming the inverter  104  to increase a drive power. In other words, the threshold voltage of the N-channel transistor usually decreases in proportion to temperature, and hence the overheat detection circuit can be configured such that the threshold voltage of the inverter  104  decreases in proportion to temperature. The circuit described above is an example, and the present invention is not necessarily limited to this mode.