Abstract:
A semiconductor integrated circuit apparatus includes an overheat protection circuit including a voltage generating circuit, a voltage comparing circuit, and a voltage outputting circuit. The voltage generating circuit generates two reference voltages having substantially equivalent responsiveness to an input voltage and different variation gradients with respect to a temperature change such that the different variation gradients intersect with each other at a predetermined temperature. The voltage comparing circuit compares the two reference voltages generated by the voltage generating circuit. The voltage outputting circuit outputs an output voltage when the different variation gradients do not intersect and changes the output voltage to an inverse output voltage upon intersection of the different variation gradients to stop an operation of circuits included in the semiconductor integrated circuit apparatus. An overheat protection method is also described.

Description:
BACKGROUND  
       [0001]     1. Field  
         [0002]     This patent specification relates to a semiconductor integrated circuit apparatus having an overheat protection circuit and an overheat protection method. More particularly, this patent specification relates to a semiconductor integrated circuit apparatus having an overheat protection circuit and an overheat protection method capable of desirably setting a detection temperature, with two input terminals of a comparator being connected to elements of similar characteristics to keep a constant relationship between two voltages input in the two input terminals despite a change in an input voltage input in the semiconductor integrated circuit.  
         [0003]     2. Discussion of the Background  
         [0004]      FIG. 1  illustrates an example of a background semiconductor circuit. A voltage regulator circuit  1  shown in  FIG. 1  includes a background overheat protection circuit  2 . The voltage regulator circuit  1  includes a reference voltage circuit (RV)  31 , a differential amplifier circuit (DA)  41 , an output driver M 20 , resistors Ra and Rb, and an output terminal OUT. The overheat protection circuit  2  includes a temperature monitoring circuit  30  and a cut-off circuit  20 . The temperature monitoring circuit  30  includes a reference voltage circuit (RV)  11 , a comparator circuit (CMP)  21 , a constant current circuit  51 , and a diode D 1 , while the cut-off circuit  20  includes a p-channel transistor M 10 . The overheat protection circuit  2  and the voltage regulator circuit  1  receive an input voltage Vin input in an input terminal IN, and the voltage regulator circuit  1  outputs an output voltage Vout from the output terminal OUT.  
         [0005]     In the voltage regulator circuit  1  of  FIG. 1 , an output terminal of the overheat protection circuit  2  is connected to a gate of the output driver M 20  which is a p-channel transistor. An output voltage output from the reference voltage circuit  31  is input in an inverting input terminal of the differential amplifier circuit  41 , and an output voltage output from the differential amplifier circuit  41  is input in the gate of the output driver M 20 . A drain of the output driver M 20  is connected to the output terminal OUT of the voltage regulator circuit  1 . The output terminal OUT of the voltage regulator circuit  1  is also connected to the resistors Ra and Rb. The resistors Ra and Rb form a voltage divider circuit which divides the output voltage Vout to generate and input a feedback voltage in a non-inverting input terminal of the differential amplifier circuit  41 .  
         [0006]     In the overheat protection circuit  2 , a reference voltage Vref 1  output from the reference voltage circuit  11  is input in an non-inverting input terminal of the comparator circuit  21 . Meanwhile, an inverting input terminal of the comparator circuit  21  is connected to a connection point A 1  between the constant current circuit  51  and the diode D 1  which are connected in series. An output terminal of the comparator circuit  21  is connected to a gate of the p-channel transistor M 10 , and a drain of the p-channel transistor M 10  is connected to the gate of the output driver M 20  in the voltage regulator circuit  1 . Since a relatively low power consuming CMOS (Complementary Metal Oxide Semiconductor) circuit is included in the reference voltage circuit  11  and the constant current circuit  51 , the reference voltage circuit  11  and the constant current circuit  51  may have a problem which does not occur in a relatively high power consuming bipolar transistor circuit.  
         [0007]     In the overheat protection circuit  2 , the reference voltage Vref 1  is kept at a constant value irrespective of a temperature of a semiconductor integrated circuit including the overheat protection circuit  2 . Since a constant current flows through the diode D 1 , a voltage at the connection point A 1  increases at a rate of two millivolts per degree Celsius as the temperature of the semiconductor integrated circuit increases.  
         [0008]      FIG. 2  illustrates temperature characteristics of the reference voltage Vref 1  and the voltage of the connection point A 1  in the overheat protection circuit  2  shown in  FIG. 1 . As observed in  FIG. 2 , when the temperature of the semiconductor integrated circuit increases and the voltage of the connection point A 1  exceeds the reference voltage Vref 1 , the output voltage output from the comparator circuit  21  shifts from a high level (HIGH) to a low level (LOW). As a result, the p-channel transistor M 10  is turned on, and the output driver M 20  of the voltage regulator circuit  1  is turned off. Accordingly, the output driver M 20  stops outputting the output voltage Vout.  
         [0009]     In this manner, the overheat protection circuit  2  detects an increase in the temperature of the semiconductor integrated circuit and turns off the output driver M 20 , so that overheat of the semiconductor integrated circuit can be prevented. A detection temperature detected by the overheat protection circuit  2  may be set based on a difference between the reference voltage Vref 1  and the voltage of the connection point A 1  which are measured at room temperature (e.g., 25 degrees Celsius).  
         [0010]     The reference voltage Vref 1  and the voltage of the connection point A 1 , however, are generated by substantially different circuits. Accordingly, the reference voltage Vref 1  and the voltage of the connection point A 1  respond to an instantaneous change of the input voltage Vin at different response speeds. Therefore, if the input voltage Vin instantaneously changes in a state in which the temperature of the semiconductor integrated circuit is below the detection temperature of the overheat protection circuit  2  (i.e., when the reference voltage Vref 1  is higher than the voltage of the connection point A 1 ), there is a moment when the reference voltage Vref 1  falls below the voltage of the connection point A 1 . In this event, the output voltage output from the comparator circuit  21  shifts in the level, and the cut-off circuit  20  is turned on and the output driver M 20  is turned off. This type of operational error frequently occurs in the CMOS circuit. Despite this disadvantage, the CMOS circuit is used as the reference voltage circuit  11  for its relatively low power consuming characteristics.  
         [0011]     Turning on of the cut-off circuit  20  at a temperature below the detection temperature is an operational error. In light of this, there has been a demand for an overheat protection circuit unaffected by the instantaneous change of the input voltage.  
         [0012]     Operational errors caused by noise are discussed in the Japanese Laid-Open Patent Publication No. 2000-311985, for example. A semiconductor device described in the patent publication includes a first protection circuit for detecting a first temperature T 1  and a second protection circuit for detecting a second temperature T 2  which is higher than the first temperature T 1 . In this semiconductor device, the first protection circuit forces the semiconductor device to be turned off when a temperature of the semiconductor device continues to exceed the first temperature T 1  for a predetermined time period. The second protection circuit forces the semiconductor device to be turned off immediately after detecting that the temperature of the semiconductor device exceeds the second temperature T 2 . Accordingly, erroneous turn-off of the semiconductor device can be prevented in a normal operation state in which the temperature of the semiconductor device is lower than the first temperature T 1 , even when a noise occurs in the first protection circuit.  
         [0013]     The semiconductor device, however, includes two protection circuits and an additional circuit which sets the predetermined time period. This increases a circuit size. Further, in this semiconductor device, the predetermined time period is set to be longer than a pulse width of an expected noise. In a general-purpose semiconductor integrated circuit used for a variety of purposes, however, it is difficult to determine the pulse width of the expected noise.  
       SUMMARY  
       [0014]     This patent specification describes a novel semiconductor integrated circuit apparatus. In one example, a novel semiconductor integrated circuit apparatus includes an overheat protection circuit which includes a voltage generating circuit, a voltage comparing circuit, and a voltage outputting circuit. The voltage generating circuit is configured to generate two reference voltages having substantially equivalent responsiveness to an input voltage and different variation gradients with respect to a temperature change such that the different variation gradients intersect with each other at a predetermined temperature. The voltage comparing circuit is configured to compare the two reference voltages generated by the voltage generating circuit. The voltage outputting circuit is configured to output an output voltage when the different variation gradients do not intersect and changes the output voltage to an inverse output voltage upon intersection of the different variation gradients to stop an operation of circuits included in the semiconductor integrated circuit apparatus.  
         [0015]     This patent specification further describes another novel semiconductor integrated circuit apparatus. In one example, this novel semiconductor integrated circuit apparatus includes an input terminal and an overheat protection circuit. The overheat protection circuit includes a temperature monitoring circuit and a cut-off circuit. The temperature monitoring circuit is configured to monitor a temperature of the semiconductor integrated circuit apparatus. The cut-off circuit is configured to stop an operation of circuits included in the semiconductor integrated circuit apparatus according to an output signal output from the temperature monitoring circuit. The temperature monitoring circuit includes a first series circuit, a second series circuit, and a differential amplifier circuit. The first series circuit is configured to connect a first resistor to a first diode group including plurality of series-connected diodes and to connect the first diode group to a first constant current circuit, and the first resistor is connected to the input terminal. The second series circuit is configured to connect a second resistor to a second diode group including another plurality of series-connected diodes and to connect the second diode group to a second constant current circuit, and the second resistor is connected to the input terminal. The differential amplifier circuit includes a first input terminal to receive a first forward output voltage of the first diode group and a second input terminal to receive a second forward output voltage of the second diode group.  
         [0016]     In the semiconductor integrated circuit apparatus, the temperature monitoring circuit may have a thermal hysteresis.  
         [0017]     In the semiconductor integrated circuit apparatus, laser trimming may be performed in a post-process to adjust a resistance value of one of the first and second resistors or a constant current value of one of the first and second constant current circuits.  
         [0018]     In the semiconductor integrated circuit apparatus, the first and second resistors may be replaced by a constant voltage circuit which outputs two different voltages.  
         [0019]     In the semiconductor integrated circuit apparatus, the first input terminal of the differential amplifier circuit may be connected to a connection point between two diodes included in the first diode group, and the second input terminal of the differential amplifier circuit may be connected to a connection point between two diodes included in the second diode group.  
         [0020]     In the semiconductor integrated circuit apparatus, the temperature monitoring circuit may include a complementary metal oxide semiconductor circuit.  
         [0021]     This patent specification further describes another novel semiconductor integrated circuit apparatus. In one example, this novel semiconductor integrated circuit apparatus includes an input terminal and an overheat protection circuit. The overheat protection circuit includes a temperature monitoring circuit and a cut-off circuit. The temperature monitoring circuit is configured to monitor a temperature of the semiconductor integrated circuit apparatus. The cut-off circuit is configured to stop an operation of circuits included in the semiconductor integrated circuit apparatus according to an output signal output from the temperature monitoring circuit. The temperature monitoring circuit includes a first series circuit, a second series circuit, and a differential amplifier circuit. The first series circuit is configured to connect a first constant current circuit to a first diode group including a plurality of series-connected diodes and to connect the first diode group to a first resistor, and the first constant current circuit is connected to the input terminal. The second series circuit is configured to connect a second constant current circuit to a second diode group including another plurality of series-connected diodes and to connect the second diode group to a second resistor, and the second constant current circuit is connected to the input terminal. The differential amplifier circuit includes a first input terminal to receive a first forward output voltage of the first diode group and a second input terminal to receive a second forward output voltage of the second diode group.  
         [0022]     In the semiconductor integrated circuit apparatus, the temperature monitoring circuit may have a thermal hysteresis.  
         [0023]     In the semiconductor integrated circuit apparatus, the first and second resistors may be replaced by a constant voltage circuit which outputs two different voltages.  
         [0024]     In the semiconductor integrated circuit apparatus, the first input terminal of the differential amplifier circuit may be connected to a connection point between two diodes included in the first diode group, and the second input terminal of the differential amplifier circuit may be connected to a connection point between two diodes included in the second diode group.  
         [0025]     In the semiconductor integrated circuit apparatus, the temperature monitoring circuit may include a complementary metal oxide semiconductor circuit.  
         [0026]     This patent specification further describes a novel overheat protection method for protecting a semiconductor integrated circuit apparatus from overheat. In one example, a novel overheat protection method for protecting a semiconductor integrated circuit apparatus from overheat includes: generating two reference voltages having substantially equivalent responsiveness to an input voltage and different variation gradients with respect to a temperature change such that the different variation gradients intersect with each other at a predetermined temperature; comparing the two reference voltages; outputting an output voltage when the different variation gradients do not intersect; and changing the output voltage to an inverse output voltage upon intersection of the different variation gradients to stop an operation of circuits included in the semiconductor integrated circuit apparatus.  
         [0027]     This patent specification further describes another novel overheat protection method for protecting a semiconductor integrated circuit apparatus from overheat. In one example, this novel overheat protection method for protecting a semiconductor integrated circuit apparatus from overheat includes: providing an input terminal and an overheat protection circuit configured to include a temperature monitoring circuit and a cut-off circuit; providing the temperature monitoring circuit with a first series circuit, a second series circuit, and a differential amplifier circuit configured to have first and second input terminals; forming the first series circuit by connecting a first resistor to a first diode group including a plurality of series-connected diodes and connecting the first diode group to a first constant current circuit; connecting the first resistor to the input terminal; forming the second series circuit by connecting a second resistor to a second diode group including another plurality of series-connected diodes and connecting the second diode group to a second constant current circuit; connecting the second resistor to the input terminal; inputting a first forward output voltage of the first diode group into the first input terminal of the differential amplifier circuit; inputting a second forward output voltage of the second diode group into the second input terminal of the differential amplifier circuit; causing the temperature monitoring circuit to monitor a temperature of the semiconductor integrated circuit apparatus; and causing the cut-off circuit to stop an operation of circuits included in the semiconductor integrated circuit apparatus according to an output signal output from the temperature monitoring circuit.  
         [0028]     The overheat protection method may further include providing the temperature monitoring circuit with a thermal hysteresis.  
         [0029]     The overheat protection method may further include performing laser trimming in a post-process to adjust a resistance value of one of the first and second resistors or a constant current value of one of the first and second constant current circuits.  
         [0030]     In the overheat protection method, the first and second resistors may be replaced by a constant voltage circuit which outputs two different voltages.  
         [0031]     The overheat protection method may further include: connecting the first input terminal of the differential amplifier circuit to a connection point between two diodes included in the first diode group; and connecting the second input terminal of the differential amplifier circuit to a connection point between two diodes included in the second diode group.  
         [0032]     The overheat protection method may further include including a complementary metal oxide semiconductor circuit in the temperature monitoring circuit.  
         [0033]     This patent specification further describes another novel overheat protection method for protecting a semiconductor integrated circuit apparatus from overheat. In one example, this novel overheat protection method for protecting a semiconductor integrated circuit apparatus from overheat includes: providing an input terminal and an overheat protection circuit configured to include a temperature monitoring circuit and a cut-off circuit; providing the temperature monitoring circuit with a first series circuit, a second series circuit, and a differential amplifier circuit configured to have first and second input terminals; forming the first series circuit by connecting a first constant current circuit to a first diode group including a plurality of series-connected diodes and connecting the first diode group to a first resistor; connecting the first constant current circuit to the input terminal; forming the second series circuit by connecting a second constant current circuit to a second diode group including another plurality of series-connected diodes and connecting the second diode group to a second resistor; connecting the second constant current circuit to the input terminal; inputting a first forward output voltage of the first diode group into the first input terminal of the differential amplifier circuit; inputting a second forward output voltage of the second diode group into the second input terminal of the differential amplifier circuit; causing the temperature monitoring circuit to monitor a temperature of the semiconductor integrated circuit apparatus; and causing the cut-off circuit to stop an operation of circuits included in the semiconductor integrated circuit apparatus according to an output signal output from the temperature monitoring circuit.  
         [0034]     The overheat protection method may further include providing the temperature monitoring circuit with a thermal hysteresis.  
         [0035]     In the overheat protection method, the first and second resistors may be replaced by a constant voltage circuit which outputs two different voltages.  
         [0036]     The overheat protection method may further include: connecting the first input terminal of the differential amplifier circuit to a connection point between two diodes included in the first diode group; and connecting the second input terminal of the differential amplifier circuit to a connection point between two diodes included in the second diode group.  
         [0037]     The overheat protection method may further include including a complementary metal oxide semiconductor circuit in the temperature monitoring circuit. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0038]     A more complete appreciation of the disclosure and many of the advantages thereof are readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:  
         [0039]      FIG. 1  is a circuit diagram illustrating a configuration of a voltage regulator circuit including a background overheat protection circuit;  
         [0040]      FIG. 2  is a graph illustrating temperature characteristics of the background overheat protection circuit shown in  FIG. 1 ;  
         [0041]      FIG. 3  is a circuit diagram illustrating a schematic view of an overheat protection circuit according to an embodiment.  
         [0042]      FIG. 4  is a circuit diagram illustrating a configuration of an overheat protection circuit according to an embodiment;  
         [0043]      FIG. 5  is a graph illustrating temperature characteristics of the overheat protection circuit shown in  FIG. 4  and an overheat protection circuit shown in  FIG. 6 ;  
         [0044]      FIG. 6  is a circuit diagram illustrating a configuration of the overheat protection circuit according to another embodiment;  
         [0045]      FIG. 7  is a circuit diagram illustrating a configuration of an overheat protection circuit according to still another embodiment;  
         [0046]      FIG. 8  is a graph illustrating temperature characteristics of the overheat protection circuit shown in  FIG. 7  and an overheat protection circuit shown in  FIG. 9 ; and  
         [0047]      FIG. 9  is a circuit diagram illustrating a configuration of the overheat protection circuit according to still yet another embodiment. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0048]     In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the purpose of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so used and it is to be understood that substitutions for each specific element can include any technical equivalents that operate in a similar manner.  
         [0049]     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, more particularly to  FIG. 3 , a circuit diagram illustrating a schematic view of an overheat protection circuit  100  according to an embodiment is described.  
         [0050]     The overheat protection circuit  100  includes a temperature monitoring circuit  102  and a cut-off circuit  101 . The temperature monitoring circuit  102  is connected to the cut-off circuit  101 , and an output voltage Vout is output from an output terminal OUT. In the temperature monitoring circuit  102 , two circuits which generate two voltages input in a comparator circuit (shown in  FIG. 4 ) are formed by elements of approximately similar characteristics. Accordingly, a relationship between the two voltages input in the comparator circuit is kept constant even when the input voltage Vin changes, so that the operational error does not occur. Further, a resistance value of the temperature monitoring circuit  102  is changed so that the detection temperature can be set at a desired value. Upon a change in the relationship between the two voltages input in the comparator circuit, the level of the output voltage output from the comparator circuit shifts, and the cut-off circuit  101  is turned on.  
         [0051]      FIG. 4  illustrates a configuration of an overheat protection circuit  200  according to an embodiment. The overheat protection circuit  200  includes a cut-off circuit  201  and a temperature monitoring circuit  202 . The temperature monitoring circuit  202  receives an input voltage Vin input in an input terminal IN and outputs the output voltage Vout from the output terminal OUT.  
         [0052]     The cut-off circuit  201  includes the p-channel transistor M 12 . The temperature monitoring circuit  202  includes a comparator circuit (CMP)  22 , constant current circuits  11  and  12 , resistors R 1  and R 2 , and a diode group Ds including “s” number of diodes (s is an integer number larger than 1) and a diode group Dt including “t” number of diodes (t is an integer number larger than 1 and other than s). The resistor R 1 , the diode group Ds, and the constant current circuit I 1  form a series circuit connected to a non-inverting input terminal of the comparator circuit  22 . Meanwhile, the resistor R 2 , the diode group Dt, and the constant current circuit I 2  form another series circuit connected to an inverting input terminal of the comparator circuit  22 . A connection point B between the diode group Ds and the constant current circuit I 1  is connected to the non-inverting input terminal of the comparator circuit  22 , while a connection point C between the diode group Dt and the constant current circuit I 2  is connected to the inverting input terminal of the comparator circuit  22 . Accordingly, a voltage Vs at the connection point B is input in the non-inverting input terminal of the comparator circuit  22 , and a voltage Vt at the connection point C is input in the inverting input terminal of the comparator circuit  22 .  
         [0053]     In the temperature monitoring circuit  202 , Rs indicates a resistance value of the resistor R 1 , and Rt indicates a resistance value of the resistor R 2 . Further, Is indicates a value of current flowing through the series circuit including the resistor R 1 , the diode group Ds, and the constant current circuit I 1 , while It indicates a value of current flowing through the series circuit including the resistor R 2 , the diode group Dt, and the constant current circuit I 2 .  
         [0054]      FIG. 5  is a graph illustrating voltage and temperature characteristics of the temperature monitoring circuit  202 . An operation of the temperature monitoring circuit  202  shown in  FIG. 4  is described with reference to the graph of  FIG. 5 . In  FIG. 5 , the horizontal axis represents temperature (degrees Celsius) of a surface of the semiconductor integrated circuit apparatus including the overheat protection circuit  200 , and the vertical axis represents voltage (volts). A line Vs indicates a relationship between the voltage Vs and a temperature of a semiconductor integrated circuit apparatus including the overheat protection circuit  200 , and a line Vt indicates a relationship between the voltage Vt and the temperature. Gradients of the lines Vs and Vt are determined by the number of diodes provided. The line Vt is steeper than the line Vs, since t is larger than s in the present embodiment.  
         [0055]     When the temperature is T 1 , the voltage Vs input in the non-inverting input terminal of the comparator circuit  22  and the voltage Vt input in the inverting input terminal of the comparator circuit  22  are expressed as Vs=Vin−(Is*Rs+Vs 1 ) and Vt=Vin−(It*Rt+Vt 1 ), respectively, wherein Vs 1  is a forward output voltage of the diode group Ds as measured when the constant current Is is flowed through the diode group Ds at the temperature T 1 , and Vt 1  is a forward output voltage of the diode group Dt as measured when the constant current It is flowed through the diode group Dt at the temperature T 1 . In this state, an output voltage output from the comparator circuit  22  is at the HIGH level, and the p-channel transistor M 12  is turned off.  
         [0056]     Meanwhile, when the temperature is T 2 , the voltages Vs and Vt are expressed as Vs=Vin−(Is*Rs+Vs 1 −2*s*(T 2 −T 1 )) and Vt=Vin−(It*Rt+Vt 1 −2*t*(T 2 −T 1 )), respectively. That is, the Vs value measured at the temperature T 2  is equal to the Vs value measured at the temperature T 1  added with a change in the forward output voltage of the diode group Ds, and the Vt value measured at the temperature T 2  is equal to the Vt value measured at the temperature T 1  added with a change in the forward output voltage of the diode group Dt. Since a forward voltage of each diode decreases at a rate of two millivolts per degree Celsius, the Vs value measured at the temperature T 2  is obtained by subtracting, from the Vs value measured at the temperature T 1 , a voltage value obtained by multiplying a difference between the temperatures T 1  and T 2  by the number of the diodes provided (i.e., s). Similarly, the Vt value measured at the temperature T 2  is obtained by subtracting, from the Vt value measured at the temperature T 1 , a voltage value obtained by multiplying the difference between the temperatures T 1  and T 2  by the number of the diodes provided (i.e., t). Depending on values of s, t, Rs, and Rt, relationship between the voltages Vs and Vt measured at the temperature T 2  can be expressed as one of Vs&gt;Vt, Vs=Vt, and Vs&lt;Vt.  
         [0057]     If Vs is smaller than Vt (i.e., Vs&lt;Vt) at the temperature T 2 , the output voltage output from the comparator circuit  22  (i.e., an output voltage output from the temperature monitoring circuit  202 ) changes from the HIGH level to the LOW level. Therefore, the p-channel transistor M 12  of the cut-off circuit  201  is turned on, and an output driver of a circuit such as a regulator connected to the p-channel transistor M 12  is turned off. Accordingly, the semiconductor integrated circuit apparatus including the overheat protection circuit  200  can be protected from overheat. The detection temperature of the overheat protection circuit  200  is a temperature at which the voltages Vs and Vt become equal.  
         [0058]     The overheat protection circuit  200  according to the present embodiment has a relatively simply configuration, in which the circuits generating the two voltages Vs and Vt input in the comparator circuit  22  are formed by resistors and diodes. Therefore, the voltages Vs and Vt similarly change in response to a change in the input voltage Vin. Accordingly, the relationship between the voltages Vs and Vt is kept constant while the input voltage Vin changes.  
         [0059]     In a region of the graph in  FIG. 5  in which the lines of Vs and Vt cross and the voltage Vt exceeds the voltage Vs, the output voltage output from the comparator  22  is in an unstable state to cause heat oscillation. Therefore, it is preferable to provide a thermal hysteresis circuit in the temperature monitoring circuit  202  to prevent oscillation of the output voltage. The thermal hysteresis circuit can prevent the heat oscillation by increasing the voltage Vt to a higher voltage Vt′ at a moment when the voltage Vt reaches the voltage Vs (i.e., at a point where the Vt line crosses the Vs line). Instead of increasing the voltage Vt, the voltage Vs may be decreased. A circuit in which the voltage input in the non-inverting input terminal of the comparator is decreased is described later.  
         [0060]     It is also preferable to make the resistance values Rs and Rt of the resistors R 1  and R 2  changeable by performing laser trimming. Accordingly, the detection temperature detected by the overheat protection circuit  200  can be set at an arbitrary value.  
         [0061]     The resistors R 1  and R 2  may be replaced by a constant voltage circuit that receives the input voltage Vin and keeps output voltages constant. The voltage regulator circuit  1  shown in  FIG. 1 , for example, may be used as the constant voltage circuit.  
         [0062]      FIG. 6  illustrates an overheat protection circuit  300  according to another embodiment. Description is omitted for components shown in  FIG. 6  which are also components shown in  FIG. 4 , and differences between the circuit configuration of  FIG. 4  and the circuit configuration of  FIG. 6  are described. The overheat protection circuit  300  includes a temperature monitoring circuit  302  and the cut-off circuit  201 . The overheat protection circuit  300  is different from the overheat protection circuit  200  in that, in the temperature monitoring circuit  302 , the non-inverting input terminal of the comparator circuit  22  is connected to a connection point D, which is a node between two diodes included in the diode group Ds, and the inverting input terminal of the comparator circuit  22  is connected to a connection point E, which is a node between two diodes included in the diode group Dt.  
         [0063]     In the present embodiment, the resistor R 1 , the diode group Ds, and the constant current circuit I 1  are connected in series, and the connection point D between a (s−q)-th diode and a (q+1)-th diode is connected to the non-inverting input terminal of the comparator circuit  22  (q is a positive integer number smaller than s). Meanwhile, the resistor R 2 , the diode group Dt, and the constant current circuit I 2  are connected in series, and the connection point E between a (t−r)-th diode and a (r+1)-th diode is connected to the inverting input terminal of the comparator circuit  22  (r is a positive integer number smaller than t, and q and r may be or may not be the same number). In  FIG. 6 , at least one diode is placed between the connection point D and the constant current circuit I 1  and between the connection point E and the constant current circuit I 2 . Voltage and temperature characteristics of the temperature monitoring circuit  302  are illustrated in the graph of  FIG. 5 .  
         [0064]      FIG. 7  illustrates a configuration of an overheat protection circuit  400  according to still another embodiment. Description is omitted for components shown in  FIG. 7  which are also components shown in  FIG. 4 , and differences between the circuit configuration of  FIG. 4  and the circuit configuration of  FIG. 7  are described. The overheat protection circuit  400  includes the cut-off circuit  201  and a temperature monitoring circuit  402 . The constant current circuit I 1 , the diode group Ds, and the resistor R 1  form a series circuit connected to the non-inverting input terminal of the comparator circuit  22 . Meanwhile, the constant current circuit I 2 , the diode group Dt, and the resistor R 2  form another series circuit connected to the inverting input terminal of the comparator circuit  22 . A connection point F between the constant current circuit I 1  and the diode group Ds is connected to the non-inverting input terminal of the comparator circuit  22 , while a connection point G between the constant current circuit I 2  and the diode group Dt is connected to the inverting input terminal of the comparator circuit  22 . The voltage Vs is input from the connection point F to the non-inverting input terminal of the comparator circuit  22 , and the voltage Vt is input from the connection point G to the inverting input terminal of the comparator circuit  22 . Rs indicates a resistance value of the resistor R 1 , and Rt indicates a resistance value of the resistor R 2 . Is indicates a value of current flowing through the series circuit including the constant current circuit I 1 , the diode group Ds, and the resistor R 1 , while It indicates a value of current flowing through the series circuit including the constant current circuit I 2 , the diode group Dt, and the resistor R 2 .  
         [0065]      FIG. 8  is a graph illustrating voltage and temperature characteristics of the temperature monitoring circuit  400  shown in  FIG. 7 . An operation of the temperature monitoring circuit  402  is described with reference to the graph of  FIG. 8 . In  FIG. 8 , the horizontal axis represents temperature (degrees Celsius) of a surface of a semiconductor integrated circuit apparatus including the overheat protection circuit  400 , and the vertical axis represents voltage (volts). A line Vs indicates a relationship between the voltage Vs and the temperature of the surface of the semiconductor integrated circuit apparatus including the overheat protection circuit  400 , and a line Vt indicates a relationship between the voltage Vt and the temperature.  
         [0066]     The voltage Vs input in the non-inverting input terminal of the comparator circuit  22  and the voltage Vt input in the inverting input terminal of the comparator circuit  22  are expressed as Vs=Is*Rs+Vs 1  and Vt=It*Rt+Vt 1 , respectively, wherein Vs is larger than Vt (i.e., Vs&gt;Vt). Vs 1  is a forward output voltage of the diode group Ds as measured when the constant current Is is flowed through the diodes Ds at the temperature T 1 , and Vt 1  is a forward output voltage of the diode group Dt as measured when the constant current It is flowed through the diode group Dt at the temperature T 1 . In this state, the output voltage output from the comparator circuit  22  is at the HIGH level, and the p-channel transistor M 12  is turned off.  
         [0067]     Meanwhile, when the temperature is T 2 , the voltages Vs and Vt are expressed as Vs=Is*Rs+Vs 1 −2*s*(T 2 −T 1 ) and Vt=It*Rt+Vt 1 −2*t*(T 2 −T 1 ), respectively. That is, the Vs value measured at the temperature T 2  is equal to the Vs value measured at the temperature T 1  added with a change in the forward output voltage of the diode group Ds, and the Vt value measured at the temperature T 2  is equal to the Vt value measured at the temperature T 1  added with a change in the forward output voltage of the diode group Dt. Since the forward voltage of each diode decreases at the rate of two millivolts per degree Celsius, the Vs value measured at the temperature T 2  is obtained by subtracting, from the Vs value measured at the temperature T 1 , a voltage value obtained by multiplying a difference between the temperatures T 1  and T 2  by the number of the diodes provided (i.e., s). Similarly, the Vt value measured at the temperature T 2  is obtained by subtracting, from the Vt value measured at the temperature T 1 , a voltage value obtained by multiplying the difference between the temperatures T 1  and T 2  by the number of the diodes provided (i.e., t). Depending on values of s, t, Rs, and Rt, relationship between the voltages Vs and Vt measured at the temperature T 2  can be expressed as one of Vs&gt;Vt, Vs=Vt, and Vs&lt;Vt.  
         [0068]     If Vs is smaller than Vt (i.e., Vs&lt;Vt) at the temperature T 2 , the output voltage output from the comparator circuit  22  (i.e., an output voltage output from the temperature monitoring circuit  402 ) shifts from the HIGH level to the LOW level. Therefore, the p-channel transistor M 12  of the cut-off circuit  201  is turned on, and an output driver of a circuit such as a regulator connected to the p-channel transistor M 12  is turned off. Accordingly, the semiconductor integrated circuit apparatus including the overheat protection circuit  400  can be protected from overheat. The detection temperature of the overheat protection circuit  400  is a temperature at which the voltages Vs and Vt become equal. Vs&#39; is a value decreased from Vs due to a thermal hysteresis, and Vt′ is a value increased from Vt due to the thermal hysteresis. A hysteresis circuit is provided in the temperature monitoring circuit  402 , and when the output voltage output from the comparator circuit  22  is shifted in level, the voltage Vt is increased to the voltage Vt′ or the voltage Vs is decreased to the voltage Vs′. Accordingly, the unstable state of the output voltage output from the comparator circuit  22  due to the heat oscillation can be prevented.  
         [0069]      FIG. 9  illustrates a configuration of an overheat protection circuit  500  according to still yet another embodiment. Description is omitted for components shown in  FIG. 9  which are also components shown in  FIG. 7 , and differences between the circuit configuration of  FIG. 7  and the circuit configuration of  FIG. 9  are described. The overheat protection circuit  500  includes a temperature monitoring circuit  502  and the cut-off circuit  201 . The overheat protection circuit  500  is different from the overheat protection circuit  400  in that the overheat protection circuit  500  includes an n-channel transistor  24  to form a hysteresis circuit in the temperature monitoring circuit  502 .  
         [0070]     In the overheat protection circuit  500 , the hysteresis circuit is formed by connecting a drain of the n-channel transistor  24  to an arbitrary point in the resistor R 2 . Further, a source of the n-channel transistor  24  is connected to the ground (GND), and a gate of the n-channel transistor  24  is connected to a gate of the p-channel transistor M 12  of the cut-off circuit  201 .  
         [0071]     When it is assumed that R 1  is a resistance value of a portion of the resistor RN on a ground side from the arbitrary point and R 2  is a resistance value of a portion of the resistor RN on a power-source side from the arbitrary point, Rt is expressed as Rt=R 1 +R 2 . Since the output voltage output from the comparator circuit  22  is at the HIGH level in a state in which the voltages Vs and Vt input in the comparator circuit  22  are not yet shifted, the n-channel transistor  24  is turned on. In this state, a resistance value of a portion of the resistor R 2  on the side of the inverting input terminal of the comparator circuit  22  is R 2 . Therefore, a voltage of the resistor R 2  is expressed as R 2 *It. When the voltages Vs and Vt input in the comparator circuit  22  shift, however, the output voltage output from the comparator circuit  22  shifts from the HIGH level to the LOW level. As a result, the n-channel transistor  24  is turned off, and the voltage of the resistor R 2  is expressed as (R 1 +R 2 )*It which is higher, by a value R 1 *It, than the voltage of the resistor R 2  measured before the shift of the voltages Vs and Vt. The value R 1 *It is equal to Vt′−Vt.  
         [0072]     Voltage and temperature characteristics of the temperature monitoring circuit  502  are illustrated in the graph of  FIG. 8 . In the present embodiment shown in  FIG. 9 , Vt is increased to Vt′ due to the hysteresis. Alternatively, the voltage Vs may be decreased to Vs&#39; due to the hysteresis when the levels of the voltages Vs and Vt are shifted.  
         [0073]     As described above, in the temperature monitoring circuits according to the above embodiments, the circuits which generate the two voltages input in the comparator circuit are formed by the constant current circuits, the resistors, and the diodes. Further, the resistors and the diodes are connected to the two input terminals of the comparator circuit. Therefore, the two voltages input in the comparator circuit similarly change to the change in the input voltage Vin. As a result, the relationship between the two voltages are kept constant while the input voltage Vin changes. Preferably, the comparator circuit may have a thermal hysteresis effect or the laser trimming may be performed to obtain the desired detection temperature.  
         [0074]     In the above embodiments, the circuits which generate the two voltages input in the comparator circuit of the temperature monitoring circuit are approximately similar in characteristics, and the elements connected to the two input terminals of the comparator circuit are similar in characteristics. Accordingly, even when the input voltage Vin changes, the relationship between the two voltages input in the comparator circuit is kept constant, and the operational errors can be prevented.  
         [0075]     Further, the resistance values of the temperature monitoring circuits according to the above embodiments can be changed by performing the laser trimming. Accordingly, the detection temperature can be set at the desired value.  
         [0076]     In the above embodiments, the constant currents are flowed through the resistors to generate voltages. If the resistance values and the constant current values are affected by manufacturing variation and temperature dependence of the resistors and the constant current circuits, the voltages generated by the resistors are varied. In order to reduce this variation, there is a method of adjusting the resistance values and the constant current values by performing the laser trimming in post-processes. Alternatively, the constant voltage circuit may be used. For example, if a voltage regulator is used, a relatively accurate output voltage can be obtained, and thus the adjustment by the laser trimming performed in the post-processes is not necessary. Accordingly, manufacturing costs can be reduced.  
         [0077]     The above-described embodiments are illustrative, and numerous additional modifications and variations are possible in light of the above teachings. For example, elements and/or features of different illustrative and exemplary embodiments herein may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. It is therefore to be understood that within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein.  
         [0078]     This patent specification is based on Japanese patent application No. 2004-162941 filed on Jun. 1, 2004 in the Japan Patent Office, the entire contents of which are incorporated by reference herein.