Abstract:
A semiconductor integrated circuit comprises a transistor which has a first electrode, a second electrode and a third electrode, said transistor conducting a current of a first power source from the second electrode to the third electrode by a power supplied to the first electrode; a driver to supply said first electrode with power for driving said transistor; a reference voltage circuit to generate a reference voltage which is variable in response to temperature of said transistor, said reference voltage being used as the reference for comparison; a comparative voltage circuit to generate a comparative voltage which is variable in response to a current flowing from said second electrode to said third electrode, said comparative voltage being compared with said reference voltage; and a controller which receives said reference voltage and said comparative voltage and which supplies a control signal to said driver, said control signal being based on a result of the comparison between the comparative voltage and the reference voltage to control the power supplied to said first electrode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-309829, filed on Oct. 5, 2001, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to a semiconductor integrated circuit. 
   2. Related Background Art 
   In general, Semiconductor integrated circuits having a power transistor suffer from heat generated by electric current flowing through the power transistor. The heat may raise the temperature of the junction of the power transistor, and thereby may break the power transistor and elements placed around it. 
   As a countermeasure, an overheat detector circuit has been provided heretofore to detect the temperature of the junction of the power transistor. 
     FIG. 8  is a schematic diagram of a conventional semiconductor integrated circuit  800  having an overheat detector circuit. The semiconductor integrated circuit  800  includes a bipolar transistor  10 , driver  20  for driving the transistor  10 , reference voltage circuit  30  for generating a reference voltage, resistor R 1  connected between a power source and an emitter, an over-current detector circuit  50  which detects the emitter current by comparing a comparative voltage from the power source via the resistor R 1  with the reference voltage and thereby controls the driver  20 , and an overheat detector circuit  65  for controlling the driver  20  in accordance with the temperature of the junction of the transistor  10 . 
   Operation of the semiconductor integrated circuit  800  is explained below with reference to  FIGS. 8 and 9 .  FIG. 9  shows a graph of changes in the quantity of emitter current IE with temperature Tj at the junction of the transistor  10 . 
   Since the comparative voltage is applied from the power source VCC through the resistor R 1 , it varies with the emitter current IE. The reference voltage is a constant voltage determined by the electromotive force of the reference voltage circuit  30 . 
   The over-current detector circuit  50  compares the comparative voltage with the reference voltage, and controls the driver  20  to prevent the emitter current IE from increasing beyond a predetermined value Ioc (see FIG.  9 ). 
   If the emitter current Ioc continuously flows in the transistor  10 , the junction temperature Tj of the transistor  10  rises because the energy VCE*Ioc continuously generates at the junction of the transistor  10 , where VCE is the collector-emitter voltage. Continuous rise of the junction temperature Tj will break the transistor  10 . Therefore, the overheat protective circuit  65  controls the driver  20  to prevent the junction temperature Tj from surpassing a predetermined value Tot. 
   That is, in  FIG. 9 , when the junction temperature Tj reaches Tot, the driver  20  shuts the transistor  10  off so as not to move it into the state of the shadowed region Sb. As a result, the transistor  10  turns OFF and the emitter current IE stops. 
   In this manner, the overheat protective circuit  65  was heretofore used to protect the transistor  10  and its peripheral elements from destruction by overheating. 
   However, in case the rising rate (for example, K/s (Kelvin per second)) of the junction temperature Tj per unit time is too high for the overheat detector circuit  65  to catch up to for detection, or in case the junction temperature Tj is already high when the transistor  10  is activated, the transistor  10  may undesirably move into the state of the shadowed region Sb. 
   For example, assume here that the junction temperature Tj is high when the transistor  10  is activated and that the transistor  10  is in the state of the region Sa of FIG.  9 . Even in this case, the over-current detector circuit  50  permits the emitter current Ioc to flow into the transistor  10 . Therefore, the transistor l heretofore taking the state of the region Sa shifts to the state of the region Sb before the overheat detector circuit  65  can detect it and turns OFF the transistor  10 . 
   Therefore, the semiconductor integrated circuit  800  even with the overheat detector circuit  65  was still unable to reliably protect internal elements from heat of the power transistor. 
   It is therefore desirable to provide a semiconductor integrated circuit which is capable of reliably protecting the power transistor and its peripheral elements against heat of the power transistor. 
   BRIEF SUMMARY OF THE INVENTION 
   A semiconductor integrated circuit according to an embodiment of the invention comprises: a transistor which has a first electrode, a second electrode and a third electrode, said transistor conducting a current of a first power source from the second electrode to the third electrode by a power supplied to the first electrode; a driver to supply said first electrode with power for driving said transistor; a reference voltage circuit to generate a reference voltage which is variable in response to temperature of said transistor, said reference voltage being used as the reference for comparison; a comparative voltage circuit to generate a comparative voltage which is variable in response to a current flowing from said second electrode to said third electrode, said comparative voltage being compared with said reference voltage; and a controller which receives said reference voltage and said comparative voltage and which supplies a control signal to said driver, said control signal being based on a result of the comparison between the comparative voltage and the reference voltage to control the power supplied to said first electrode. 
   A semiconductor integrated circuit according to a further embodiment of the invention comprises: a transistor which has a first electrode, a second electrode and a third electrode, said transistor conducting a current of a first power source from the second electrode to the third electrode by a power supplied to the first electrode; a driver to supply said first electrode with power for driving said transistor; a reference voltage circuit to generate a reference voltage which is used as the reference for comparison; a comparative voltage circuit to generate a comparative voltage which is variable in response to both the temperature of said transistor and a value of the current flowing from said second electrode to said third electrode, said comparative voltage being compared with said reference voltage; and a controller which receives said reference voltage and said comparative voltage and which supplies a control signal to said driver, said control signal being based on a result of comparison between the comparative voltage and the reference voltage to control the power supplied to said first electrode. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram of a semiconductor integrated circuit  100  according to an embodiment of the invention; 
       FIG. 2  is a diagram of a graph showing changes in the quantity of emitter current IE with temperature Tj at the junction of a transistor  10 ; 
       FIG. 3  is a circuit diagram of a semiconductor integrated circuit  300  using an NPN bipolar transistor as the transistor  10 ; 
       FIG. 4  is a circuit diagram of a semiconductor integrated circuit  400  as an embodiment of the semiconductor integrated circuit  100 ; 
       FIG. 5  is a circuit diagram of a semiconductor integrated circuit  500  as another embodiment of the semiconductor integrated circuit  100 ; 
       FIG. 6  is a circuit diagram of a semiconductor integrated circuit  600  according to a further embodiment of the invention; 
       FIG. 7  is a circuit diagram of a semiconductor integrated circuit  700  as an embodiment of the semiconductor integrated circuit  600 ; 
       FIG. 8  is a block diagram of a conventional semiconductor integrated circuit  800  having an overheat detector circuit; and 
       FIG. 9  is a diagram of a graph showing changes in the quantity of emitter current IE with temperature Tj at the junction of a transistor  10 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Some embodiments of the invention will now be explained below with reference to the drawings. These embodiments, however, should not be construed to limit the invention. 
     FIG. 1  is a circuit diagram of a semiconductor integrated circuit  100  according to an embodiment of the invention. The semiconductor integrated circuit  100  includes a bipolar transistor  10  and a driver circuit  20 . The bipolar transistor  10  permits a current from a power source VCC to flow from the emitter to the collector as the emitter current IE depending on a current supplied to the base. The driver circuit  20  drives the transistor  10 . 
   The semiconductor integrated circuit further includes a reference voltage source  32  and a resistor R 1 . The reference voltage source  32  generates a reference voltage Va variable with temperature Tj of a junction formed inside the transistor  10 . The resistor R 1  generates a comparative voltage vb compared with the reference voltage Va and variable with the emitter current (flowing in the arrow-marked direction). 
   The semiconductor integrated circuit  100  further includes an over-current detector circuit  50 . The over-current detector circuit  50  outputs a control signal to the driver circuit  20  to interrupt the current to the base when the comparative voltage Vb input from the resistor R 1  is equal to or lower than the reference voltage Va input from the reference voltage source  32 . 
   In conjunction with  FIGS. 1 and 2 , operation of the semiconductor integrated circuit  100  is explained below.  FIG. 2  is a diagram of a graph showing changes in the quantity of emitter current IE with temperature Tj at the junction of a transistor  10 . 
   Assume here that the junction temperature Tj of the transistor  10  is relatively low and the emitter current IE is relatively small. That is, the transistor  10  is assumed to currently take the state of the region Sd in FIG.  2 . 
   The reference voltage Va is a voltage obtained by voltage drop from the voltage of the power source VCC by the reference voltage source  32 . The reference voltage source  32  sets the reference voltage Va relatively low when the junction temperature Tj is relatively low. 
   Since the comparative voltage Vb is applied from the power source VCC through the resistor R 1 , it varies with the emitter current IE. When the emitter current IE is relatively small, voltage drop by the resistor R 1  is relatively small, and accordingly, the comparative voltage Vb is relatively high. 
   Therefore, when the emitter current IE is relatively small, the comparative voltage Vb is higher than the reference voltage Va, and the potential difference between the comparative voltage Vb and the reference voltage Va is relatively large. As a result, the transistor  10  can afford to conduct larger emitter current IE. 
   Next assume that the emitter current IE has become relatively large, and the reference voltage Va and the comparative voltage Vb have been approximately equalized. That is, the transistor  10  is assumed to currently take the state of the region Sc in FIG.  2 . 
   Since the junction temperature Tj is still relatively low, the reference voltage Va remains relatively low as well. However, since voltage drop by the resistor R 1  increases as the emitter current IE increases, the comparative voltage Vb drops. When the comparative voltage Vb goes down to become approximately equal to the reference voltage Va, the over-current detector circuit  50  gives a control signal to the driver circuit  20  to interrupt the voltage to the base. As a result, the transistor  10  is switched OFF. 
   That is, in  FIG. 2 , when the emitter current IE becomes substantially equal to a predetermined current value (herein below called current restrictive value Ioc), the transistor  10  is switched OFF, and the emitter current IE can no longer flow between the emitter and the collector of the transistor  10 . Therefore, the emitter current IE never surpasses the current restrictive value Ioc. 
   On the other hand, when the emitter current IE decreases below the current restrictive value Ioc, the transistor  10  is switched ON, and the emitter current IE is permitted to flow between the emitter and the collector of the transistor  10 . Therefore, the emitter current IE is controlled to be equal to the current restrictive value Ioc. 
   Next assume that the junction temperature Tj of the transistor  10  is relatively high. That is, let the transistor  10  currently take the state of the region Sa. 
   The reference voltage source  32  is configured to set the reference voltage Va higher as the junction temperature Tj rises. When the reference voltage Va becomes high, voltage breadth permitting the comparative voltage Vb to lower is narrowed greatly. That is, larger emitter current IE can no longer flow. Therefore, in response to the rise of the junction temperature Tj, the current restrictive value Ioc decreases. In the embodiment shown here, the junction temperature Tj and the current restrictive value Ioc are approximately inversely proportional. 
   For example, when the thermal resistance is Rth (° C./W) and the emitter-collector voltage is VCE, the current restrictive value Ioc is expressed as
 
 Ioc =( Tot−Tj )/( Rth*VCE )  (1)
 
It can be understood from Equation (1) that the junction temperature Tj is inversely proportional to the current restrictive value Ioc.
 
   In conventional techniques, the current restrictive value Ioc was constant independently from the junction temperature Tj as shown in FIG.  9 . 
   In  FIG. 2 , however, the current restrictive value Ioc varies with the junction temperature Tj. In the instant embodiment, when the junction temperature Tj is near to the predetermined value Tot, the current restrictive value Ioc is nearly zero. Therefore, the emitter current IE can reach the current restrictive value Ioc in a short time. 
   Accordingly, even if the rising rate of the junction temperature Tj per unit time is high or even if the junction temperature Tj is already high when activation of the transistor  10  is started, the transistor  10  never becomes the state of the shadowed region Sb, and it does not break. Therefore, the configuration of the semiconductor integrated circuit  100  reliably protects the transistor  10  from excessive emitter current IE. 
   The semiconductor integrated circuit  100  need not include the overheat detector circuit  65  required in the conventional semiconductor integrated circuit  800 . Thus the semiconductor integrated circuit  100  can be smaller than the conventional semiconductor integrated circuit  800 . 
   The transistor  10  used in the semiconductor integrated circuit  100  is a PNP bipolar transistor, but an NPN bipolar transistor may be used instead. 
     FIG. 3  is a circuit diagram of a semiconductor integrated circuit  300  using an NPN bipolar transistor as the transistor  10 . 
   In the semiconductor integrated circuit  300  shown in  FIG. 3 , respective components may be arranged similarly to those of the semiconductor integrated circuit  100  while using the ground as a power source. The change of the conduction type is similarly possible also for other embodiments explained later. 
   In addition to typical bipolar transistors, MOS transistors, IGBT (insulated gate bipolar transistor) are also usable as the transistor  10 . In case a MOS transistor is used as the transistor  10 , the emitter and the collector may be replaced by combination of gate, source and drain or combination of gate drain and source, in this and other embodiments. 
     FIG. 4  is a circuit diagram of a semiconductor integrated circuit  400  as a more detailed embodiment of the semiconductor integrated circuit  100 . The semiconductor integrated circuit  400  includes a constant current circuit  60  and a reference voltage generator  34 . The reference voltage generator  34  includes a diode D 1  connected in series in the way from the power source VCC to the constant current circuit  60 , resistors R 2  and R 3  connected in series to each other and connected in parallel to the diode D 1 , and diode DD 1  connected in series in the way from the power source VCC and the diode D 1 . The constant current circuit  60  is connected to the ground. 
   The diode D 1  is placed on a common chip with the transistor  10 . Temperature of the diode D 1  is proportional to the temperature of the transistor  10 . More preferably, the temperature of the diode D 1  is equal to the temperature of the transistor  10 . Therefore, the diode D 1  is preferably located near the transistor  10 . 
   Operation of the semiconductor integrated circuit  400  is explained in conjunction with its configuration. 
   In general, operation voltage of a diode has a constant temperature property about −2 mV/° C. Accordingly, the diode D 1  generates a potential difference approximately proportional to the temperature of the transistor  10 . 
   Potential difference across opposite ends of the diode D 1  is divided by the resistors R 2 , R 3  connected in parallel to the diode D 1 . For example, if the resistance ratio of the resistors R 2  and R 3  is 1:1, the potential difference across opposite ends of the diode D 1  is divided to one-half. In this case, the reference voltage Va corresponds to a value obtained by subtracting the operation voltage of the diode DD 1  and one-half of the operation voltage of the diode D 1  from the voltage of the power source VCC. That is, when the operation voltage of the diode is VF, it becomes
 
 Va=VCC−VD− (½)* VF 
 
When the resistance ratio of the resistors R 2  ad R 3  is adjusted to be 1:1, it remains unchanged regardless of the temperature properties of the resistors R 2  and R 3 .
 
   If the resistance values of the resistors R 2  and R 3  are R 2  and R 3 , respectively, Va can be expressed as
 
 Va=VCC−VD −( R   2 /( R   2 + R   3 )* VF   (2)
 
   As such, since the operation voltage VF has a temperature property, it is appreciated that the reference voltage Va also has a temperature property. 
   The semiconductor integrated circuit  400  includes a comparative voltage generator  44  having a resistor R 1 , a diode DD 2  and a resistor R 4 . An end of the resistor R 4  is connected to a junction between the emitter and the resistor R 1 , and the other end of the resistor R 4  is connected to the constant current circuit  60  via the diode DD 2 . 
   Electric current from the power source VCC flows into the transistor  10  through the resistor R 1 . After electric current flows through the resistor R 1 , it flows through the diode DD 2  and the resistor R 4 . Therefore, the comparative voltage Vb becomes a value obtained by subtracting a voltage drop by the resistor R 1 , operation voltage of the diode DD 2  and a voltage drop by the resistor R 4  from the voltage of the power source VCC. When the current flowing into the resistor R 4  is Ia, the comparative voltage vb can be expressed by
 
 Vb=VCC −( IE+Ia )* R   1 − VF−Ia*R   4 ≈ VCC−IE*R   1 − VF−Ia*R   4   (3)
 
where IE&gt;&gt;Ia.
 
   A comparator  52  compares the comparative voltage Vb with the reference voltage Vb, and outputs its result to the driver  20 . 
   The constant current circuit  60  has a current mirror circuit including transistors CM 1 , CM 2  and CM 3  having emitters grounded and having a common base. The collector of the transistor CM 1  is connected to the base and further to a constant current source  70 . The collector of the transistor CM 2  is connected to the diode D 1 , and the collector of the transistor CM 3  to the resistor R 4 . 
   Collector potential of the transistor CM 1  and base potential of the transistors CM 1 , CM 2  and CM 3  are equal. Therefore, if the transistors CM 1 , CM 2 , CM 3  are equal in size, a current equal to the current Ia from the constant current source  70  flows between collectors and emitters of the transistors CM 1 , CM 2 , and CM 3 . Accordingly, the equal constant current Ia flows in both the diode D 1  and the resistor R 4 . Therefore, resistance value of the resistor R 4  can be easily set, and the diodes DD 1  and DD 2  can be well balanced as well. 
   When the emitter current IE is small, potential of the comparative voltage Vb is higher than the potential of the reference voltage Va. As the emitter current IE decreases, the comparative voltage Vb lowers and approaches the reference voltage Va. When the comparative voltage Vb becomes equal to the reference voltage Va, the difference between the comparative voltage Vb and the reference voltage Va compared by the comparator  52  becomes smaller than a predetermined value. As a result, the driver circuit  20  interrupts the current to the base of the transistor  10 . Therefore, the transistor  10  turns OFF and the emitter current IE does not flow. 
   When the comparative voltage Vb becomes equal to the reference voltage Va, that is, when Vb=Va, the following equation is established from Equations 2 and 3.
 
 VCC−VF −( R   2 /( R   2 + R   3 ))* VF=VCC−Ioc*R   1 − VF−Ia*R   4   (4)
 
Note here that IE is replaced by Ioc in Equation 4 because the emitter current IE equals the current restrictive value Ioc under Vb=Va.
 
   When Equation (4) is rearranged and differentiated by temperature, the following equation is established. 
     dIoc/dT= 1/ R   1 *( R   2 /( R   2 + R   3 ))* dVF/dT   (5) 
   where dIa/dT=0, and dR 1 /dT=0. 
   In general, dVF/dT≈−2 mV/° C. Therefore, according to Equation (5), Ioc is inversely proportional to the temperature. That is, Ioc has a negative temperature property. As a result, the semiconductor integrated circuit  400  according to the instant embodiment exhibits the property as shown by the graph of FIG.  2 . That is, Ioc varies with the junction temperature Tj of the transistor  10 . 
     FIG. 5  is a circuit diagram of a semiconductor integrated circuit  500  as another detailed embodiment of the semiconductor integrated circuit  100 . 
   The semiconductor integrated circuit  500  includes a reference voltage generator  36  having a resistor R 5 , resistors R 6  and R 7  and bipolar transistor  80 . One end of the resistor R 5  is connected to the power source VCC. One end of the resistors R 6  and R 7  is connected, respectively, to the other end of the resistor R 5 . The other end of the resistors R 6  and R 7  is grounded. The bipolar transistor  80  is connected between a constant current circuit  62  and a ground connection. The base and the collector of the transistor  80  are connected to each other, and the transistor  80  functions as a diode. The transistor  80  has a temperature proportional or equal to the junction temperature of the transistor  10 . Thus the transistor  80  functions as a heat-sensitive element. The transistor  80  is preferably located near the transistor  10  to accurately detect the junction temperature of the transistor  10 . In the semiconductor integrated circuit  500 , the reference voltage Va is the result of the current passing through the resistor R 5 . 
   The comparative voltage circuit has a interconnection connected from a node between the resistor R 1  and the transistor  10  to the constant current circuit  62 . In the semiconductor integrated circuit  500 , the comparative voltage Vb is the voltage of the current passing through the resistor R 1 . 
   The constant current circuit  62  supplies a constant current to the reference voltage generator  36  and a comparative voltage generator  46  in order to have the reference voltage generator  36  and the comparative voltage generator  46  generate the reference voltage and the comparative voltage, respectively. The constant voltage circuit  62  receives the reference voltage and the comparative voltage, and supplies a current based on the comparative voltage to a bipolar transistor  54 . The bipolar transistor  54  switches the drive signal in response to the current from the constant current circuit  62 . 
   Operation of the semiconductor integrated circuit  500  is explained below in conjunction with its configuration. 
   The constant current circuit  62  includes a first current mirror circuit having transistors CM 4 , CM 5  and CM 6  having a common base. The collector of the transistor CM 4  is connected to the base of its own and further to a constant current source  70 , and its emitter is connected to the collector of the transistor  80 . Emitters of the transistors CM 5  and CM 6  are connected to resistors R 6  and R 7 , respectively. 
   Since the transistors CM 4 , CM 5  and CM 6  have a common base potential, potential difference across the collector and the emitter of the transistor  80 , the potential difference between both ends of the resistor R 6  and the potential difference between both ends of the resistor R 7  are equal. Additionally, in case the resistance value of the resistor R 6  and that of the resistor R 7  are equalized, an equal reference current Ia flows in the transistor CM 6  and CM 7 . Further, since the transistor  80  has a potential difference relying upon the junction temperature of the transistor  10 , the reference current Ia also relies on the junction current Tj of the transistor  10 . 
   On the other hand, the constant current circuit  62  includes a second current mirror circuit having transistors CM 7  and CM 8  using a common base. The collector of the transistor CM 7  is connected to the collector of the transistor CM 5 . The collector of the transistor CM 8  is connected to the base of its own and to the transistor CM 6  via a bipolar transistor  90 . The emitter of the transistor CM 7  is connected to the resistor R 5 , and the emitter of the transistor CM 8  is connected to a node between the resistor R 1  and the transistor  10 . 
   Therefore, in the second current mirror circuit, the transistor CM 7  receives the reference voltage Va from the reference voltage generator  36 , and the transistor CM 8  receives the comparative voltage Vb from the comparative voltage generator  46 . Additionally, since the transistors CM 7  and CM 8  are equal in potential of the base, they supply the reference current Ia and the comparative current Ik corresponding to the reference voltage Va and the comparative voltage Vb, respectively. 
   The circuit is designed such that a current equal to the current Ia controlled by the first current mirror circuit flows as the reference current. On the other hand, the comparative current does not always coincide with the current Ia, but it corresponds to the comparative voltage Vb. Therefore, if the comparative voltage Vb is higher than the reference voltage Va, a larger comparative current Ik than the reference current Ia will flow. That is, in this case, the reference current Ik flowing from the transistor  90  to the base of the transistor  54  is larger than the reference current Ia flowing in the resistor R 7 . 
   By the difference between the reference current Ia and the comparative current Ik, the transistor  54  is controlled to be ON or OFF. 
   In case the comparative current Ik is larger than the reference current Ia, no base current is generated in the transistor  54 . Therefore, the driver circuit  20  maintains the base of the transistor  10  ON, and the emitter current IE continues to flow. 
   In case the comparative current Ik is smaller than the reference current Ia, the base current is generated in the transistor  54 . Therefore, the transistor  54  is switched ON. As a result, the driver circuit  20  switches the transistor  10  OFF, and the emitter current IE does not flow. 
   In this manner, the transistor  54  is switched when the comparative current Ik becomes equal to the reference current Ia, that is, when the comparative voltage Vb becomes equal to the reference voltage Va. Further, the reference current Ia is changed by the transistor  80 . And the reference current Ia depends upon the junction temperature Tj of the transistor  10 . Therefore, the semiconductor integrated circuit  500  exhibits the same property as that of the graph of FIG.  2 . 
   Using equations, it is explained that the semiconductor integrated circuit  500  exhibits the property as shown by the graph of FIG.  2 . 
   Since the reference voltage Va is the voltage of the power source VCC after passing the resistor R 5 , it can be expressed by
 
 Va=VCC−Ia*R   5   (6)
 
The reference current Ia is determined by the potential difference across both ends of the resistor R 6 , i.e. the potential difference between the base and the emitter of the transistor  80 , and the resistance value of the resistor R 6 . That is,
 
 Ia=VF/R   6   (7)
 
   From Equations 6 and 7,
 
 Va=VCC −( R   5 / R   6 )* VF   (8)
 
   Since the comparative voltage Vb is the voltage of the power source VCC after passing the resistor R 1 , it can be expressed by
 
 Vb=VCC −( IE+Ik )* R   1   (9)
 
If the emitter current IE is an over-current, Ik is as small as negligible relative to IE. Therefore, Equation (9) can be expressed by
 
 Vb=VCC−IE*R   1   (10)
 
   When the comparative voltage Vb equals the reference voltage Va, the transistor  54  detects an over-current. Therefore, the following equation is established from Equations (8) and (10). 
     VCC −( R   5 / R   6 )* VF=VCC−Ioc*R   1   (11) 
   Note that IE is replaced by Ioc in Equation (11) because the emitter current IE is the current restrictive value Ioc when Vb=Va. 
   When Equation (11) is rearranged and differentiated by temperature, the following equation is established.
 
 dIoc/dT= 1/ R   1 *( R   5 + R   6 )* dVF/dT   (12)
 
where dR 1 /dT=0.
 
   In general, dVF/dT=−2 mV/° C. Therefore, according to Equation (12) similarly to Equation (5), Ioc is inversely proportional to the temperature. That is, Ioc has a negative temperature property. As a result, the semiconductor integrated circuit  500  according to the instant embodiment exhibits the property as shown by the graph of FIG.  2 . That is, Ioc varies with the junction temperature Tj of the transistor  10 . 
   The transistor  90  is used to correct the base current and thereby keep the reference current Ia and the comparative current Ik in precise correspondence to the reference voltage Va and the comparative voltage Vb. The second current mirror circuit including this transistor  90  forms a so-called Wilson constant current circuit. 
     FIG. 6  is a circuit diagram of a semiconductor integrated circuit  600  according to a further embodiment of the invention. Similarly to the semiconductor integrated circuit  100 , the semiconductor integrated circuit  600  includes the bipolar transistor  10 , driver circuit  20  and over-current detector circuit  50 . 
   The semiconductor integrated circuit  600  additionally includes a reference voltage circuit  30  connected between the power source VCC and the over-current detector circuit  50 , and a resistor R 8  connected in series between the power source VCC and the collector of the transistor  10 . 
   The difference between the semiconductor integrated circuit  600  and the semiconductor integrated circuit  100  is in that the reference voltage circuit  30  has no temperature property but the resistor R 8  has a temperature property. That is, the reference voltage circuit  30  supplies a constant reference voltage Va to the over-current detector circuit  50  independently from the junction temperature of the transistor  10 . In contrast, the resistor R 8  supplies the comparative voltage Vb to the over-current detector circuit  50 . The comparative voltage Vb is variable with the junction temperature of the transistor  10 . 
   The resistor R 8  changes the comparative voltage Vb in response to the emitter current IE. Therefore, the comparative voltage Vb varies depending upon both the junction temperature Tj of the transistor  10  and the emitter current IE. 
   In the instant embodiment, the resistor R 8  should be formed on the common chip together with the transistor  10  and located near the junction of the transistor  10 . 
     FIG. 7  is a circuit diagram of a semiconductor integrated circuit  700  as a more detailed embodiment of the semiconductor integrated circuit  600 . The semiconductor integrated circuit  700  includes a reference voltage generator  38  having a resistor R 9  connected in series between the power source VCC and the constant current source  64 . Therefore, the reference voltage Va is a result of a current from the power source VCC flowing through the resistor R 9 . 
   The semiconductor integrated circuit  700  additionally includes a comparative voltage generator  48  having a resistor R 8  that produces a potential difference depending upon the junction temperature of the transistor  10  and functions as a heat-sensitive element. The comparative voltage generator  48  further includes resistors R 10  and R 11  connected in series to each other and connected in parallel to the resistor R 8 . 
   The reference voltage Vb is the voltage at the junction of the resistors R 10  and R 11 , and corresponds to a voltage obtained by dividing the potential difference across both ends of the resistor R 8 . 
   The semiconductor integrated circuit  700  includes a current mirror circuit  64  having transistors CM 9  and CM 10  that have emitters grounded and a common base. The collector of the transistor CM 9  is connected to the base of its own and further to a constant current source  70 . The collector of the transistor CM 10  is connected to the resistor R 9 . 
   Collector potential of the transistor CM 9  and base potential of the transistors CM 9 , CM 10  are equal. Therefore, the transistors CM 9  and CM 10  are equally sized such that a current equal to the current Ia from the constant current source  70  flows between the collector and the emitter of the transistor CM 10 . Thus, the constant current Ia flows in the resistor R 9 , and the reference voltage Va is kept constant. 
   Using equations, operation of the semiconductor integrated circuit  700  is explained below. 
   Since the reference voltage Va is the voltage of the power source VCC after passing the resistor R 9 , it is expressed by
 
 Va=VCC−Ia*R   9   (13)
 
   The comparative voltage Vb is a voltage obtained by dividing the potential difference between both ends of the resistor R 8  divided by resistors R 10  and R 11 . Therefore, it is expressed by
 
 Vb=VCC−[R   10 /( R   10 + R   11 )]*( IE*R   8 )  (14)
 
   When the comparative voltage Vb equals the reference voltage Va, the comparator  52  makes the driver circuit  20  interrupt the transistor  10 . Therefore, from Equations (13) and (14), the following equation is established.
 
 VCC−Ia*R   9 = VCC−[R   10 /( R   10 + R   11 )]*( Ioc*R   8 )  (15)
 
IE is replaced by Ioc because the emitter current IE is the current restrictive value Ioc when Vb=Va. When it is rearranged and differentiated by temperature, the following equation is established.
 
 dIoc/dT=[R   9 *( R   10 + R   11 )/ R   10 ]* Ia*d (1 /R   8 )/ dT   (16)
 
In this case, dR 9 /dT=0.
 
   A potential difference across both ends of the resistor R 8  increases with temperature. That is, the resistor R 8  has a positive temperature property. Therefore, d(1/R 8 )/dT in Equation 16 is negative. Thus the current restrictive value Ioc is inversely proportional to the temperature. That is, the current restrictive value Ioc has a negative temperature property. Therefore, the semiconductor integrated circuit  700  according to the instant embodiment can also exhibit the same property as shown by the graph of FIG.  2 . 
   Note here that reference numerals of respective components are used as their voltage values, current values and resistance values in the equations. 
   The resistors in the foregoing embodiments may be load elements having resistance components. For example, transistors may be used as loads. Additionally, diodes in the foregoing embodiments may be any elements having PN junctions. For example, bipolar transistors, which have the base of its own connected to the collector of its own, are usable. 
   Furthermore, transistors in the foregoing embodiments may be MOS transistors as well. 
   As such, the semiconductor integrated circuits explained as the foregoing embodiments can reliably protect power transistors and their peripheral elements from heat of the transistors. 
   Moreover, any of the semiconductor integrated circuits according to the above-explained embodiments need not over-heat protective circuit, and can be smaller than conventional ones.