Overheat detecting circuit

1. An overheat detecting circuit, comprising:

a current source for generating a constant current;

an overheat detecting element unit that operates with a first current generated in accordance with the constant current and generates a first voltage based on a semiconductor substrate temperature; and

a detecting circuit unit that operates a second current generated in accordance with the constant current, and generates a second voltage corresponding to a predetermined semiconductor substrate temperature to detect overheating based on a voltage difference between the first voltage and a reference voltage and a voltage difference between the second voltage and the reference voltage.

2. The overheat detecting circuit according to claim 1 , wherein when the constant current increases, the voltage difference between the first voltage and the reference voltage and the voltage difference between the second voltage and the reference voltage increase, and

when the constant current is reduced, the voltage difference between the first voltage and the reference voltage and the voltage difference between the second voltage and the reference voltage decrease.

3. The overheat detecting circuit according to claim 1 , wherein the overheat detecting element unit changes the first voltage based in a temperature characteristic of an element.

4. The overheat detecting circuit according to claim 1 , wherein a temperature characteristic of the first voltage involves a larger change amount than a temperature characteristic of the second voltage.

5. The overheat detecting circuit according to claim 1 , wherein a change amount of the first voltage based on a change in the constant current and a change amount of the second voltage are substantially the same.

6. The overheat detecting circuit according to claim 1 , wherein the first voltage and the second voltage are generated by elements that are produced through substantially the same process.

7. The overheat detecting circuit according to claim 1 , wherein the constant current is larger than the first current and the first current is larger than the second current.

8. The overheat detecting circuit according to claim 1 , wherein when the voltage difference between the second voltage and the reference voltage is larger than the voltage difference between the first voltage and the reference voltage, the detecting circuit unit outputs a signal indicating that the semiconductor substrate temperature exceeds the detection temperature.

9. The overheat detecting circuit according to claim 1 , wherein the overheat detecting circuit has hysteresis characteristic of the overheat detection temperature.

10. The overheat detecting circuit of claim 9 , wherein said hysteresis characteristic is due to a hysteresis circuit comprising:

a PMOS transistor and an NMOS transistor serially interconnected such as to be in parallel with said current source, a gate of said PMOS transistor and a gate of said NMOS transistor receiving an overtemperature output signal of said overheat detecting;

a third current generated in accordance with said constant current; and

a transistor through which flows said third current, a source of said transistor connected to said first voltage, a gate of said transistor being connected to a node between said serially interconnected PMOS and NMOS transistors.

11. The overheat detection circuit of claim 1 , wherein said detecting circuit unit receives said first voltage as an input signal.

12. The overheat detection circuit of claim 1 , wherein said detecting circuit detects said overheating based on comparing, relative to said reference voltage, said first voltage with said second voltage.

13. The overheat detection circuit of claim 1 , wherein said current source is connected between a power supply voltage and said reference voltage, said power supply being referenced to a ground, and said reference voltage comprises a voltage preset a predetermined amount below said power supply voltage.

14. The overheat detection circuit of claim 1 , wherein said overheat detection element comprises a diode.

15. The overheat detection circuit of claim 14 , wherein said overheat detection element further comprises a transistor serially connected with said diode.

16. The overheat detection circuit of claim 1 , wherein said detecting circuit unit comprises a transistor having a gate controlled by said first voltage.

17. The overheat detecting circuit of claim 1 , wherein said constant current, said first current, and said second current are generated by a plurality of PMOS transistors interconnected as current mirrors.

18. The overheat detecting circuit of claim 1 , wherein said constant current, said first current, and said second current are generated by a plurality of NMOS transistors interconnected as current mirrors.

19. An overheat detecting circuit, comprising:

a current source for generating a constant current;

an overheat detecting element unit that operates with a first current generated in accordance with the constant current and generates a first voltage based on a semiconductor substrate temperature; and

a detecting circuit unit that operates a second current generated in accordance with the constant current, said detecting circuit generating a second voltage corresponding to a predetermined semiconductor substrate temperature to detect overheating, said detecting circuit being activated by said first voltage when an overheat condition has been sensed by said overheat detecting element.

20. A method of detecting an overheat condition, said method comprising:

generating constant current;

operating an overheat detecting element unit with a first current generated in accordance with the constant current;

generating a first voltage by said overheat detecting element, based on a semiconductor substrate temperature;

operating a detecting circuit unit with a second current generated in accordance with the constant current, said detecting circuit generating a second voltage corresponding to a predetermined semiconductor substrate temperature to detect overheating; and

activating said detecting circuit by said first voltage when an overheat condition has been sensed by said overheat detecting element.