Power supply voltage monitoring circuit and electronic circuit including the power supply voltage monitoring circuit

1. A power supply voltage monitoring circuit for monitoring a power supply voltage supplied to an electronic circuit, comprising:

a signal output circuit for outputting a signal voltage which exhibits a saturation characteristic with respect to an increase in the power supply voltage, wherein the signal output circuit outputs, when power is turned on, a signal voltage which increases along with an increase in the power supply voltage until the power supply voltage makes a transition to a predetermined voltage, and outputs a voltage which monotonously increases and has a saturation characteristic in a case where the power supply voltage continues to increase after exceeding the predetermined voltage; and

a signal voltage monitoring circuit for comparing the power supply voltage with the signal voltage, and for outputting a signal indicating that the signal voltage is normal when there is a predetermined voltage difference between the power supply voltage and the signal voltage; wherein, the signal voltage monitoring circuit compares the signal voltage output from the signal output circuit with the power supply voltage, and outputs a signal indicating that the power supply voltage has exceeded a minimum operating power supply voltage of the electronic circuit when there is the predetermined voltage difference between the power supply voltage and the signal voltage.

2. A power supply voltage monitoring circuit according to claim 1 , wherein:

the signal output circuit includes an impedance circuit supplied with current from a power source via a current supply circuit, generates the signal voltage through the impedance circuit, and outputs the signal voltage thus generated; and

the signal voltage monitoring circuit outputs a signal indicating that the power supply voltage has become equal to or larger than a minimum operating power supply voltage of the electronic circuit when there is the predetermined voltage difference between the power supply voltage and the signal voltage.

3. A power supply voltage monitoring circuit according to claim 2 , wherein the signal output circuit generates the signal voltage through the impedance circuit, as a reference voltage to be compared with the power supply voltage and further as a voltage which exhibits temperature dependence, and outputs the signal voltage thus generated.

4. A power supply voltage monitoring circuit according to claim 1 , wherein the signal voltage monitoring circuit comprises a comparison circuit which sets an offset potential as one of two signals input thereto, and outputs a signal indicating that the power supply voltage has exceeded a minimum operating power supply voltage in a case where a potential difference between the two signals satisfies the offset potential.

5. A power supply voltage monitoring circuit according to claim 1 , wherein the signal output circuit comprises a current supply circuit and an impedance circuit, which are connected in series, and outputs the signal voltage from a circuit connection point between the current supply circuit and the impedance circuit.

6. A power supply voltage monitoring circuit according to claim 5 , wherein the impedance circuit comprises a sensor for inputting a current output from the current supply circuit and for outputting a voltage signal corresponding to detected physical quantities.

7. A power supply voltage monitoring circuit according to claim 6 , wherein the sensor comprises a temperature sensor for detecting temperatures.

8. An electronic circuit, comprising the power supply voltage monitoring circuit according to claim 1 .