Voltage regulator

1. A voltage regulator, comprising:

a first power source terminal at which an input voltage can be applied;

a second power source terminal at which a power source reference voltage can be applied;

an output terminal at which an output voltage can be output;

an operational amplifier configured to compare a predetermined reference voltage to a feedback voltage that is proportional to the output voltage and output an output signal corresponding to the comparison of the predetermined reference voltage and the feedback voltage;

a detecting circuit configured to detect an operating state of the operational amplifier and output a control signal according to the operating state detected; and

an output transistor connected between the first power source terminal and the output terminal, a conductance state of the output transistor being changeable based on the output signal of the operational amplifier and the control signal from the detecting circuit,

wherein the control signal from the detecting circuit changes the conductance state of the output transistor to a non-conductive state when the operational amplifier is not operating while the input voltage is being applied to the first power source terminal,

wherein the output transistor comprises a first p-channel metal oxide semiconductor (PMOS) transistor that has a source electrode connected to the first power source terminal and a drain electrode connected to the output terminal,

wherein the control signal from the detecting circuit is supplied to a gate electrode of the first PMOS transistor, and

wherein the operation amplifier includes:

a second PMOS transistor that has a source electrode connected to the first power source terminal;

a third PMOS transistor that has a gate electrode at which the predetermined reference voltage is applied and a source electrode connected to a drain electrode of the second PMOS transistor;

a fourth PMOS transistor that has a gate electrode at which the feedback voltage is applied and a source electrode connected to the drain electrode of the second PMOS transistor;

a first n-channel metal oxide semiconductor (NMOS) transistor that has a drain electrode connected to a drain electrode of the third PMOS transistor and a source electrode connected to the second power source terminal; and

a second NMOS transistor that has a drain electrode connected to a drain of the fourth PMOS transistor, a source electrode connected to the second power source terminal, and a gate electrode connected to a gate electrode of the first NMOS transistor.

2. The voltage regulator according to claim 1 , wherein the detecting circuit includes:

a fifth PMOS transistor that has a source electrode connected to the first power source terminal and a gate electrode connected to a gate electrode of the second PMOS transistor;

a sixth PMOS transistor that has a source electrode connected to a drain electrode of the fifth PMOS transistor, a drain electrode connected to an amplifier circuit, and a gate electrode at which the predetermined reference voltage is applied; and

a seventh PMOS transistor that has a source electrode connected to the first power source terminal, a drain electrode connected to the gate electrode of the first PMOS transistor, and a gate electrode connected to the amplifier.

3. The voltage regulator according to claim 2 , wherein the detecting circuit further includes a resistor connected between the drain electrode of the sixth PMOS transistor and the second power source terminal.

4. The voltage regulator according to claim 2 , wherein the detecting circuit further includes a constant current source connected between the drain electrode of the sixth PMOS transistor and the second power source terminal.

5. The voltage regulator according to claim 1 , wherein the detecting circuit includes:

a third NMOS transistor that has a source electrode connected to the second power source terminal and a gate electrode connected to the drain electrode of the third PMOS transistor;

a resistor connected between the first power source terminal and a drain electrode of the third NMOS transistor;

a fifth PMOS transistor that has a source electrode connected to the first power source terminal and a drain electrode connected to the gate electrode of the first PMOS transistor; and

an inverter that has an inverter input terminal connected to the drain electrode of the third NMOS transistor and an inverter output terminal connected to a gate electrode of the fifth PMOS transistor.

6. The voltage regulator according to claim 1 , wherein the detecting circuit includes:

a third NMOS transistor that has a source electrode connected to the second power source terminal and a gate electrode connected to the drain electrode of the third PMOS transistor;

a constant current source connected between the first power source terminal and a drain electrode of the third NMOS transistor;

a fifth PMOS transistor that has a source electrode connected to the first power source terminal and a drain electrode connected to the gate electrode of the first PMOS transistor; and

an inverter that has an inverter input terminal connected to the drain electrode of the third NMOS transistor and an inverter output terminal connected to a gate electrode of the fifth PMOS transistor.

7. A voltage regulator, comprising:

an output transistor that is a p-channel metal oxide semiconductor (PMOS) transistor connected between a first power source terminal and an output terminal;

an operational amplifier configured to control a conductance of the output transistor to thereby control an output voltage level supplied at the output terminal to be a predetermined output level that is less than an input voltage level applied at the first power source terminal; and

a detecting circuit configured to place the output transistor in a non-conductive state when the input voltage level is a positive level that is less a level at which the operational amplifier operates,

wherein the operational amplifier includes:

a second PMOS transistor that has a source electrode connected to the first power source terminal;

a third PMOS transistor that has a gate electrode at which a predetermined reference voltage is applied and a source electrode connected to a drain electrode of the second PMOS transistor;

a fourth PMOS transistor that has a gate electrode at which a feedback voltage that is proportional to the output voltage level is applied and a source electrode connected to the drain electrode of the second PMOS transistor;

a first n-channel metal oxide semiconductor (NMOS) transistor that has a drain electrode connected to a drain electrode of the third PMOS transistor and a source electrode connected to a ground terminal; and

a second NMOS transistor that has a drain electrode connected to a drain of the fourth PMOS transistor, a source electrode connected to the ground terminal, and a gate electrode connected to a gate electrode of the first NMOS transistor.

8. The voltage regulator of claim 7 , wherein the detecting circuit supplies a signal directly to a gate electrode of the output transistor to place the output transistor in the non-conductive state when the input voltage is the positive level that is less than the level at which the operational amplifier operates.

9. The voltage regulator according to claim 7 , wherein the detecting circuit includes:

a fifth PMOS transistor that has a source electrode connected to the first power source terminal and a gate electrode connected to a gate electrode of the second PMOS transistor;

a sixth PMOS transistor that has a source electrode connected to a drain electrode of the fifth PMOS transistor, a drain electrode connected to an amplifier circuit, and a gate electrode at which the predetermined reference voltage is applied; and

a seventh PMOS transistor that has a source electrode connected to the first power source terminal, a drain electrode connected to a gate electrode of the output transistor, and a gate electrode connected to the amplifier.

10. A voltage regulator, comprising:

a first power source terminal at which an input voltage can be applied;

a second power source terminal at which a power source reference voltage can be applied;

an output terminal at which an output voltage can be output;

a first p-channel metal oxide semiconductor (PMOS) transistor that has a source electrode connected to the first power source terminal and a drain electrode connected to the output terminal;

a second PMOS transistor that has a gate electrode at which a predetermined bias voltage can be applied and a source electrode connected to the first power source terminal;

a third PMOS transistor that has a gate electrode at which a predetermined reference voltage can be applied and a source electrode connected to a drain electrode of the second PMOS transistor;

a fourth PMOS transistor that has a gate electrode at which a feedback voltage that is proportional to the output voltage can be applied and a source electrode connected to the drain electrode of the second PMOS transistor;

a first n-channel metal oxide semiconductor (NMOS) transistor that has a drain electrode connected to a drain electrode of the third PMOS transistor, a source electrode connected to the second power source terminal, and a gate electrode connected to the drain electrode of the third PMOS transistor;

a second NMOS transistor that has a drain electrode connected a drain electrode of the fourth PMOS transistor, a source electrode connected to the second power source terminal, and a gate electrode connected to the drain electrode of the third PMOS transistor;

a third NMOS transistor that has a drain electrode connected to a gate electrode of the first PMOS transistor and a gate electrode connected to the drain electrode of the fourth PMOS transistor;

a fourth NMOS transistor that has a drain electrode connected to a source electrode of the third NMOS transistor and a source electrode connected to the second power source terminal;

a fifth NMOS transistor that has a gate electrode connected to the drain electrode of the third PMOS transistor and a source electrode connected to the second power source terminal; and

an inverter that has an input terminal connected to a drain electrode of the fifth NMOS transistor and an output terminal connected to a gate electrode of the fourth NMOS transistor.

11. The voltage regulator according to claim 10 , further comprising:

a first constant current source connected between the first power source terminal and the drain electrode of the fifth NMOS transistor; and

a second constant current source connected between the first power source terminal and the source electrode of the third NMOS transistor.

12. The voltage regulator according to claim 10 , wherein the feedback voltage is provided by a voltage dividing circuit connected between the output terminal and the second power source terminal.

13. The voltage regulator according to claim 10 , further comprising:

a first resistor connected between the first power source terminal and the drain electrode of the fifth NMOS transistor; and

a second resistor connected between the first power source terminal and the drain electrode of the third NMOS transistor.