Power supply circuit

1. A power supply circuit for generating an internal power supply voltage from a first power supply capable of supplying a first power supply voltage if connected to a first power supply terminal and a second power supply capable of supplying a second power supply voltage, which is lower than the first power supply voltage, if connected to a second power supply terminal, the first power supply and the second power supply being separate from each other, the power supply circuit comprising:

a first transistor provided between the first power supply terminal and an output node;

a second transistor provided between the second power supply terminal and the output node;

a first supply unit having an input for receiving a voltage representative of a voltage of the first power supply terminal and configured to continuously supply an inverted value of the input to a gate input of said first transistor; and

a second supply unit configured to continuously supply the voltage representative of the voltage of the first power supply to a gate input of said second transistor.

2. A power supply circuit according to claim 1 , wherein the first power supply is an external power supply, and when the first power supply is connected to said power supply circuit, said first supply unit receives an input of the first power supply voltage and outputs a low-level voltage to the gate input of the said first transistor, and

wherein when the first power supply is not connected to said power supply circuit, said first supply unit receives no input of the first power supply voltage and outputs a high-level voltage to the gate input of said first transistor.

3. A power supply circuit, according to claim 1 , for generating the internal power supply voltage from the first power supply, the second power supply and a third power supply capable of supplying a third power supply voltage, which is lower than the second power supply voltage, the power supply circuit further comprising a third transistor provided between the third power supply and the output node,

wherein said second supply unit supplies the output voltage of the first power supply or the voltage corresponding to the output voltage of the first power supply, to a gate input of said third transistor.

4. A power supply circuit, according to claim 2 , for generating the internal power supply voltage from the first power supply, the second power supply and a third power supply capable of supplying a third power supply voltage, which is lower than the second power supply voltage, the power supply circuit further comprising a third transistor provided between the third power supply and the output node,

wherein said second supply unit supplies the output voltage of the first power supply or the voltage corresponding to the output voltage of the first power supply, to a gate input of said third transistor.

5. A power supply circuit according to claim 3 , further comprising:

a fourth transistor provided between the first power supply and the output node;

a fifth transistor provided between the second power supply and the output node; and

a sixth transistor provided between the third power supply and the output node,

wherein a source of said first transistor is connected to the first power supply, a drain of said first transistor is connected to a source of said fourth transistor, and a drain of said fourth transistor is connected to the output node,

wherein a source of said second transistor is connected to the second power supply, a drain of said second transistor is connected to a source of said fifth transistor, and a drain of said fifth transistor is connected to the output node, and

wherein a source of said third transistor is connected to the third power supply, a drain of said third transistor is connected to a source of said sixth transistor, and a drain of said sixth transistor is connected to the output node.

6. A power supply circuit according to claim 4 , further comprising:

a fourth transistor provided between the first power supply and the output node;

a fifth transistor provided between the second power supply and the output node; and

a sixth transistor provided between the third power supply and the output node,

wherein a source of said first transistor is connected to the first power supply, a drain of said first transistor is connected to a source of said fourth transistor, and a drain of said fourth transistor is connected to the output node,

wherein a source of said second transistor is connected to the second power supply, a drain of said second transistor is connected to a source of said fifth transistor, and a drain of said fifth transistor is connected to the output node, and

wherein a source of said third transistor is connected to the third power supply, a drain of said third transistor is connected to a source of said sixth transistor, and a drain of said sixth transistor is connected to the output node.

7. A power supply circuit according to claim 5 , further comprising:

a third supply unit configured to supply the inverted value of an output voltage of the second power supply or the inverted value of a voltage corresponding to the output voltage of the second power supply, to a gate input of said fifth transistor; and

a fourth supply unit configured to supply the output voltage of the second power supply or the voltage corresponding to the output voltage of the second power supply, to a gate input of said sixth transistor.

8. A power supply circuit according to claim 6 , further comprising:

a third supply unit configured to supply the inverted value of an output voltage of the second power supply or the inverted value of a voltage corresponding to the output voltage of the second power supply, to a gate input of said fifth transistor; and

a fourth supply unit configured to supply the output voltage of the second power supply or the voltage corresponding to the output voltage of the second power supply, to a gate input of said sixth transistor.

9. A power supply circuit for generating an internal power supply voltage from one of first and second power supply voltage terminals, comprising:

an input node for continuously receiving a voltage representative of a voltage of the first power supply voltage terminal;

an inverter having an input coupled to the input node, and an output;

a first transistor having a first current electrode coupled to the first power supply voltage terminal, a gate coupled to the output of the inverter, and a second current electrode coupled to an output node, the output node providing the internal power supply voltage; and

a second transistor having a first current electrode coupled to the second power supply voltage terminal, a gate coupled to the input node, and a second current electrode coupled to the output node.

10. The power supply circuit of claim 9 wherein:

the inverter has a power supply input coupled to the second power supply voltage terminal.

11. The power supply circuit of claim 9 wherein:

the first and second transistors are P-channel MOS transistors.

12. The power supply circuit of claim 9 wherein:

the voltage at the first power supply terminal is greater than the voltage at the second power supply terminal when both the first and second power supply voltages are present.

13. The power supply circuit of claim 9 wherein:

the first transistor has a back gate connected to a back gate of the second transistor.

14. The power supply circuit of claim 9 , further comprising:

a third transistor having a first current electrode coupled to a third power supply voltage terminal, a gate coupled to the input node, and a second current electrode coupled to the output node.

15. The power supply circuit of claim 14 , further comprising:

a fourth transistor coupled between the second current electrode of the first transistor and the output node and having a gate for receiving a voltage corresponding to an inverse of a level of the first supply voltage;

a fifth transistor coupled between the second current electrode of the second transistor and the output node and having a gate for receiving a voltage corresponding to an inverse of a level of the second supply voltage; and

a sixth transistor coupled between the second current electrode of the third transistor and the output node and having a gate for receiving a voltage corresponding to the level of the second supply voltage.

16. A method for generating an internal power supply voltage on an output node from one of first and second power supply voltage terminals, comprising:

continuously receiving a voltage representative of a first power supply at a first power supply voltage terminal if the first power supply is connected to the first power supply terminal;

continuously receiving a voltage representative of a second power supply at a second power supply voltage terminal if the second power supply is connected to the second power supply terminal;

if the first power supply is connected to the first power voltage terminal, switching the first power voltage terminal to the output node using a first transistor; and

if the second power supply is connected to the second power voltage terminal and the first power supply is not connected to the first power supply voltage terminal, switching the second power supply voltage terminal to the output node using a second transistor.

17. The method of claim 16 , wherein:

the switching the first power voltage terminal to the output node using the first transistor comprises driving a low voltage on a gate of a first P-channel MOS transistor.

18. The method of claim 17 , wherein:

the driving the low voltage on a gate of the first P-channel MOS transistor comprises inverting a level of the first power supply voltage on the gate of the first P-channel MOS transistor.

19. The method of claim 16 , wherein:

the switching the second power voltage terminal to the output node using the second transistor comprises driving a low voltage on a gate of a second P-channel MOS transistor.

20. The method of claim 19 , wherein:

the driving the low voltage on the gate of the second P-channel MOS transistor comprises providing a level of the first power supply voltage on the gate of the second P-channel MOS transistor.