Power source generation circuit and integrated circuit

1. An integrated circuit comprising:

a regulator circuit which receives an external power source voltage from an external power source and provides a predetermined power source voltage generated internally, and includes a first switching transistor, a source of the first switching transistor being connected to the external power source; and

a charging circuit which includes a second switching transistor having a source connected to the external power source and a drain connected to a first terminal, wherein:

the second switching transistor turns ON to provide the external power source voltage received from the external power source to the first terminal via the second switching transistor when the external power source voltage is below a predetermined threshold voltage,

the first switching transistor is a first p-type Metal Oxide Semiconductor (MOS) transistor,

the second switching transistor is a second p-type MOS transistor having a gate that is controlled based on the external power source voltage and a variable input signal, and

a source-drain path of the first p-type MOS transistor and a source-drain path of the second p-type MOS transistor are not connected in series.

2. The integrated circuit according to claim 1 , wherein a drain of the first p-type MOS transistor and a drain of the second p-type MOS transistor are connected in common to a capacitor.

3. The integrated circuit according to claim 2 , wherein the drain of the first p-type MOS transistor and the drain of the second p-type MOS transistor are connected in common to a capacitor via a third terminal.

4. The integrated circuit according to claim 1 , wherein the second switching transistor can be controlled by a single external input signal.

5. The integrated circuit according to claim 4 , wherein the second switching transistor can be turned OFF by the single external input signal.

6. The integrated circuit according to claim 1 , wherein the second switching transistor can be automatically turned OFF based on a voltage detected internally.

7. The integrated circuit according to claim 1 , wherein:

the variable input signal is applied to a second terminal, and

the second terminal is not on a path between the gate of the second p-type MOS transistor and the external power source.

8. The integrated circuit according to claim 1 , wherein

a gate signal applied to the gate of the second p-type MOS transistor controlled based on the external power source voltage and the variable input signal is not supplied to the regulator circuit.

9. An integrated circuit comprising:

a regulator circuit which receives an external power source voltage from an external power source and provides a predetermined power source voltage generated internally, and includes a first switching transistor, one of a source and a drain of the first switching transistor being connected to the external power source; and

a charging circuit which includes a second switching transistor with one of a source and a drain connected to the external power source and another of the source and the drain connected to a first terminal, wherein:

the second switching transistor turns ON to provide the external power source voltage received from the external power source to the first terminal via the second switching transistor when the external power source voltage is below a predetermined threshold voltage,

the first switching transistor is a first Metal Oxide Semiconductor (MOS) transistor,

the second switching transistor is a second MOS transistor having a gate controlled based on the external power source voltage and a variable input signal applied to a second terminal, and

the second terminal is not on a path between the gate of the second MOS transistor and the external power source.

10. The integrated circuit according to claim 9 , wherein

a gate signal applied to the gate of the second MOS transistor controlled based on the external power source voltage and the variable input signal is not supplied to the regulator circuit.

11. An integrated circuit comprising:

a first circuit which receives an external power source voltage from an external power source and provides a predetermined power source voltage generated internally, and includes a first Metal Oxide Semiconductor (MOS) transistor, one of a source and a drain of the first MOS transistor being connected to the external power source; and

a second circuit which includes a second MOS transistor with one of a source and a drain connected to the external power source and another of the source and the drain connected to a first terminal, wherein:

the second MOS transistor turns ON to provide the external power source voltage received from the external power source to the first terminal via the second MOS transistor when the external power source voltage is below a predetermined threshold voltage,

a gate signal applied to a gate of the second MOS transistor is controlled based on the external power source voltage and a variable signal, and

the gate signal is not supplied to the first circuit.