Contactless communication circuit and portable terminal

1. A contactless communication circuit having both a contactless communication card function and a contactless communication reader/writer function, comprising:

a capacitor section that forms a parallel-resonant circuit with a contactless communication antenna; and

a capacitance switching section that makes a capacitance of the capacitor section effective in a mode of the contactless communication card function, and reduces the capacitance of the capacitor section in a mode of the contactless communication reader/writer function.

2. The contactless communication circuit according to claim 1 , wherein:

the capacitor section is a capacitor connected at one end thereof to one end of the contactless communication antenna, and

the capacitance switching section has:

a first field-effect transistor inserted between the other end of the capacitor and a ground,

a second field-effect transistor inserted between the other end of the contactless communication antenna and a ground, and

a control section that turns on the first and second field-effect transistors in the mode of the contactless communication card function and turns off the first and second field-effect transistor in the mode of the contactless communication reader/writer function.

3. A contactless communication circuit having both a contactless communication card function and a contactless communication reader/writer function, comprising:

a main antenna connected to a main circuit for contactless communication;

a sub-antenna coupled with the main antenna;

a capacitor section that forms a parallel-resonant circuit with the sub-antenna; and

a capacitance switching section that makes a capacitance of the capacitor section effective in a mode of the contactless communication card function, and reduces the capacitance of the capacitor section in a mode of the contactless communication reader/writer function.

4. The contactless communication circuit according to claim 3 , wherein:

the capacitor section is a capacitor that forms the parallel-resonant circuit with the sub-antenna, and

the capacitance switching section has:

a field-effect transistor connected within the parallel-resonant circuit formed of the capacitor and the sub-antenna, and

a control section that turns on the field-effect transistor in the mode of the contactless communication card function and turns off the field-effect transistor in the mode of the contactless communication reader/writer function.

5. The contactless communication circuit according to claim 3 , wherein:

the capacitor section is a capacitor that forms the parallel-resonant circuit with the sub-antenna, and

the capacitance switching section has:

a field-effect transistor inserted between a ground and a node between the capacitor and the sub-antenna, and

a control section that turns on the field-effect transistor in the mode of the contactless communication card function and turns off the field-effect transistor in the mode of the contactless communication reader/writer function.

6. A contactless communication circuit having at least a contactless communication reader/writer function, comprising:

a capacitor connected at one end thereof to one end of a contactless communication antenna;

a first field-effect transistor inserted between the other end of the capacitor and a ground;

a second field-effect transistor inserted between the other end of the contactless communication antenna and a ground; and

a control section that turns on the first and second field-effect transistors when communicating with an anticollision contactless communication card, and turns off the first and second field-effect transistors when communicating with a non-anticollision contactless communication card.

7. A contactless communication circuit having at least a contactless communication reader/writer function, comprising:

a main antenna connected to a main circuit for contactless communication;

a sub-antenna coupled with the main antenna;

a capacitor that forms a parallel-resonant circuit with the sub-antenna;

a field-effect transistor connected within the parallel-resonant circuit formed of the capacitor and the sub-antenna; and

a control section that turns on the field-effect transistor when communicating with an anticollision contactless communication card, and turns off the field-effect transistor when communicating with a non-anticollision contactless communication card.

8. A contactless communication circuit having at least a contactless communication reader/writer function, comprising:

a main antenna connected to a main circuit for contactless communication;

a sub-antenna coupled with the main antenna;

a capacitor that forms a parallel-resonant circuit with the sub-antenna;

a field-effect transistor inserted between a ground and a node between the capacitor and the sub-antenna; and

a control section that turns on the field-effect transistor when communicating with an anticollision contactless communication card, and turns off the field-effect transistor when communicating with a non-anticollision contactless communication card.

9. A portable terminal having both a contactless communication card function and a contactless communication reader/writer function, comprising:

a capacitor section that forms a parallel-resonant circuit with a contactless communication antenna;

a capacitance switching section that switches a capacitance of the capacitor section; and

a control section that controls the capacitance switching section to make the capacitance of the capacitor section effective in a mode of the contactless communication card function, and controls the capacitance switching section to reduce the capacitance of the capacitor section in a mode of the contactless communication reader/writer function.

10. A portable terminal having both a contactless communication card function and a contactless communication reader/writer function, comprising:

a main antenna connected to a main circuit for contactless communication;

a sub-antenna coupled with the main antenna;

a capacitor section that forms a parallel-resonant circuit with the sub-antenna;

a capacitance switching section that switches a capacitance of the capacitor section; and

a control section that controls the capacitance switching section to make the capacitance of the capacitor section effective in a mode of the contactless communication card function, and controls the capacitance switching section to reduce the capacitance of the capacitor section in a mode of the contactless communication reader/writer function.

11. A portable terminal having at least a contactless communication reader/writer function, comprising:

a capacitor connected at one end thereof to one end of a contactless communication antenna;

a first field-effect transistor inserted between the other end of the capacitor and a ground;

a second field-effect transistor inserted between the other end of the contactless communication antenna and a ground; and

a control section that turns on the first and second field-effect transistors when communicating with an anticollision contactless communication card, and turns off the first and second field-effect transistors when communicating with a non-anticollision contactless communication card.

12. A portable terminal having at least a contactless communication reader/writer function, comprising:

a main antenna connected to a main circuit for contactless communication;

a sub-antenna coupled with the main antenna;

a capacitor that forms a parallel-resonant circuit with the sub-antenna;

a field-effect transistor connected within the parallel-resonant circuit formed of the capacitor and the sub-antenna; and

a control section that turns on the field-effect transistor when communicating with an anticollision contactless communication card, and turns off the field-effect transistor when communicating with a non-anticollision contactless communication card.

13. A portable terminal having at least a contactless communication reader/writer function, comprising:

a main antenna connected to a main circuit for contactless communication;

a sub-antenna coupled with the main antenna;

a capacitor that forms a parallel-resonant circuit with the sub-antenna;

a field-effect transistor inserted between a ground and a node between the capacitor and the sub-antenna; and

a control section that turns on the field-effect transistor when communicating with an anticollision contactless communication card, and turns off the field-effect transistor when communicating with a non-anticollision contactless communication card.