Communication circuit
In a communication circuit, an RFIC includes an IO terminal and a control IC includes an IO terminal. A variable capacitance element includes control terminals, a capacitance element with a capacitance value that is determined according to a control voltage, and a resistance voltage divider circuit configured to generate the control voltage by dividing a voltage inputted to the control terminals. One of the RFIC and the control IC supplies control data to the variable capacitance element via a signal line. The variable capacitance element, along with an antenna coil, constitutes an antenna circuit of an LC parallel resonance circuit, and sets a resonant frequency of the antenna circuit to be a predetermined frequency.
1. A communication circuit comprising:
an RFIC configured to perform modulation and demodulation between a baseband signal and a high-frequency signal;
a control IC configured to control the RFIC by receiving and transmitting data including communication data;
an antenna coil; and
a variable capacitance element configured to change a resonant frequency of an antenna circuit including the antenna coil; wherein
an IO terminal of the RFIC and an IO terminal of the control IC are connected by a signal line, and control terminals of the variable capacitance element are connected to at least a portion of the signal line;
the variable capacitance element includes a capacitance element with a capacitance value that is determined according to a control voltage, and a resistance voltage divider circuit configured to generate the control voltage by dividing a voltage inputted to the control terminals; and
the resistance voltage divider circuit includes a plurality of resistances each including a first terminal connected to each of the control terminals, and a common line to which second terminals of the resistances are connected in common and through which the control voltage is outputted, and resistance values of the plurality of resistances are determined to be in a ratio based on powers of 2 based on a lowest value among the resistance values.
2. The communication circuit according to claim 1 , further comprising an external IO terminal connected to the signal line and configured to receive and transmit a signal from and to an external circuit.
3. The communication circuit according to claim 1 , wherein the variable capacitance element and the RFIC are configured as a single monolithic IC.
4. The communication circuit according to claim 1 , wherein the RFIC includes an operational mode switch configured to switch an operational mode of the RFIC based on a signal outputted from the IO terminal of the control IC when the RFIC is turned on.
5. The communication circuit according to claim 1 , wherein one of the RFIC and the control IC is configured to determine a capacitance setting mode of the variable capacitance element when power is turned on, and both of the RFIC and the control IC are configured to output control data for the variable capacitance element to the IO terminal.
6. The communication circuit according to claim 1 , wherein the RFIC and the control IC are connected to each other via data transmission lines.
7. The communication circuit according to claim 1 , wherein the antenna circuit is an LC parallel resonance circuit.
8. The communication circuit according to claim 1 , further comprising capacitors configured and arranged to adjust a degree of coupling between the RFIC and the antenna coil.
9. The communication circuit according to claim 1 , wherein an impedance from the RFIC to the antenna coil is configured to change based on an operation mode of the communication circuit.
10. The communication circuit according to claim 1 , further comprising electrostatic discharge elements connected between the antenna coil and ground.
11. The communication circuit according to claim 1 , wherein the variable capacitance element includes a control voltage application circuit and a variable capacitance unit.
12. The communication circuit according to claim 11 , wherein the control voltage application circuit includes resistance elements connected to the RFIC and in common.
13. The communication circuit according to claim 11 , wherein the variable capacitance unit includes capacitance elements including a ferroelectric film between a pair of electrodes, and resistance elements having the same or substantially the same resistance value.
14. The communication circuit according to claim 3 , wherein the single monolithic IC includes a Si substrate including a circuit unit of the RFIC and a circuit unit of the variable capacitance element provided thereon.
15. The communication circuit according to claim 3 , further comprising a static capacitance element connected in series to the variable capacitance element.
16. The communication circuit according to claim 1 , wherein one of the RFIC and the control IC is configured to output control data for the variable capacitance element to the IO terminal.
17. A Radio Frequency Identification system comprising the communication circuit according to claim 1 .
18. A Near Field Communication system comprising the communication circuit according to claim 1 .