Variable-capacitance device

A variable-capacitance device comprising:

• a first capacitance element coupled between a first power supply terminal and an output terminal;
• a capacitance selection switch configured to turn on or off in accordance with a capacitance switching signal;
• a second capacitance element coupled in parallel to the first capacitance element and in series to the capacitance selection switch;
• and
• an error correction circuit configured to operate such that in a state in which the capacitance selection switch is in an OFF state, in response to a charge reset signal that causes a voltage at the output terminal to be reset to a reset voltage, the error correction circuit substantially eliminates a difference between the voltage at the output terminal and a voltage at a capacitance switching node at which the second capacitance element is coupled to the capacitance selection switch,
• wherein the variable-capacitance device is used as a capacitance element in an oscillation circuit whose oscillation frequency is set by a constant current set by resistance and capacitance of the capacitance element, and
• wherein the oscillation circuit includes:
• a filter capacitor configured to generate an oscillation frequency control voltage depending on the amount of a charge stored in the filter capacitor based on an oscillation frequency setting current;
• an oscillator configured to output an oscillation signal with a frequency varying according to the oscillation frequency control voltage;
• a control circuit configured to generate a timing control signal with a logical level that is switched based on the period of the oscillation signal;
• a frequency detection circuit including the variable-capacitance device and configured to generate a frequency detection voltage with a voltage level varying according to the length of the period of the oscillation signal and a charging/discharging current of the variable-capacitance device based on the timing control signal;
• and
• a differential amplifier configured to continuously changing the oscillation frequency setting current according to a voltage difference between the frequency detection voltage and a reference voltage and output the resultant oscillation frequency setting current to the filter capacitor.