Self calibrating RC oscillator
There is disclosed a self-calibrating resistor-capacitor (RC) oscillator in which a resistor has a resistance value varied minimally by temperature change and process variation, a capacitor has a capacitance value selected adequately according to needs, and the resistor and capacitor are configurable as a one-chip by a complementary metal-oxide semiconductor (CMOS) process. The self-calibrating RC oscillator comprises a resistor part including a first resistor having a resistance value reduced with increase in temperature and a second resistor connected in series with the first resistor and having a resistance value increasing with increase in temperature.
1. A self-calibrating resistor-capacitor oscillator for generating an oscillation signal comprising:
a resistor part including a first resistor having a resistance value reduced with increase in temperature and a second resistor connected in series with the first resistor and having a resistance value increasing with increase in temperature, the resistor part having a resistance value formed by the first and second resistors connected in series with each other;
a bias circuit generating a preset first reference voltage, and a ramp current to be proportional to the first reference voltage and inversely proportional to the resistance value of the resistor part;
a capacitor part having a capacitance value;
a ramp and hold circuit supplying the ramp current to the capacitor part to charge the capacitor part during a charging time set according to a period of the oscillation signal, and maintaining a both-end voltage of the capacitor part after the charging;
a first comparator comparing a size of the first reference voltage with a size of the both-end voltage of the capacitor part;
a first charge pump for outputting a control voltage varying in response to a control signal;
a control logic outputting the control signal for increasing and decreasing the control voltage outputted from the first charge pump in response to a comparison result of the first comparator, receiving the oscillation signal through feed-back, and setting the charging time during which the ramp and hold circuit supplies the ramp current to the capacitor part, in response to the received oscillation signal; and
an oscillator part for outputting the oscillation signal having a frequency controlled in response to the control voltage of the first charge pump.
2. The self-calibrating resistor-capacitor oscillator of claim 1 , wherein the capacitor part comprises:
a plurality of switches connected in parallel with one another to a terminal to which the ramp current is inputted; and
a plurality of capacitors each having one end connected to a corresponding one of the switches and another end grounded,
wherein the control logic controls on/off of the switches.
3. The self-calibrating resistor-capacitor oscillator of claim 2 , further comprising:
a crystal resonance part generating a reference frequency from a resonance signal generated from a crystal resonator;
a phase frequency detector for comparing a frequency of the oscillation signal with the reference frequency;
a second charge pump for outputting a comparison voltage controlled in response to a comparison result of the phase frequency detector; and
a second comparator for comparing the comparison voltage outputted from the second charge pump with a preset second reference voltage to provide to the control logic,
wherein the control logic controls on/off of the switches in response to a comparison result of the second comparator to determine the capacitance value of the capacitor part.
4. The self-calibrating resistor-capacitor oscillator of claim 3 , further comprising a storage for storing on/off status of the switches in response to the comparison result of the second comparator.