Drive circuit for a three-phase brushless motor

A three-phase brushless motor drive circuit that switches its output voltages by producing drive signals displaced in phase relative to one another, comprising:

• voltage comparators, provided one for each phase, for comparing output voltages appearing across coils of a three-phase motor individually with a voltage at a terminal common to three phases;
• a reference clock generating circuit for generating a reference clock based on voltages outputted from the voltage comparators, said reference clock being displaced in phase by 60.degree. relative to switching timing of the output voltages and having one third of a period of the output voltages;
• a detecting circuit for detecting edges of the reference clock;
• a noise reduction circuit for reducing noise components that are generated by noises included in the output voltages when the reference clock is subjected to edge detection;
• an RS flip-flop for forming a pulse-train signal by using as a set signal an edge detection output obtained through the noise reduction circuit;
• a circuit for controlling charging and discharging of one capacitor in synchronism with the pulse-train signal;
• a first voltage comparator for performing voltage comparison between a sawtooth voltage appearing across the capacitor and a first reference voltage to reset the RS flip-flop in accordance with a result of the voltage comparison;
• means for forming three-phase drive signals in synchronism with the pulse-train signal outputted from the RS flip-flop;
• a second voltage comparator for comparing the sawtooth voltage with a second reference voltage;
• and
• means for feeding pulses outputted from the second voltage comparator to the noise reduction circuit to achieve noise reduction.