Patent ID: 7342367

Claim:
A motor drive circuit for driving a sensorless brushless DC motor provided with a rotor and a plurality of coils for rotating the rotor, providing: a voltage controlled type variable frequency oscillation circuit operating by a frequency in accordance with a voltage applied to a control terminal and generating a clock signal; a drive pulse generator for generating a plurality of drive signals having a cycle defined by the frequency of the clock signal and indicating drive circuit phases having sequentially different phase differences based on the clock signal from the variable frequency oscillation circuit; a rotor phase detector for detecting the rotor phase from phases of induction voltages induced in the plurality of coils when supplying said plurality of drive signals to said plurality of coils and rotating said rotor; and a phase comparison circuit for comparing said detected rotor phase and a drive circuit phase included in the drive signal generated by said drive pulse generator, outputting an advance phase difference signal to said voltage controlled type variable frequency oscillation circuit within a range not exceeding a defined advance maximum duration when said rotor phase is advanced from said drive circuit phase, and outputting a delay phase difference signal to said voltage controlled type variable frequency oscillation circuit within a range not exceeding a defined delay maximum duration when said rotor phase is later than said drive circuit phase, wherein said maximum duration is defined longer than the advance maximum duration.