Patent ID: 8084971

Claim:
A motor drive circuit which supplies a drive current to a multiphase motor to drive the motor, the circuit comprising: an energization signal generating circuit which respectively compares a back electromotive force generated in each phase coil of the multiphase motor, with a midpoint voltage of each phase, and which generates an energization signal; a pulse signal generating circuit which generates a pulse signal whose duty ratio is controlled according to torque; a ramp signal generating circuit which divides, into a plurality of times, a period of a frequency generation signal obtained by synthesizing the energization signal and, which for each divided time unit, generates a ramp signal whose pulse width gradually changes and whose frequency is the same as the pulse signal; and an output circuit which supplies a drive current to each phase coil of the multiphase motor based on the energization signal generated by the energization signal generating circuit, the pulse signal generated by the pulse signal generating circuit, and the ramp signal generated by the ramp signal generating circuit; wherein the output circuit selects a coil to be energized from phase coils of the multiphase motor, based on the energization signal, and in a first period the output circuit provides an output circuit signal in response to the ramp signal in which pulse width gradually increases to a control terminal of a transistor connected to one end of the selected coil; in a second period the output circuit provides the output circuit signal in response to the pulse signal; and in a third period the output circuit provides the output circuit signal in response to the ramp signal in which pulse width gradually decreases.