Patent ID: 7235938

Claim:
A motor driver comprising: a back electromotive force detection section that detects a back electromotive force appearing in a motor coil of a direct-current motor and outputs a detection signal; and a speed control section that, based on the back electromotive force detected by the back electromotive force detection section, sets and outputs a current value of a drive current fed to the motor coil, wherein the motor driver further comprises: an offset calculation section that first establishes a state in which the direct-current motor cannot be driven via the motor coil and thus no back electromotive force appears, that then reads, as a first output, an output of the back electromotive force detection section as obtained when the current value of the drive current fed to the motor coil is made equal to zero, that then reads, as a second output, the output of the back electromotive force detection section as obtained when the current value of the drive current fed to the motor coil is made equal to a predetermined value, that then calculates, based on a difference between the first and second outputs, an offset that appears in the output of the back electromotive force detection section as a result of a change in an internal resistance of the motor coil, and that then eliminates the offset by changing an amplification factor of the back electromotive force detection section.