Driving device for three-phase brushless motor

A three-phase brushless motor driving device for supplying a three-phase brushless motor with drive currents, comprising:

• a comparator circuit for comparing back electromotive forces appearing in three coils provided one for each phase in the three-phase brushless motor individually with a common-terminal voltage of the three coils to produce square-wave signals;
• gate circuits for controlling passage of the square-wave signals individually;
• a drive waveform synthesizing circuit for synthesizing together outputs of the gate circuits to produce drive signals;
• an FG circuit for producing a speed signal from the outputs of the gate circuits;
• a phase shifting circuit for producing from the speed signal a phase signal having a phase shifted by 30 degrees relative to the speed signal;
• a current supply circuit for supplying the drive currents to the three-phase brushless motor in accordance with the drive signals and the phase signal;
• and
• a selector circuit for feeding a select signal back to the gate circuits in accordance with the drive signals and the phase signal,
• wherein the gate circuits allow passage of the square-wave signals individually only in a predetermined period.