Driving circuit for direct-current (dc) motor

Abstract

A motor driving circuit capable of controlling a motor speed by a current controlled method using negative impedance is constituted by a constant-current circuit, a reference voltage generating circuit, a comparator circuit, an output driving circuit, a symmetric current circuit, and a switching circuit. The reference voltage generating circuit outputs a voltage of a value derived from addition of a reference voltage and a voltage proportional to a value of a current flowing to a motor. The comparator circuit compares a voltage outputted from the reference voltage generating circuit and a voltage at an output terminal. The output driving circuit amplifies an output of the comparator circuit. The symmetric current circuit amplifies the output from the output driving circuit. The switching circuit performs switching of connections of one of transistors in the symmetric current circuit between the input of the reference voltage generating circuit and the output terminal, and can release the motor from a normal state of operation upon starting the motor. Thus, it is possible to achieve sufficient torque during the starting operation of the motor.