Semiconductor integrated circuit for sensorless driving and sensorless driving system

A sensorless driving system for sensorless driving of a brushless motor, comprising:

• an induced voltage detecting circuit which includes a comparator for comparing a voltage induced in an exciting coil by a rotation of a rotor of the brushless motor with a midpoint voltage of the rotor of the brushless motor and outputting a detection signal corresponding to a comparison result, and detects a zero cross point where the induced voltage crosses the midpoint voltage;
• a command circuit that outputs a command signal for regulating an operation of the brushless motor, a logic circuit that outputs a control signal for controlling the brushless motor, in response to the command signal and an output signal of the comparator;
• and
• a power transistor circuit that supplies a driving current to the exciting coil, the power transistor including a first transistor having one end connected to a power supply and the other end connected to the exciting coil and being controlled in response to the control signal, and a second transistor connected between the other end of the first transistor and a ground and being controlled in response to the control signal,
• wherein the comparator has an in-phase input voltage range in which a voltage not larger than a ground voltage can be detected, the comparator being at least so sensitive as to detect a change of a voltage waveform generated according to the induced voltage during the short brake, and
• during a short brake in which the brushless motor is braked by turning off the first transistor and turning on the second transistor to short-circuit the exciting coil of the brushless motor with the ground,
• the comparator compares a voltage generated according to the induced voltage and the midpoint voltage of the rotor of the brushless motor and outputs the detection signal according to a comparison result, and the logic circuit decides a rotational position of the rotor based on the detection signal.

A semiconductor integrated circuit for sensorless driving of a three-phase brushless motor, comprising:

• an induced voltage detecting circuit which includes three comparators for comparing voltages induced in three exciting coils by a rotation of a rotor of the three-phase brushless motor with a midpoint voltage of the rotor of the brushless motor and outputting detection signals corresponding to comparison results, and detects zero cross points where the induced voltages cross the midpoint voltage;
• a logic circuit that outputs a control signal for controlling the three-phase brushless motor, in response to a command signal for regulating an operation of the three-phase brushless motor and output signals of the comparators;
• and
• a power transistor circuit that supplies a driving current to the exciting coil, the power transistor including a first transistor having one end connected to a power supply and the other end connected to the exciting coil and being controlled in response to the control signal, and a second transistor connected between the other end of the first transistor and a ground and being controlled in response to the control signal,
• wherein the comparator has an in-phase input voltage range in which a voltage not larger than a ground voltage can be detected, the comparator being at least so sensitive as to detect a change of a voltage waveform generated according to the induced voltage during the short brake, and
• during a short brake in which the brushless motor is braked by turning off the first transistor and turning on the second transistor to short-circuit the exciting coil of the brushless motor with the ground,
• the comparator compares a voltage generated according to the induced voltage and the midpoint voltage of the rotor of the brushless motor and outputs the detection signal according to a comparison result, and the logic circuit decides a rotational position of the rotor based on the detection signal.