Index signal generating circuit

An index signal generating circuit comprising a magnet attached onto the circumference of the rotor of a spindle motor, a magnetic flux detecting device for detecting magnetic flux of the magnet when the magnet moves past the magnetic flux detecting device, a Hall effect device for switching a current in the driving coil of the spindle motor, a waveform shaping circuit for shaping the output of the Hall effect device into a rectangular-wave signal, and a feedback type comparator circuit for comparing the rectangular-wave signal fed from the waveform shaping circuit to a first input of the comparator circuit with the detected output of the magnetic flux detecting device fed to a second input of the comparator circuit to produce a comparison output, wherein the level of the detected output of the magnetic flux detecting device fed to the second input with no magnetic flux being detected is set to be lower than the negative polarity level of the rectangular-wave signal;

• the level of the detected output of the magnetic flux detecting device obtained during the negative polarity duration of the rectangular-wave signal is set to be higher than the negative polarity level of the rectangular-wave signal;
• and an index signal is generated based on the timing of a transition of the comparison output of the comparator;
• a transistor connected to switch the rectangular-wave signal that is periodically transitioned between a voltage level V1 and a voltage level V2 to the comparator circuit;
• a bias voltage power supply connected to superimpose a direct current voltage on the detected output of the magnetic flux detecting device such that the voltage level of the magnetic flux detecting device with no magnetic flux detected is lower than the two voltage levels V1 and V2 of the rectangular-wave signal and such that the voltage level of a positive going peak of the magnetic flux detecting device with a magnetic flux detected is between the two voltage levels V1 and V2.