Stepping motor drive device and method

1. A stepping motor drive device comprising:

a pulse-width modulation control unit operable to pulse-width modulate a supply current from a power supply circuit to a coil that is included in a stepping motor, according to a reference signal showing a current limit value;

a discharge instruction signal output unit operable to output a discharge instruction signal when the current limit value decreases; and

a discharge control unit operable to cause a regenerative current from the coil to flow back into the power supply circuit during a time the discharge instruction signal is outputted.

2. The stepping motor drive device of claim 1 , further comprising

a bridge rectification circuit including a plurality of switching units, and operable to rectify a current through the coil, wherein

the discharge control unit causes the regenerative current from the coil to flow back into the power supply circuit by putting each of the switching units individually in a predetermined state that is one of a conducting state and a non-conducting state.

3. The stepping motor drive device of claim 2 , further comprising

a synchronous rectification control unit operable to cause the regenerative current from the coil to circulate around the bridge rectification circuit by putting at least two of the switching units into the conducting state so as to form a closed circuit with the coil.

4. The stepping motor drive device of claim 1 , wherein

the discharge instruction signal output unit acquires a period-specifying signal that specifies a length of a discharge period, and outputs the discharge instruction signal for the specified discharge period starting when the current limit value decreases.

5. The stepping motor drive device of claim 1 , further comprising:

a coil current measurement unit operable to measure the regenerative current from the coil flowing back into the power supply circuit; and

a comparison unit operable to compare the measured regenerative current with the current limit value, wherein

the discharge instruction signal output unit outputs the discharge instruction signal for a duration starting when the current limit value decreases until the measured regenerative current falls below the decreased current limit value.

6. The stepping motor drive device of claim 5 , further comprising

a resistor connected in series with the coil and the power supply circuit, the regenerative current from the coil flowing back into the power supply circuit through the resistor, wherein

the coil current measurement unit measures the regenerative current based on a voltage across the resistor.

7. The stepping motor drive device of claim 6 , wherein

the supply current from the power supply circuit to the coil, as well as the regenerative current, flows through the resistor,

the coil current measurement unit measures the supply current to the coil based on a voltage across the resistor, and

the pulse-width modulation control unit compares the measured supply current with the current limit value, and pulse-width modulates the supply current according to the comparison.

8. The stepping motor drive device according to claim 6 , further comprising:

a bridge rectification circuit including a plurality of switching units, and operable to rectify a current through the coil, one of the switching units that is put into the conducting state being a first semiconductor element that serves as the resistor;

a reference current supply circuit operable to supply a current at the current limit value; and

a second semiconductor circuit that is connected in series with the reference current supply circuit and is in the conducting state, wherein

the comparison unit compares a first voltage across the first semiconductor element with a second voltage across the second semiconductor voltage, and

the discharge instruction signal output unit outputs the discharge instruction signal for a duration until the second voltage falls below the first voltage.

9. The stepping motor driving device of claim 1 , further comprising

a digital signal reception unit operable to receive a digital signal showing the current limit value, wherein

the stepping motor drive device acquires the reference signal by digital-to-analog converting the received digital signal.

10. The stepping motor drive device of claim 1 , wherein

the stepping motor includes a plurality of coils which correspond one-to-one with a plurality of phases,

the pulse-width modulation control unit pulse-width modulates a supply current to each coil individually, according to a reference signal showing a current limit value for a corresponding coil,

the discharge instruction output signal outputs a discharge instruction signal for each coil of which a current limit value decreases, and

the discharge control signal causes, for a duration of the discharge instruction signal, a regenerative current from a corresponding coil to flow back into the power supply circuit.

11. A stepping motor drive method comprising:

a pulse-width modulation step of pulse-width modulating a supply current from a power supply circuit to a coil that is included in a stepping motor, according to a reference signal showing a current limit value;

a discharge instruction signal output step of outputting a discharge instruction signal when the current limit value decreases; and

a discharge control step of causing a regenerative current from the coil to flow back into the power supply circuit during a time the discharge instruction signal is outputted.