Motor driving circuit and electronic apparatus using the same

1. A motor driving circuit for driving a motor, comprising:

a detecting circuit configured to detect a detection signal indicative of a current state of the motor;

a command value generating unit configured to generate a command value indicative of a target state of the motor;

an error amplifier configured to generate an error signal by amplifying an error between the detection signal and the command value;

a pulse width modulator configured to generate a pulse signal having a duty cycle corresponding to the error signal; and

an output circuit configured to apply a switching voltage having a duty cycle corresponding to the pulse signal across the motor,

wherein the command value generating unit is configured to provide a variation to the command value,

wherein the command value generating unit includes:

a set voltage generating part configured to generate a set voltage; and

a first D/A converter configured to generate the command value, the first D/A converter having a reference voltage terminal receiving the set voltage and an input terminal receiving a digital value representing the command value,

wherein the set voltage generating part is configured to provide a variation to the set voltage, and

wherein the set voltage generating part modulates the set voltage with a predetermined frequency.

2. The motor driving circuit of claim 1 , wherein the set voltage generating part includes:

a set data generating portion configured to generate digital set data corresponding to the set voltage; and

a second D/A converter configured to convert the set data into an analog set voltage, and

wherein the set data generating portion is configured to provide a variation to the set data.

3. The motor driving circuit of claim 2 , wherein the set data generating portion varies the set data by one LSB (Least Significant Bit) at a predetermined period.

4. The motor driving circuit of claim 1 , wherein the command value generating unit includes:

a micro step generating part configured to generate a digital value representing the command value and

wherein the micro step generating part is configured to provide a variation to the digital voltage.

5. The motor driving circuit of claim 1 , wherein the detecting circuit generates a detection signal representing a voltage applied to the motor.

6. The motor driving circuit of claim 5 , wherein the detecting circuit is configured to have a first output terminal from which one of voltages of first and second terminals of the motor is output, the one voltage having a fixed voltage level, and a second output terminal from which the other of voltages of first and second terminals of the motor is output, the other voltage being switched, and

wherein the error amplifier includes:

an operational amplifier;

a first capacitor connected between an inverted input terminal of the operational amplifier and a non-inverted input terminal thereof;

a first resistor connected between the non-inverted input terminal of the operational amplifier and the first output terminal of the detecting circuit;

a second resistor connected between the non-inverted input terminal of the operational amplifier and an output terminal of the command value generating unit;

third and fourth resistors connected in series between the second output terminal of the detecting circuit and the inverted input terminal of the operational amplifier; and

a fifth resistor connected between a node between the third resistor and the fourth resistor and a ground terminal.

7. The motor driving circuit of claim 1 , wherein the detecting circuit generates a detection signal representing a current flowing into the motor.

8. The motor driving circuit of claim 1 , wherein the detecting circuit generates a detection signal representing a speed of the motor.

9. A motor driving circuit for driving a motor, comprising:

a detecting circuit configured to detect a detection signal indicative of a current state of the motor;

a command value generating unit configured to generate a command value indicative of a target state of the motor;

an error amplifier configured to generate an error signal by amplifying an error between the detection signal and the command value;

a pulse width modulator configured to generate a pulse signal having a duty cycle corresponding to the error signal; and

an output circuit configured to apply a switching voltage having a duty cycle corresponding to the pulse signal across the motor,

wherein a feedback loop including the error amplifier is configured to vary a time constant of the feedback loop,

wherein the command value generating unit includes:

a set voltage generating part configured to generate a set voltage; and

a first D/A converter configured to generate the command value, the first D/A converter having a reference voltage terminal receiving the set voltage and an input terminal receiving a digital value representing the command value,

wherein the set voltage generating part is configured to provide a variation to the set voltage, and

wherein the set voltage generating part modulates the set voltage with a predetermined frequency.

10. The motor driving circuit of claim 9 , wherein the time constant is set to have a larger value if the command value remains at a constant value for a predetermined period.

11. The motor driving circuit of claim 9 , wherein the error amplifier includes:

an operational amplifier; and

a variable feedback capacitor connected between an output of the operational amplifier and an input thereof.

12. The motor driving circuit of claim 9 , wherein the error amplifier includes:

an operational amplifier having a non-inverted input terminal to which a reference voltage is input;

a sixth resistor and a second capacitor connected in series between an output terminal of the operational amplifier and an inverted input terminal thereof;

a third capacitor connected between the output terminal of the operational amplifier and an inverted terminal thereof;

a seventh resistor connected between a terminal to which the command value is input and the inverted input terminal of the operational amplifier; and

an eighth resistor connected between a terminal to which the detection signal is input and the inverted input terminal of the operational amplifier.

13. The motor driving circuit of claim 9 , wherein the detecting circuit generates a detection signal representing a current flowing into the motor.

14. The motor driving circuit of claim 13 , wherein the detecting circuit includes:

a detecting resistor connected in series to the motor; and

a sense amplifier configured to amplify a voltage drop of the detecting resistor.

15. The motor driving circuit of claim 9 , wherein the detecting circuit generates a detection signal representing a voltage applied to the motor.

16. The motor driving circuit of claim 9 , wherein the detecting circuit generates a detection signal representing a speed of the motor.

17. The motor driving circuit of claim 1 , wherein the motor driving circuit is integrated into a single semiconductor substrate.

18. An electronic apparatus comprising:

a motor; and

a motor driving circuit of claim 1 for driving the motor.