Sensing motor current

1. A circuit configured to sense the driving current of a motor, the circuit comprising:

a driver configured to generate a driving current for a motor;

a first current sensor configured to sense the driving current thereby forming a first sensed current;

a second current sensor configured to sense the driving current thereby forming a second sensed current, the second current sensor being more thermally stable than the first current sensor;

a comparator configured to compare the first sensed current and the second sensed current below a threshold frequency to generate a thermal calibration output; and

a calibrator configured to calibrate the first sensed current by the thermal calibration output to form a sensed driving current.

2. A circuit as claimed in claim 1 , wherein the driver is configured to generate the driving current in response to a control input, wherein the control input is the sensed driving current.

3. A circuit as claimed in claim 1 , wherein the first current sensor has a higher operational frequency bandwidth than the second current sensor.

4. A circuit as claimed in claim 3 , wherein the driving current frequency range is within the operational frequency range of the first current sensor and outside the operational frequency range of the second current sensor.

5. A circuit as claimed in any claim 1 , further comprising:

a first low pass filter configured to attenuate frequencies above the threshold frequency to form a filtered first sensed current; and

a second low pass filter configured to attenuate frequencies above the threshold frequency to form a filtered second sensed current;

wherein the comparator is configured to compare the filtered first sensed current and the filtered second sensed current to generate the thermal calibration output.

6. A circuit as claimed in claim 1 , wherein the thermal calibration output is a linear offset, and the calibrator is configured to calibrate the first sensed current by adding the linear offset to the first sensed current to form the sensed driving current.

7. A circuit as claimed in claim 1 , further comprising a fault detector configured to:

compare the thermal calibration output to a predetermined threshold; and

generate a fault signal if the thermal calibration output is greater than the predetermined threshold.

8. A circuit as claimed in claim 1 , wherein the second current sensor is a current sense resistor.

9. A circuit as claimed in claim 8 , wherein the current sense resistor comprises:

a shunt resistor connected between the driver and the motor; and

a differential amplifier connected across the shunt resistor, the differential amplifier configured to generate a differential mode signal proportional to the driving current.

10. A circuit as claimed in claim 9 , wherein the current sense resistor further comprises a third low pass filter connected between the shunt resistor and the differential amplifier, the third low pass filter configured to attenuate components common to the inputs to the differential amplifier.

11. A circuit as claimed in claim 10 , wherein the third low pass filter comprises a common-mode choke.

12. A circuit as claimed in claim 1 , wherein the third low pass filter further comprises a shunt capacitor which connects the common-mode choke to ground.

13. A circuit as claimed in claim 1 , wherein the first current sensor is a Hall sensor.

14. A method of sensing the driving current of a motor comprising:

at a driver, generating a driving current for a motor;

at a first current sensor, sensing the driving current thereby forming a first sensed current;

at a second current sensor, sensing the driving current thereby forming a second sensed current, wherein the second current sensor is more thermally stable than the first current sensor;

generating a thermal calibration output by comparing the first sensed current and the second sensed current below a threshold frequency;

calibrating the first sensed current by the thermal calibration output to form a sensed driving current.

15. A method as claimed in claim 14 , further comprising generating the driving current in response to a control input, wherein the control input is the sensed driving current.

16. A method as claimed in claim 14 , wherein the first current sensor has a higher operational frequency bandwidth than the second current sensor.

17. A method as claimed in claim 16 , wherein the driving current frequency range of the driver is within the operational frequency range of the first current sensor and outside the operational frequency range of the second current sensor.

18. A method as claimed in claim 14 , further comprising:

at a first low pass filter, attenuating frequencies above the threshold frequency to form a filtered first sensed current;

at a second low pass filter, attenuating frequencies above the threshold frequency to form a filtered second sensed current; and

generating the thermal calibration output by comparing the filtered first sensed current and the filtered second sensed current.

19. A method as claimed in claim 14 , wherein the thermal calibration output is a linear offset, the method comprising calibrating the first sensed current by adding the linear offset to the first sensed current to form the sensed driving current.

20. A method as claimed in claim 14 , further comprising:

comparing the thermal calibration output to a predetermined threshold; and

generating a fault signal if the thermal calibration output is greater than the predetermined threshold.