Control circuit for a brushless dc motor and method therefor

A control circuit for a brushless direct current (DC) motor comprising:

• a current drive circuit having an input for receiving a control signal, a first output adapted to be coupled to the brushless DC motor, and a second output for providing a current sense signal;
• a current loop regulator having a first input for receiving said current sense signal, a second input for receiving a polarity signal, a first output for providing said control signal, and a second output for providing a duty cycle signal indicative of a duty cycle of said control signal;
• and
• a commutation loop regulator having an first input for receiving said duty cycle signal, and an output for providing said polarity signal, wherein said commutation loop regulator changes said polarity signal to commutate the brushless DC motor in response to a comparison of a pre-commutation duty cycle value and a post-commutation duty cycle value.

A method of controlling a brushless direct current (DC) motor comprising:

• regulating current through a winding of the brushless DC motor to substantially equal a positive reference current;
• determining a pre-commutation duty cycle value of a control signal provided to a current drive circuit;
• commutating said current;
• regulating said current through said winding of the brushless DC motor to substantially equal a negative reference current;
• determining a post-commutation duty cycle value of said control signal;
• decreasing a next pre-commutation duty cycle if said post-commutation duty cycle value is less than said pre-commutation duty cycle value;
• and
• increasing said next pre-commutation duty cycle value if said pre-commutation duty cycle is greater than said post-commutation duty cycle value.