Electronic circuit and method for synchronizing electric motor drive signals between a start-up mode of operation and a normal mode of operation

1. A method of driving, with a motor control circuit, a multi-phase motor having a plurality of motor windings, the method comprising:

generating, with the motor control circuit, a plurality of drive signals comprising determined start-up signal characteristics to start the multi-phase motor spinning, wherein the determined start-up signal characteristics of the plurality of drive signals comprise a ramped start-up of the electric motor during a ramp-up time period during a start-up mode of operation, wherein the plurality of drive signals results in a corresponding plurality of sinusoidal current signals carried by the plurality of motor windings, and wherein the generating comprises generating the plurality of drive signals such that, at an end of the ramp-up time period, a last one of the plurality of cycles of the sinusoidal current signal ends at a zero current, the method further comprising

stopping the plurality of drive signals for a coasting time period following the end of the ramp-up time period;

detecting a back EMF signal indicative of a zero back EMF voltage on the selected one of the plurality of motor windings while the plurality of drive signals are stopped, the detected back EMF signal indicative of an end of the coasting time period; and

driving the plurality of motor windings with the plurality of sinusoidal current signals in a normal mode of operation in response to the detecting.

2. The method of claim 1 , wherein, at a beginning of the normal mode of operation, a first cycle of the plurality of sinusoidal current signals on the selected one of the plurality of motor windings begins at the zero current and at a phase continuous with a phase of the last one of the plurality of cycles of the sinusoidal current signal on the selected one of the plurality of motor windings at the end of the ramp-up time period.

3. The method of claim 2 , further comprising:

adjusting, at the start of the normal mode of operation, the plurality of drive signals to result in smaller amplitudes of the plurality of sinusoidal current signals than at the end of the ramp-up time period.

4. The method of claim 2 , wherein the generating the plurality of drive signals comprises:

generating a corresponding plurality of modulated PWM waveforms, the plurality of modulated PWM waveforms having respective duty cycles that change with time according to a sine-drive modulation envelope, wherein the sine-drive modulation envelope has a frequency, wherein each one of the plurality of modulated PWM waveforms has a peak duty cycle, wherein the peak duty cycle of each one of the plurality of modulated PWM waveforms is related to an amplitude of a respective one of a plurality of sinusoidal current signals carried by the plurality of motor windings, and wherein the frequency of the modulation envelope is related to a frequency of the plurality of sinusoidal current signals.

5. The method of claim 2 , wherein the coasting time period has a coasting time duration corresponding to an integer number of cycles of the plurality of sinusoidal current signals proximate to the end of the ramp-up time period.

6. The method of claim 1 , wherein the generating the plurality of drive signals comprises:

generating a corresponding plurality of modulated PWM waveforms, the plurality of modulated PWM waveforms having respective duty cycles that change with time according to a sine-drive modulation envelope, wherein the sine-drive modulation envelope has a frequency, wherein each one of the plurality of modulated PWM waveforms has a peak duty cycle, wherein the peak duty cycle of each one of the plurality of modulated PWM waveforms is related to an amplitude of a respective one of a plurality of sinusoidal current signals carried by the plurality of motor windings, and wherein the frequency of the modulation envelope is related to a frequency of the plurality of sinusoidal current signals.

7. The method of claim 1 , wherein the coasting time period has a coasting time duration corresponding to a sum of periods of an integer number of cycles of the plurality of sinusoidal current signals proximate to the end of the ramp-up time period.

8. A motor control circuit for driving a multi-phase motor having a plurality of motor windings, the motor control circuit configured to:

generate a plurality of drive signals comprising determined start-up signal characteristics to start the multi-phase motor spinning, wherein the determined start-up signal characteristics of the plurality of drive signals comprise a ramped start-up of the electric motor during a ramp-up time period during a start-up mode of operation, wherein the plurality of drive signals results in a corresponding plurality of sinusoidal current signals carried by the plurality of motor windings, and wherein the plurality of drive signals are generated such that, at an end of the ramp-up time period, a last one of the plurality of cycles of the sinusoidal current signal ends at a zero current;

stop the plurality of drive signals for a coasting time period following the end of the ramp-up time period;

detect a back EMF signal indicative of a zero back EMF voltage on the selected one of the plurality of motor windings while the plurality of drive signals are stopped, the detected back EMF signal indicative of an end of the coasting time period; and

drive the plurality of motor windings with the plurality of sinusoidal current signals in a normal mode of operation in response to the detecting.

9. The motor control circuit of claim 8 , wherein, at a beginning of the normal mode of operation, a first cycle of the plurality of sinusoidal current signals on the selected one of the plurality of motor windings begins at the zero current and at a phase continuous with a phase of the last one of the plurality of cycles of the sinusoidal current signal on the selected one of the plurality of motor windings at the end of the ramp-up time period.

10. The motor control circuit of claim 9 , wherein the motor control circuit is further configured to:

adjust, at the start of the normal mode of operation, the plurality of drive signals to result in smaller amplitudes of the plurality of sinusoidal current signals than at the end of the ramp-up time period.

11. The motor control circuit of claim 9 , wherein the motor control circuit is configured to generate the plurality of drive signals by:

generating a corresponding plurality of modulated PWM waveforms, the plurality of modulated PWM waveforms having respective duty cycles that change with time according to a sine-drive modulation envelope, wherein the sine-drive modulation envelope has a frequency, wherein each one of the plurality of modulated PWM waveforms has a peak duty cycle, wherein the peak duty cycle of each one of the plurality of modulated PWM waveforms is related to an amplitude of a respective one of a plurality of sinusoidal current signals carried by the plurality of motor windings, and wherein the frequency of the modulation envelope is related to a frequency of the plurality of sinusoidal current signals.

12. The motor control circuit of claim 9 , wherein the coasting time period has a coasting time duration corresponding to an integer number of cycles of the plurality of sinusoidal current signals proximate to the end of the ramp-up time period.

13. The motor control circuit of claim 8 , wherein the motor control circuit is configured to generate the plurality of drive signals by:

generating a corresponding plurality of modulated PWM waveforms, the plurality of modulated PWM waveforms having respective duty cycles that change with time according to a sine-drive modulation envelope, wherein the sine-drive modulation envelope has a frequency, wherein each one of the plurality of modulated PWM waveforms has a peak duty cycle, wherein the peak duty cycle of each one of the plurality of modulated PWM waveforms is related to an amplitude of a respective one of a plurality of sinusoidal current signals carried by the plurality of motor windings, and wherein the frequency of the modulation envelope is related to a frequency of the plurality of sinusoidal current signals.

14. The motor control circuit of claim 8 , wherein the coasting time period has a coasting time duration corresponding to a sum of periods of an integer number of cycles of the plurality of sinusoidal current signals proximate to the end of the ramp-up time period.