Variable commutation can be configured by setting COMM_CONTROL to 01b. 120^o commutation may result in acoustic noise due to the long Hi-Z period causing some torque ripple in the motor. In order to reduce this torque ripple and acoustic noise, the MCT8315A uses variable commutation to reduce the phase current ripple at commutation by extending 120^o driving time and gradually decreasing duty cycle prior to entering Hi-Z state. In this mode, the phase is Hi-Z between 30^o and 60^o and this window size is dynamically adjusted based on speed. A smaller window size will typically give better acoustic performance. Figure 7-31 shows 150^o commutation with 30^o window size.

Figure 7-31 150^o commutation