SLLSFH8B August 2021 – February 2022 MCT8316A
The BEMF_THRESHOLD1 and BEMF_THRESHOLD2 values used for commutation instant detection in MCT8316A can be computed from the motor phase voltage waveforms during coasting. For example, consider the three-phase voltage waveforms of a BLDC motor while coasting as in Figure 9-6. The motor phase voltage during coasting is the motor back-EMF.
In Figure 9-6, one floating phase voltage interval is denoted by the vertical markers on channel 3. The Vpeak (peak-peak back-EMF) on channel 3 is 208-mV and Tc (commutation interval) is 2.22-ms as denoted by the horizontal and vertical markers on channel 3. The digital equivalent counts for Vpeak and Tc are calculated as follows.
In MCT8316A, a 3-V analog input corresponds to 4095 counts(12-bit) and phase voltage is scaled down by 10x factor before ADC input; therfore, Vpeak of 208-mV corresponds to an ADC input of 20.8mV, which in turn equals 29 ADC counts. Assuming the PWM switching frequency is 25-kHz, one back-EMF sample is available every 40-μs. So, in a time interval of 2.22-ms, a total of 55 back-EMF samples are integrated. Therefore, the BEMF_THRESHOLD1 or BEMF_THRESHOLD2 value calculated as per Equation 8 is (½) * (29/2) * (55/2) = 199. Hence, in this example, BEMF_THRESHOLD1 and BEMF_THRESHOLD2 are set to 8h (corresponding to 200 which is the closest value to 199) for commutation instant detection using back-EMF integration method during fast start-up. The exact speed at which the Vpeak and Tc values are measured to calculate the BEMF_THRESHOLD1 and BEMF_THRESHOLD2 values is not critical (as long as there is sufficient resolution in digital counts) since the product (Vpeak * Tc) is, largely, a constant for a given BLDC motor.