MCT8317A datasheet(147/161 Pages) TI1

147 / 161 page

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 MCT8317A, 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 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.

9.2.1.5 Maximum speed

Figure 9-7 shows phase current, phase voltage and FG of a motor that spins at maximum electrical speed of 3

kHz.

Phase current

Phase voltage

Figure 9-7. Phase current, Phase voltage and FG at Maximum speed

9.2.1.6 Faster deceleration

MCT8317A has features to decelerate the motor quickly. Figure 9-8 shows phase current and motor electrical

speed waveform when the motor decelerates from 100% duty cycle to 10% duty cycle. Time taken for the motor

to decelerate from 100% duty cycle to 10% duty cycle when fast deceleration is disabled is around 10 seconds.

Figure 9-9 shows phase current and motor electrical speed waveform when the motor decelerates from 100%

duty cycle to 10% duty cycle. Time taken for the motor to decelerate from 100% duty cycle to 10% duty cycle

when fast deceleration is enabled is around 1.5 seconds.

www.ti.com

MCT8317A

SLLSFQ2 – JANUARY 2023

Submit Document Feedback

147

Product Folder Links: MCT8317A

MCT8317A Datasheet(PDF) 147 Page - Texas Instruments