MCT8317A datasheet(47/161 Pages) TI1

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Duty cycle

Commutation method

ZC based

Integration based

Duty 2

Duty 1

INTEG_DUTY

_THR_LOW

INTEG_DUTY

_THR_HIGH

Duty 1- Duty cycle at which motor speed is such that number

of BEMF samples per 30

o is > INTEG_CYCL_THR_HIGH

Duty 2 - Duty cycle at which motor speed is such that number

of BEMF samples per 30

o is < INTEG_CYCL_THR_LOW

Figure 8-34. Commutation Method Transition

8.3.15.1 BEMF Threshold

Figure 8-35 shows the three-phase voltages during 120o trapezoidal operation. It is seen that one of the phases

will always be floating within a 60o commuation interval and MCT8317A integrates this floating phase voltage

(which denotes the motor back-EMF) in the back-EMF integration method to detect the next commutation

instant. The floating phase voltage can either be increasing or decreasing and the algorithm starts the integration

after the zero cross detection in order to eliminate integration errors due to variable degauss time. The

floating phase voltage is periodically sampled (after zero cross) and added (discrete form of integration).

BEMF threshold (BEMF_THRESHOLD1 and BEMF_THRESHOLD2) value is set such that the integral value

of the floating phase voltage crosses the BEMF_THRESHOLD1 or BEMF_THRESHOLD2 value at (or very

near) to the commutation instant. BEMF_THRESHOLD1 is the threshold for rising floating phase voltage and

BEMF_THRESHOLD2 is the threshold for falling floating phase voltage. If BEMF_THRESHOLD2 is set to 0,

then BEMF_THRESHOLD1 is used as the threshold for both rising and falling floating phase voltage.

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MCT8317A

SLLSFQ2 – JANUARY 2023

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