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Degauss time(shown by double-sided arrow) after commutation during which the outgoing(floating) phase

voltage is clamped to VM(by negative outgoing phase current) during increasing back-EMF; sampling of

back-EMF(denoted by *) should start after degauss time is over for accurate zero cross instant detection

Degauss time(shown by double-sided arrow) after commutation during which the outgoing(floating) phase

voltage is clamped to PGND(by positive outgoing phase current) during decreasing back-EMF; sampling of

back-EMF(denoted by *) should start after degauss time is over for accurate zero cross instant detection

PGND

Figure 8-36. Degauss Time

8.3.16 Fast Deceleration

MCT8317A has the capability to decelerate a motor quickly (100% to 10% speed reduction within tens of ms)

without pumping energy back into the input DC supply using the fast deceleration feature in conjunction with

the AVS feature. The fast deceleration feature can be enabled by setting FAST_DECEL_EN to 1b; AVS_EN

should be set to 1b to prevent energy pump-back into the input DC supply. This combination enables a linear

braking effect resulting in a fast and smooth speed reduction without energy pump-back into the DC input supply.

This feature combination can also be used during reverse drive (see Reverse Drive) or motor stop (see Active

Spin-Down) to reduce the motor speed quickly without energy pump-back into the DC input supply.

The deceleration time can be controlled by appropriately configuring the current limit during deceleration,

FAST_DECEL_CURR_LIM. A higher current limit results in a lower deceleration time and vice-versa. A

higher than necessary current limit setting may result in motor stall faults, at low target speeds, due to

excessive braking torque. This can also lead to higher losses in MCT8317A, especially in repeated acceleration-

deceleration cycles. Therefore, the FAST_DECEL_CURR_LIM should be chosen appropriately, so as to

decelerate within the required time without resulting in stall faults or overheating.

FAST_BRK_DELTA is used to configure the target speed hysteresis band to exit the fast deceleration mode and

re-enter motoring mode when motor reaches the target speed. For example, if FAST_BRK_DELTA is set to 1%,

the fast deceleration is deemed complete when motor speed reaches within 1% of target speed. Setting a higher

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MCT8317A

SLLSFQ2 – JANUARY 2023

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