MCT8315A 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 MCT8315A, 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 value for FAST_BRK_DELTA may eliminate motor stall faults, especially when high FAST_DECEL_CURR_LIM values are used. Setting a higher value for FAST_BRK_DETLA will also result in higher speed error between target speed and motor speed at the end of deceleration mode - motor will eventually reach the target speed once motoring mode is resumed. FAST_DECEL_CURR_LIM and FAST_BRK_DELTA should be configured in tandem to optimize between lower deceleration time and reliable (no stall faults) deceleration profile.

FAST_DEC_DUTY_THR configures the speed below which fast deceleration will be implemented. For example, if FAST_DEC_DUTY_THR is set to 70%, any deceleration from speeds above 70% will not use fast deceleration until the speed goes below 70%. FAST_DEC_DUTY_WIN is used to set the minimum deceleration window (initial speed - target speed) below which fast deceleration will not be implemented. For example, if FAST_DEC_DUTY_WIN is set to 15% and 50%->40% deceleration command is received, fast deceleration is not used to reduce the speed from 50% to 40% since the deceleration window (10%) is smaller than FAST_DEC_DUTY_WIN.

MCT8315A provides a dynamic current limit option during fast deceleration to improve the stability of fast deceleration when braking to very low speeds; using this feature the current limit during fast deceleration can be reduced as the motor speed decreases. This feature can be enabled by setting DYNAMIC_BRK_CURR to 1b. The current limit at the start of fast deceleration (at FAST_DEC_DUTY_THR) is configured by FAST_DECEL_CURR_LIM and the current limit at zero speed is configured by DYN_BRK_CURR_LOW_LIM; the current limit during fast deceleration varies linearly with speed between these two operating points when dynamic current limit is enabled. If dynamic current limit is disabled, current limit during fast deceleration stays constant and is configured by FAST_DECEL_CURR_LIM.