SLOSE70 December 2020 DRV8434S
During fast decay, the polarity of the H-bridge is reversed. The H-bridge will be turned off as current approaches zero in order to prevent current flow in the opposite direction. In this mode, fast decay occurs only during decreasing current. Slow decay is used for increasing current.
Fast decay exhibits the highest current ripple of the decay modes for a given tOFF. Transition time on decreasing current steps is much faster than slow decay since the current is allowed to decrease much faster.