SLOSE70 December 2020 DRV8434S
During slow decay, both low-side MOSFETs of the H-bridge are turned on, allowing the current to be recirculated.
Slow decay exhibits the least current ripple of the decay modes for a given tOFF. However on decreasing current steps, slow decay will take a long time to settle to the new ITRIP level because the current decreases very slowly. If the current at the end of the off time is above the ITRIP level, slow decay will be extended for multiple off time duration, until the current at the end of the cumulative off time is below the ITRIP level.
When the winding current is held static for a long time (for example when no STEP input is present) or at very low step rates, slow decay may not properly regulate the current because back-EMF will be small or absent across the motor windings. The motor current can rise rapidly, and may require an extremely long off-time to regulate the current. In some cases this could result in loss of current regulation. An aggressive decay mode is recommended in such cases.