After the recharge, if the SRP-SRN voltage decreases below 5 mV, then the low-side FET is turned off for the rest of the switching cycle. During discontinuous conduction mode (DCM), the low-side FET turns on for a short period of time when the boostrap capacitor voltage drops below 4 V to provide refresh charge for the capacitor. This is important to prevent negative inductor current from causing any boost effect in which the input voltage increases as power is transferred from the battery to the input capacitors. This can lead to an overvoltage on the AVCC node and potentially cause damage to the system.