SLVAFP0 December 2023 LMR51610
To prevent the negative output from becoming too positive, a diode across the negative output supply needs to be used. Because there is a cap from Vin to negative output, if large line transients, the current can go through from input cap to LS and inductor return to GND, hence larger spike on negative side show up, adding a diode can help to protect LS from damaged by positive voltage on output. Increase the output cap can also help to solve this problem. See Figure 3-3.