It would be best to put a SCHOTTKY diode clamp from CS to GND.
Cathode to CS, Anode to GND.
Don't let it get below -0.3V.
The input impedance of CS is not specified, however it would be quite high.
Figure 14 of the data sheet shows a 77K in series for the DCM function.
It forms an RC with the 6.5pF.
We recommend a small RC filter on CS,
Usually a 1K and 470pF. A 330pF is used on the EVM.
So there is probably already a series resistor on CS that the added SCHOTTKY would clamp through.