Never assume your 5V rail is clean. Voltage spikes from hot-plugging can exceed 7V instantly. Place a 5.6V Zener diode at the input if your system is prone to transients.
Fixed at 1.8V (indicated by the "-18" in the part number). Input Voltage Range: Typically 2.5V to 5.5V . dc e2h datasheet
Protects the regulator and upstream power supply during output short circuits or overload conditions. Never assume your 5V rail is clean