Specifications to consider when selecting a polyfuse
Can anyone explain I_trip and I_hold to me?


The fuse is guaranteed not to trip if the current is less than I_hold. 

The fuse is guaranteed to trip if the current is more than I_trip.

You need to choose a polyfuse with I_hold larger than your maximum load current consumption, or it may nuisance trip.

You need to choose a polyfuse with an I_trip smaller than your power supply output if you want it to trip when a fault is presented. Otherwise it may fail to trip, and cook your supply.