Did I choose the correct fuse for my USB powered circuit?
My interpretation is slightly different to yours:


\$I_{MAX}\$ = The maximum current the fuse can handle without exploding in a shower of dust
\$I_{TRIP}\$ = The current at which the fuse will "blow" at
\$I_{HOLD}\$ = The current that is "safe" to pass through the fuse
Feed it 8 amps and it will blow within 0.1 seconds.


So yours will definitely "blow" at 750mA (give or take), but could also degrade and eventually "blow" at anything over 350mA.

Wikipedia has a good description on the different parameters:


Initial resistance: The resistance of the device as received from the factory of manufacturing.
Operating voltage: The maximum voltage a device can withstand without damage at the rated current.
Holding current: Safe current through the device.
Trip current: Where the device interrupts the current.
Time to trip: The time it takes for the device to trip at a given temperature.
Tripped state: Transition from the low resistance state to the high resistance state due to an overload.
Leakage current: A small value of stray current flowing through the device after it has switched to high resistance mode.
Trip cycle: The number of trip cycles (at rated voltage and current) the device sustains without failure.
Trip endurance: The duration of time the device sustains its maximum rated voltage in the tripped state without failure.
Power dissipation: Power dissipated by the device in its tripped state.
Thermal duration: Influence of ambient temperature.
Hysteresis: The range between where the device trips and where the device returns to a conductive state.


PTC Fuses are basically heat based.  When they get too hot they go very high resistance.  That heat could be rapidly produced by an overcurrent situation (\$I_{TRIP}\$) or a prolonged higher current (\$&gt; I_{HOLD}\$) over time.