I have a device with two voltage sources and therefore I need atleast a diode on the lowest source to secure it. The lowest source is a 5V USB, and it is protected with a schottky diode with a 0.5V drop. I will also use a polyfuse to protect the USB, but I am starting to worry that the voltage drops over the fuse and diode is too much, as the USB voltage needs to be over 4V after the fuse and diode do be accpected by my buck regulator. Is the polyfuse really necessary? I have read that the USB voltage can vary between +-5% of the 5V. This is of course worst case scenario.
I think it's necessary; what if a lot of current gets supplied, or what if a drastic temperature change causes the IV characteristics of the computer supplying the current to the USB changes?
ONE THING TO WORRY ABOUT: "If a polyfuse has been triggered ("blown"), its resistance can increase so much (and for several days) that even 4.5 volt isn't reached, and the keyboard only gets say 3.5 Volt. The device will likely fail, by e.g., start repeating keys, etc."
"With the USB polyfuses something special is going on, because these are designed to "blow" very quickly, with just a few 100mA. Because they blow with very low currents does not mean they need less energy (heat) to blow."