Can I power an IC form both 3.3v and 5.0v using the following setup:
Given that the IC can have an input range of 3-5v ofcource. I want to ensure the IC has power regardless of if only one or both of the voltages are present. Should I add a decoupling capacitor?
Your circuit will work. For sure you need to know the voltage drop in the diodes.
As said before: Capacitors are essential. Many IC manufacturers recommend a unique capacitor at each VCC pin of each IC.
Additionally a bulk capacitor will be useful.
There are special IC s for power supply applications, they switch over automatically, while avoiding the usual voltage drop of diodes.
The forward voltage drop of the MBR0520L should be less than 300mV @ 100mA and the circuit that is powered through these diodes should not consume more than 80mA.
VCC1 and VCC2 are just two different sides of the isolation in a CAN-transceiver, and in this case is used to separate two power-planes. I left out as much of the circuit as possible to focus on a simple solution as possible. Thanks for the suggestion though!