I'm designing for the first time a PCB that can be powered from an external +12V supply or from the USB connector.
In the input of the board, just after the power connector (VINLDO net is the +12V power input), I've placed two LDOs in series. The first provides regulated +5V and the other +3.3V as the schematics shows:
When VBUS = +5V and there's no voltage in VINLDO net could I damage the U5 (MCP1703AT-5002E/MB) voltage regulator, since I'm "feeding" a +5V signal into its output?
Answer
As Ignacio pointed out there is a parasitic body diode inside the regulator from the output to input.
If VBUS has a very low source impedance then the C15 will get charged quickly through U5 and there is not much limiting the current. Or, anything connected to the VINLDO input could draw current from VBUS through U5 (for example, shorting to ground could kill the regulator).
Edit: To make this a bit more clear, here is the internal diagram of U5:
One possibly practical solution if this appears to be an issue is to add a Schottky from output (across U5's body diode and in the same direction) to input and a polyfuse in series with the VINLDO input.
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