# What if I source 3.3V to the LDO(AMS1117-3.3) output pin?

We feed DC 5V into VCC_EXT_5V and then it outputs 3.3V to the 3V3 in the normal condition. The following is a part of the power circuit.

My question is followed:

How does it work if I connect the other 3.3V power source to the 3V3 leaving the VCC_EXT_5V open?

As far as I know, the internal circuit may be broken if Vout is higher than Vin. But the manual says there is some protection circuit.

How do you think about it?

Thank you.

• I don't know that I'd call the '1117 an LDO. Jul 8, 2021 at 2:32
• Your question is not very clear. What do " it outputs 3.3V to the 3V3 in the normal condition" and " if I connect the other 3.3V power source to the 3V3" mean? Jul 8, 2021 at 4:59
• @PrathikPrashanth Sorry for my poor English. Normally we feed 5V into input (VCC_EXT_5V) and the LDO outputs 3.3V. The 3.3V is used for the main power for the board. The question is .... Is it OK If I feed DC 3.3V to the 3V3 line(the LDO's output port) without feeding DC 5V into the VCC_EXT_5V? Jul 8, 2021 at 5:04
• Im very confused now. How do you expect to get 3.3V at the output of the 3.3V regulator without a 5V input? Jul 8, 2021 at 5:07
• @PrathikPrashanth I think he/she asks about any potential problems if he/she applies +3.3V from a "separate source" while the circuit is unpowered (i.e. no 5V applied). Jul 8, 2021 at 5:09

According to the paragraph you supplied, it won’t cause excess current to flow except perhaps at power-up if the 3.3V source had extremely low source impedance. Say much less than 0.1 $$\\Omega\$$.