I'm looking for low dropout three pin regulators that are designed for use when there is a voltage on it's output pin and power is disconnected.

 

I think you are looking for "mirror-image insertion protection"; these are regulators which can tolerate the input being grounded and output being at a higher potential than the input (but often not higher than the maximum input voltage of the regulator.) Be aware that only some can continuously withstand this situation; others might only manage a minute (it will usually be specified in the datasheet.)

EDIT: Are you looking for regulators where the input is grounded?

 

Actually the input will be open, no voltage present. It's for a serial adapter where when built will have either an external power supply via the regulator or be used as a daughterboard on a 3.3V motherboard.

Adding a diode on the output pin would work BUT it would drop the output by 0.2V not desired.

 

Actually the input will be open, no voltage present. It's for a serial adapter where when built will have either an external power supply via the regulator or be used as a daughterboard on a 3.3V motherboard.

Adding a diode on the output pin would work BUT it would drop the output by 0.2V not desired.

Okay, well I'd say most if not all regulators can withstand that situation. Think about it: when power is removed, the output capacitor would basically be supplying 3.3V and the input cap would be open - yet almost (if not all) regulators are fine with this.

 

The old LM78xx series needed a diode across the in & out pins. I would have figured modern regulators would have something similar built in.
And the input will not be shorted.

 

The old LM78xx series needed a diode across the in & out pins. I would have figured modern regulators would have something similar built in.
And the input will not be shorted.

I would think that you would not need a 4700µF capacitor on any modern regulator.

An example TC1017 shows a body diode across the main fet, which might cause the discharge problem as mentioned.

 

I've just went through the LD1117V33 datasheet and they show a battery charger with a pair of LD1117 outputs connected together. Seems it's not a problem for that regulator.