I've been looking at the schematic of a popular product that has two options as the power source. One source is a Vin point (pin or DC jack) that goes to a 1117-5V regulator, the other is a USB port. The USB +5V line joins the 1117 output via a Schottky diode.
Does this mean that the 1117 has no problem with this arrangement - that is, with power coming from another source to its output pin?
Sure, this issue is often brought up while discussing regulators. Apparently the makers of the product (Arduino, various versions) seem to feel that the input of the LDO getting shorted while being powered via USB is not likely enough to bother with.
Anyway, what I'm thinking about is how the LDO's internal circuit reacts to being reverse fed a voltage at the output pin without having an input but *without* the input being shorted to ground.
Section 8.4.1 of the TI datasheet (SNOS412Q) address this issue: No protection diode is needed unless the input is shorted to ground and there is a big capacitor in the output (or another power source).
then the input will equalize to the output voltage and no current will flow and nothing happens
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