Re: Power from another source fed to 1117 regulator output pin

On 9/30/2024 7:43 AM, Pimpom wrote:

On 30-09-2024 05:51 pm, Don Y wrote:

On 9/30/2024 4:37 AM, Pimpom wrote:

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.
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Does this mean that the 1117 has no problem with this arrangement - that is, with power coming from another source to its output pin?

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Short the LDO's input to GND and consider how it will (mis)behave.
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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.

If the input is powered *or* left "open" (assuming you don't have something
"ridiculous" on the input that can represent a short even for a short time),
it will likely be comfortable in that state indefinitely.
If *designing* in this sort of configuration, I tend to be more paranoid
about the types of "stuff" that will inevitably get connected to the
"input connector" -- especially if you opt for the ubiquitous "barrel
connectors".  There, it seems that the typical user just hunts for *an*
adapter (wall wart) that "fits", mechanically.  Consider that many such
devices are generic and don't even declare the device to which they
are *intended* to be mated (People's Wall Wart Company #773), the
user has very little by way of guidance.
As far as what the adapter (even if powered off!) might look like
from the LDO's point of view:  the polarity could be reversed;
there could be some bulk cap intended to make full-wave DC "look"
more like a regulated supply;  it could be a bare transformer winding
(i.e., AC); the wrong voltage/amperage (excessive ripple); etc.
I've not found a robust solution that doesn't quickly start to
look like overkill.  And, a customer will never admit to having
plugged the "wrong" adapter into it (before *possibly* finding
the right one to include with his RMA).  So, you see a tiny
insignificant part of your design proving to be the Achilles
heel of your *product* (is it worth disassembling to replace the
LDO or just scrapping the thing, whole?)
My current approach is NOT to allow external power sources (all PoE)
AND to use a non-standard connector to ensure the PSEs and PDs
aren't subjected to arbitrary device matings (there are many
things that call themselves "PoE" without being truly compliant).
And, I *still* expect to find some really clever idiot who will
manage to subvert all of my precautions!  <frown>