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From: Miguel Gimenez <me@privacy.net>
Newsgroups: sci.electronics.design
Subject: Re: Power from another source fed to 1117 regulator output pin
Date: Mon, 30 Sep 2024 16:55:32 +0200
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El 30/09/2024 a las 16:43, Pimpom escribió:
> 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.
>>>
>>> Does this mean that the 1117 has no problem with this arrangement - 
>>> that is, with power coming from another source to its output pin?
>>
>> Short the LDO's input to GND and consider how it will (mis)behave.
>>
> 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).

-- 
Regards
Miguel Gimenez