• Power from another source fed to 1117 regulator output pin

    From Pimpom@21:1/5 to All on Mon Sep 30 17:07:03 2024
    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?

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Don Y@21:1/5 to Pimpom on Mon Sep 30 05:21:28 2024
    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.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Pimpom@21:1/5 to Don Y on Mon Sep 30 20:13:43 2024
    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.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Miguel Gimenez@21:1/5 to All on Mon Sep 30 16:55:32 2024
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Lasse Langwadt@21:1/5 to Pimpom on Mon Sep 30 19:51:37 2024
    On 9/30/24 16:43, 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.

    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.

    then the input will equalize to the output voltage and no current will
    flow and nothing happens

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Don Y@21:1/5 to Don Y on Mon Sep 30 13:19:54 2024
    On 9/30/2024 1:16 PM, Don Y wrote:
    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.

    A great use for pTouch labelers is to tag each such wall wart in
    your home with a label declaring its intended mate. I have
    some with odd connectors (e.g., 4 pin) that come in two different
    INCOMPATIBLE pin configurations. So, "ONLY USE THIS ON XYZ DEVICE!!!"
    to remind myself that the other devices with similar connectors
    are not wired the same way (finding 12V on your 5V -- and 5V on the
    12V input -- input tends to leave you with toast!)

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Don Y@21:1/5 to Pimpom on Mon Sep 30 13:16:11 2024
    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.

    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.

    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>

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)