Two supply input issue

[f1netune]

Hello everyone, I am not a pcb design expert and I need your guidance to complete my design.

I’m working on an industrial IoT board. The system can run either from a 24 V input or from USB-C.

• When powered from 24 V, I use an LM2596 to generate 5 V, then a TLV1117 LDO to get 3.3 V for the MCU.
• When powered from USB, the board uses the USB 5 V (VBUS) through the same TLV1117 to 3.3 V.
• The USB interface uses a CP2104, which of course has a VBUS pin.

Here’s my confusion:

• The LM2596 output is 5 V, but I am not sure where exactly this 5 V should connect.
• At first I thought about tying the LM2596 5 V output directly to CP2104’s VBUS pin. But I realized that might violate USB rules since VBUS from USB-C and VBUS from the 24 V→5 V regulator are different sources.
• Also, I’m not 100% sure I am labeling the nets correctly. “VBUS” coming from USB-C and the “5 V” coming from LM2596 are actually not the same thing.

Important note:

• It’s not necessary for me to program the MCU while running from 24 V. I only need the board to operate normally when powered from 24 V. Programming/debugging can be done over USB power only.

So my questions are:

1. Should I keep the LM2596 5 V completely separate from CP2104’s VBUS net?
2. Is it better to treat LM2596 output simply as a “local 5 V rail” and only connect CP2104’s VBUS to the USB connector’s 5 V pin?
3. If so, what’s the recommended way to label the nets in the schematic so it’s clear these are two different 5 V sources?
4. Additional thought:

I was also considering another approach. Instead of trying to combine both 5V sources, I could label the LM2596 output as a separate net (e.g. +5VA) and then let the user select the power source with a simple jumper.

For example:

• If the board is powered from 24V, the user places a jumper to connect the LM2596 5V output to the LDO input.
• If powered from USB, the jumper connects USB VBUS to the LDO input instead.

This way only one source is connected at a time, and the user explicitly decides.

Would this be a reasonable and safe solution? Or is there a better practice for handling such dual 5V sources (USB VBUS vs. local regulator output)?

I dropped my schematics about power sections :
USB C Connector Side

Note : I guess I can only share 1 picture because of new user thing. Thanks for advices!

[retiredfeline]

Welcome. Please read this information for new members especially the first main paragraph. It also contains information on how to promote your level to be able to post attachments.

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Welcome new member. The purpose of the Kicad Info. forum is to discuss ideas, make remarks and ask questions related to the use of Kicad. This Kicad forum is not for questions regarding general electronic design. Basic information on using Kicad can be found at Getting started in Kicad Comprehensive information is at https://docs.kicad.org/ and specialised subjects are covered in the FAQs below. This is an English language forum. Please use a translator for non English posts. This Kicad For…

Unfortunately this forum has to concentrate on the use of KiCad and let other forums, e.g. eevblog, handle electronic design questions.

[paulvdh]

As retiredfeline already mentioned, this forum is not intended for electronic questions, but only for the KiCad software itself. Therefore this topic will probably be closed quickly. But in general, you should not be back feeding power into an USB bus. Adding some diodes is a common solution, but something with switching FET’s can also be done.

On a side note you connected the wires wrong to the USB ESD protection diodes. You’ve shorted the data wires, and the connector is not connected to U3 at all.

[jmk]

The OP has had time to comment, so it is time to close this thread.