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I am currently designing an SMD PCB with an atmega328P. I want to able to feed this circuit using below schematic.

enter image description here

In the past I have had certain heat development issues with arduino boards and all kinds of DC DC converters. The atmega is to be programmed and fed by an ICSP connector. That means I am going to apply 5V to the OUT/FB pin of the LM2596.

Looking at the capacitors and the schottky diode it would be safe. I have however no clue if the LM2596 will do harm. I won't be applying power to the J9 connector at the time of programming.

So with regards to the atmega chip's VCC pin (and the ICSP 5v), should I use an extra schottky diode to be safe or am I just fine without one?

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Well I'll take a stab at this. I am not an authority on the matter, but by looking at the internal schematic you should do no harm to the LM2596.

The feedback pin connects to an op-amp that will have no supply voltage.

The output pin connects the the emitter of 3 bjts (which should also cause no issue).

enter image description here

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  • \$\begingroup\$ My same opinion too, also the feedback is heavily current limited from the input divider. I would ask TI before doing that for a recommendation, their E2E forum is quite good. \$\endgroup\$ Feb 8, 2021 at 21:59
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This is a quite common issue, regulators plugged in live load. In general, you can't. In fact some 78xx for protection requires a diode from the output to the input to at least bias the circuitry (it's their only failure mode, in fact).

Since it's operative from 4.5V up, a schottky from the output to the input would protect it. However keep in mind that you'll be powering the whole 5V rail from the programmer, and that could be bad. I usually switch the supply of the programmer (don't know if the AVR can do that) and program the board running from the internal supply to avoid issues.

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That means I am going to apply 5V to the OUT/FB pin of the LM2596.

You're not really applying 5 V to the FB pin. The LM2596 uses the FB pin to sense the output voltage so that the output becomes 5 V. So the FB pin MUST be connected to the +5 V output, if you don't then the LM2596 will think the output voltage is too low (if you leave the FB pin unconnected) and you will get close to 12 V at the output and that will probably damaging the circuits that only need 5 V.

should I use an extra schottky diode to be safe or am I just fine without one?

What does the LM2596's datasheet say? I think there's no need for a 2nd diode so why would it need to be added?

Pro tip: just stick as much as possible to the schematic in the datasheet and you will increase your chance of success.

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