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I built a circuit that uses an LM2595 buck regulator to regulate a 12V battery input current to 5V; this is used to power an Arduino's on-board regulator. The Arduino's 3V3 LDO output is then used to power some other ICs in my circuit. (The Arduino and all ICs are running at 3V3.)

The Arduino does have its onboard USB port, which is useful to have connected for debugging. This would also, of course, power the 5V0 line, downstream of the buck regulator.

If the regulator's input is disconnected / open circuit, and I were to plug in the USB cable to the Arduino (thus providing 3V3 to the Arduino & its downstream IC friends, but also exposing 5V0 on the regulator output line), is this going to harm the LM2595?

I cannot find any mention in the datasheet or TI forums of issues with back-feeding this device one way or the other.

(This is already on a PCB; as such, adding a protection diode isn't an option at this point.)

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As a general rule you can't do that and it will probably harm the switcher in the long run. Reason: if you look at the block diagram you see that the internal circuitry is powered from the VIN terminal. When you back power it, you'll bias something from the feedback pin so I'd expect at least issues with the startup block.

There are switching regulators designed for that kind of service, they usually prominently say on the datasheet "can be used on prebiased loads" or something like that (the LM2745 just as an example).

That's the theory. In practice the LM2595 is quite robust and maybe you can get away simply using a back protection diode from the output to the input (like you do with the 78xx regulators).

In fact they specify the feedback pin voltage as absolute values and not referring to VIN (this isn't however saying that it will work correctly). Another possibility is that you simply need to keep it disabled when not powered up. Your best bet is to simply ask TI the recommended configuration.

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