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LT3045 crowbar

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matthuszagh:
I'll be using an LT3045 (https://www.analog.com/media/en/technical-documentation/data-sheets/LT3045.pdf) to power an expensive device that I really don't want to damage. The LT3045 has a lot of built-in protections (current limit, thermal overload, reverse polarity input, under-voltage lockout), but I didn't see anything about output overvoltage protection. So, I was planning to add a simple crowbar at the output, consisting of a fuse, a zener, a few resistors and a thyristor. The device requires 15V and a max of 350mA.

Is there any reason why I shouldn't do this?

David Hess:
The power good feedback function of the LT3045 could be used to trigger the crowbar, or disconnect the output load with a series connected power MOSFET.

matthuszagh:

--- Quote from: David Hess on June 30, 2022, 12:09:04 am ---The power good feedback function of the LT3045 could be used to trigger the crowbar, or disconnect the output load with a series connected power MOSFET.

--- End quote ---

This is a reasonable suggestion, though I would only ever expect the output voltage to sufficiently exceed the set point if the LT3045 fails in some way, so I'm a bit reluctant to rely on features provided by that regulator.

matthuszagh:

--- Quote from: evb149 on June 29, 2022, 05:15:39 pm ---More protection is a good thing generally.
They do make integrated overvoltage (and over current, polarity, ...) protection ICs
e.g. the ones linear/analog devices called "surge stopper" and similar.

If you're really worried about making sure the supply is in tolerance e.g. +/- 10% or whatever then
you could use some kind of supply supervisor / watchdog that ensures the correct limits and shuts off the
supply if there is a problem.

A zener based one will be less precise than that generally but you could always use a more precise option like TL431
or something in a similar circuit if you wanted better cut-off-voltage accuracy.

You could also add more TVS protection in this too if you want to.

--- End quote ---

Thanks for suggesting this; it was very helpful, especially since I didn't know these sorts of protection ICs exist. It seems like the LTC1696 (https://www.analog.com/media/en/technical-documentation/data-sheets/1696fb.pdf) + SCR would be a good fit here. I'll also add a TVS diode. A lot of these protection ICs reenable the voltage when it falls back in range. In my case, I only expect an excess voltage when a serious fault occurs so I'd rather go with the traditional approach of keeping the rail shorted and blowing a fuse.

imo:
It depends what is your precious equipment like.. The fuses are rather slow devices. Also when talking for example a transceiver, the circuit around the thyristor's gate should be well decoupled (what slows down the response even more). I tried that once upon a time and rejected the idea after some experiments. I put TVS diodes everywhere then..

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