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I need to implement a one time programmable (OTP) logic function. The task is to inhibit/blow a single 3.3V logic signal for safety reasons after production of the device.

I found some small fuses like e.g. 429 which could do the job, but they have rather high amperage (>1A) and I was wondering if there is a more economical solution in terms of blow current (~100mA or so)?

Or do you have other ideas to solve this OTP task? How is OTP usually done in discrete circuits?

Stefan Wyss
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  • Run the trace through a throughhole via, and then drill through this via with a size that removes the thoughhole plating (opposite side traces) or entire annular ring (either side trace). This will be slightly difficult to repair as the trace is covered in resist. Any inner layers should be cleared of the drill diameter. – tobalt May 18 '21 at 10:56

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Sure, just use really low current fuses. You can find them as SMD package toos, in the 50mA range, more or less. A general purpose transistor can blow them without issues.

Or, in a production environment, a single jumper wire to physically cut works too!

For a more, well, 'sophisticated' way you can make a poor man antifuse essentially burning a component of your choice (VGS breakdown comes to mind). Not really elegant but it works.

Lorenzo Marcantonio
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"for security reasons". Someone examining this product will see a blown fuse, trace it to the security logic signal, and simply replace the fuse. How does this give security?

OTP fuse protection is normally internal to a chip, and not replaceable.

In a discrete circuit, you may have to mimic this by hiding a good chunk of circuit (not just the fuse) under a lump of black epoxy ... preferably with coarse sand as a filler. That's a sadistic thing to do to your adversary's cutting tools, but it'll help security a little.

  • I was confusing security with safety (in fact, in my language German there is only one word for both). So it's for "safety reasons". (But only minor grade of safety required). – Stefan Wyss May 18 '21 at 13:24
  • Ah OK yes that's a big cultural difference between English and German! A 2-pin header and a jumper would work : fit it for (programming/testing/other production purpose) and remove jumper after testing. –  May 18 '21 at 13:26