Robust digital output protection
I am working on a digital output concept that I would like to have the following characteristics:
- Output voltage: 5V
- Controllable by the uC from 3.3V power supply
- I would like to be able to switch the output at <100Hz
- I don't need more than 50mA of output current drive
- Bonus, it would be nice that the output wouldn't budge (get damaged) when stressed a bit with either ESD event of 8kV or +-12V wrong wiring connection. Unit doesn't need to function properly in this case, it just can't get damaged.
I currently have something like:
How does this look like?
2 answers
Generally you can protect against most kind of harmful events with a TVS placed on the output, with a value slightly higher than your expected signal voltage so roughly 6V.
The only thing the idea of building this yourself with BJTs got going for is somewhat fast switching but <100Hz is slow. A generic buffer IC with 3.3V on one side and 5V on the other would work about the same and if you can find a fairly modern one it ought to have ESD protection. Maybe there is one which can drive 50mA even? You can also check out "analog switches" - fast switching parts usually designed for lower voltages.
Another solution is to get pre-made "smart MOSFET driver" which is essentially a MOSFET that can supply lots of current while remaining rugged and protected against all kinds of harm: shorts, ESD, polarity flips etc. The automotive industry use them for relay replacement so the main thing is that they can source a lot of current (which you don't need). I'm not sure if there are any that can work with relatively low voltages - they are designed for 5V gate voltage, but a standard buffer IC between the MCU and the driver would fix that.
This is best answered by browsing the stock of a suitable IC manufacturer. Infineon has a lot to pick from for these kind of parts so maybe check https://www.infineon.com/products/power/gate-driver-ics. Renesas, ST, Onsemi etc should have similar products.
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That will work to switch the output on and off. You can tolerate a few ms switching time, so the slow base drives should be OK. If you needed much faster, I'd suggest a smarter topology.
Some issues to consider:
- You said you want ESD protection on the output, but you haven't given it any. Q3 will likely blow up with a proper ESD event. At minimum, I suggest diodes to 5 V and ground.
- There will typically be 200 mV or so drop across Q3 when on, so the output won't get all the way to the 5 V supply. Perhaps that's OK, but you need to at least consider it.
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