Re: Plug-in interface for External stepper Drivers

Robert Benward

To the diode question.  Yes and no.  For starters, the top diode is backwards.  The voltage will never go below 3V.  Series strings are useful if you want to keep a tight reign on the voltage and keep them within the supply rails.  You can use a single zener to ground, but the breakdowns tolerances are not that great, and they vary quite a bit over temperature.  You can use a regulator diode, such as an LM4040.  Those are tight tolerance, but also expensive.  For one input, OK, but if you had 20 or 50 or 100 inputs, your boss would probably fire you. 

The Schottky diodes are used for their low forward drop, insuring conduction not much higher than VSS (or 3.3V in this case).  The other half, the threat current x the voltage is a peak PD.  A 100mA threat x a 3.5V zener is a larger dissipation than the same current through a 0.3V forward drop.  This arrangement assumes you have a robust 3.3V supply (or whatever VCC you are using), and you can absorb the pulse current via all the other loads and the inherent capacitance on a supply line.

The thing is, they are unnecessary; every IC has these intrinsic body diodes built in to the substrate and across every input.  That is why absolute ratings, which used to be specified as max current into a pin, is now specified as VCC+0.3V or VEE-0.3V.  With sufficient impedance, and understanding the threat, the body diodes and 100K or an RC, is more than enough to protect the input. 

Still need to fix the input voltage though...


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