[sdiy] Gate input protection

[Eric Brombaugh]

Seb Francis wrote:

> The digital inputs of the dsPIC don't want voltages outside the 0V to 
> 3.3V range, with low/high levels being:
> <0.66V = low
>  >2.64V = high
> 
> So there needs to be some kind of buffer to accept gate signals from the 
> 'outside' world which could be anything from say -15V to 15V.  I'd like 
> to keep it minimal component count and my 2 thoughts so far are as follows:
> 
> http://burnit.co.uk/sdiy/stuff/gate-buffer.png
> (my ASCII art wasn't good enough to include the diagram in the actual 
> email :)
> 
> The first option looks an elegant solution, but I'm not sure it's safe 
> if a negative gate signal would be present.  Then the forward voltage 
> across the zener (~1V) would allow -1V at the dsPIC input.  Or is the 
> 100K resistor enough to make this ok?  Is there a simple way to add 
> protection for -ve gate voltages?
> 
> The 2nd option I think is ok, although higher component count than I'd 
> prefer.  Am I right in thinking that I must have D2 to protect the 
> transistor against high gate voltages?  (I'm a bit confused because of 
> the transistor datasheets I've looked at some quote the max Emitter-Base 
> voltage as +ve and others as -ve.)  And what happens if the gate input 
> is negative, can the output get pulled negative?  Hmm, showing my 
> transistor ignorance here .. must be too young, I grew up on ICs ;)

Either of those looks OK, but your concerns about negative voltage and 
component count are valid.

Why not try a BAT54 Schottky diode array? Use the 'series' version, 
tying the cathode pin to +3.3V, the anode pin to GND and the center pin 
to your dsPIC pin and a series input resistor. This provides essentially 
the same protection that the normal IC ESD diodes do, but with beefier 
junctions to take the load off the internal diodes. A 100k resistor will 
keep the forward voltage on the diodes to about 200mV even at a full 15V 
input, which is well within the abs max spec of the dsPIC.

BAT54-04 parts are about $0.13/each in qty 1 and less if you buy a bunch 
of them.

Eric