[sdiy] Overdriving 4069UB

[René Schmitz]

Hi Seb and all,

Seb Francis wrote:

>  From what I can tell from the datasheet the 4049 doesn't have the 
> protection diode from input to VDD.   Instead it has a 30V zener from 
> VSS to input.  So there's still no clue as to the max current that can 
> flow through the protection diode(s) without damage.
> 
> But you have encouraged me just to try a larger input resistor (e.g. 
> 100K).  Perhaps I should leave it running for some time with a 
> full-scale-input and see if any damage occurs (i.e. if it stops clipping 
> at the input I'll know I've burnt out the diode)

If you exceeded the input current, and would fire that parasitic 
thyristor (aka SCR latchup) the 4069 could burn out, since the thyristor 
would run enough current from Vdd to Vss for a meltdown. IIRC one would 
need several mA for fireing that thyristor, you should be safe with this 
now.

The diode itself is there for static protection, and can run more 
current than those few mA, but only if there is no power applied.

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159