[sdiy] Overdriving 4069UB

[Seb Francis]

René Schmitz wrote:

> Hi Harry, Seb and all,
>
> Harry Bissell Jr wrote:
>
>> with a 10K resistor in the inout you might be over the edge. I'd be
>>   concerned. You might want to use shottky clamp diodes instead
>>   of allowinng the CMOS input diodes to clip.
>
>
> A few mA into the diodes won't hurt. There is a parasitic thyristor 
> which might fire when a certain current is exceeded. No need for extra 
> shottkys IMO, I'd rather make that resistor a little larger.
> Ok, this is stretching the ratings in the datasheet a little, but does 
> work.
>
> Btw, this sort of overdrive is explicitly allowed for the 4049 and 
> 4050, to be able to do a 15V to 5V crossvolt conversion. Check the 
> datasheet on these.
>
 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)

Seb