On Thu, 29 Jul 1999, Jim Hartmann wrote:

>        More on my problem with increasing supply current when
>        protection diodes are forward biased.   My circuit forward
>        biases the upper protection diode with a very small current.
>
>        I did some measurements and found that supply current increases
>        in proportion to input protection diode forward bias current.
>        With zero current into the protection diode my PIC draws
>        30.5uA.  With 2.5uA into each of two pins, total current
>        increases to 34.1uA (the sum of the pin currents plus supply
>        current).  With 5uA into each of two pins, total current
>        increases to 37.6uA.
>
>        The same circuit with the same pins driven as high outputs
>        results in no increase in total current when current into the
>        pins varies from 0 to 5uA each.
>
>        My conclusion is when designing micropower circuits avoid
>        forward biasing the upper input protection diodes.
>
>        Anyone seen similar results?
>
>        Regards,
>        Jim Hartmann

Could this be the onset of a latchup condition? I realize that currents
induced by latch up are catastrophic, but perhaps there's a similar
phenomenon taking place. The P-N junction of the input protection diode
may in fact be part of a parasitic transistor. By forward biasing this
junction, you may inadvertantly cause parasitic currents to flow else
where. This is only a plausible explanation - I have no experience with
situation.

Scott