CA3046 subtle design info

[Rene Schmitz]

At 10:00 01.07.99 -0700, Tim Ressel wrote:
>To all--
>This bit me as I wired in a 3046 as a heater element in my VCO design:
>
>Quote from the 3046 data sheet:
>"The collector of each transistor of the CA3045 and CA3046 is isolated
from the
>substrate by an integral diode. The substrate (Terminal 13) must be
connected to
>the most negative point in the external circuit to maintain isolation between
>transistors and to provide for normal transistor action."

Right!

>It turns out that these intrinsic diodes can wreck havoc as the partially
>conduct or leak or whatever they do.
>My solution was to not use the pin 13 transistor and just tie pin 13 to
-15V. I
>hope this info helps anyone who 
>is seeing weird stuff in their circuits.

Don't do this, unless your collector potentials are all below ground. The
max ratings for Vce are 15V! and Vcio (collector to substrate) are 20V. 

Thus (with +-15V supplies assumed.) tie the pin 13 to a voltage at least
one diode drop more negative than the most negative terminal. That is some
-2..3V or so usually for expo convertors.

However I found that most CA3046s won't fry or something when you exceed
the Vce and Vcio voltages. But it would be "bad" design if one exceeds the
limits of the datatsheets.

Bye
 René


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