[sdiy] Tricky PCB layout with CA3046

[rburnett at richieburnett.co.uk]

> Is it ok to tie the collector and emitter of an unused transistor on
> a CA3046 to -12V?
> 
> I've got a tricky PCB layout problem, and if I could route the -12V
> straight through pins 7 and 8, it'd help quite a lot.
> 
> It's just an unused transistor, right? It's not connected to anything
> else on the chip that I need to worry about?

Hi Tom,

I think it depends what you are doing with the emitter of Q5 (pin 13).  
This is connected to the bulk silicon substrate underneath the other 
transistors and must be connected to the most negative voltage present 
of all the transistor connections.  So if your emitter of Q5 (substrate 
pin 13) is already connected to -12V (or even more negative) then you 
can go ahead and connect pins 7 & 8 to -12V without any worry.

However... If the substrate pin 13 is connected to something of greater 
voltage than -12V (such as 0v ground) then you won't be able to take 
pins 7 & 8 down to -12V.  Attempting to do this will forward bias the 
parasitic diode between the Q3 transistor and the substrate and short 
the -12V supply to ground.

Thats my understanding of the issue anyway.  I hope this helps.

If you're really stuck I guess you could always just route the -12V 
track through pins 7 & 8 on the PCB, but snip those IC legs off before 
you fit the 3046 chip in the board!

-Richie,