Re: [EE] looking for negative 5V regulator or circuit

 Hi Dwayne.

If you goal is to switch hi-side ( +12V ) on and off to some load ( in reference to the ground )
why don't you try pair P-FET on high side ( instead of PNP ) and N-FET to ground controlling
that P-FET ( plus few resistors ).

What is benefit of creating that artifical "-5 below incoming +12" level instead of doing control
in reference to the ground ?


Dmitriy.
     On Wednesday, June 11, 2025 at 09:37:44 AM PDT, Dwayne Reid <[email protected]> wrote:  
 
 I see that I was not sufficiently clear about what I am doing.  My
apologies.

I am doing hi-side switching from the incoming +12 Vdc supply.  It looks
like the easiest and least expensive way to do this is to use 74HC595 shift
registers driving MMBT4403 transistors.  THe emitters of the transistors
are tied to the incoming +12V rail.  So are the VDD pins of the shift
register and the PIC that is driving the shift register.

So - I need to create a -5V rail with respect to that +12V rail.  In other
words, about 7 Vdc.  My initial thought was to use a 7905B regulator with
its input tied to incoming ground and the bias pin tied to the +12V rail.
That gives me -5 Vdc with respect to the incoming +12V rail.  The problem I
have is that the 7905 data sheet says that worst case bias current is 8 mA
which is really high for a vehicle that might be sitting idle (turned OFF)
for weeks on end.

So - I'm looking for a -5 Vdc regulator that has significantly-lower
quiescent current than the 7905.  My fall-back position is something like a
MPSA63 voltage-follower being fed with a 6.2V Zener or, as was suggested
here, one of the low-current TL431 variants.  I would use something a bit
beefier than the MPSa63 but you get the idea.  I'm wanting to get the
quiescent current down to a couple of hundred uA.

I estimate the total current on that -5V supply is less than 50 mA,
depending on how many shift register / transistor stages I need.  Might be
as many as 16 outputs, each transistor getting a bit more than 2 mA drive.

As always, suggestions and comments gratefully accepted!

dwayne

On Tue, Jun 10, 2025 at 12:43 PM Dwayne Reid <[email protected]> wrote:

> Good day to all.
>
> I'm working on a project where I need a negative 5 Vdc rail relative to
> the incoming +12 Vdc rail.  My first thought was a bog-standard 7905B
> regulator (the "B" suffix denotes -40C operation) but this is a
> battery-powered system and the worst case bias current of 8mA is way too
> high.
>
> I spent some time looking at Digikey offerings but they all seem stupid
> expensive.  Can anyone suggest a part that is reasonably inexpensive and
> works down to -40C?  Current is fairly low - should be less than 20 mA
> worst case.  I sort of prefer a package larger than SOT-23 just because of
> the fairly minimal heat dissipated.
>
> My alternative is a 6.2V Zener diode feeding a PNP Darlington pass
> transistor.
>
> Suggestions and opinions greatly appreciated!
>
> dwayne
>
-- 
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
https://mailman.mit.edu/mailman/listinfo/piclist
  
-- 
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
https://mailman.mit.edu/mailman/listinfo/piclist