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

Dmitriy -

I wondered the same thing but I imagine that it's cost / real estate.
And an LDO regulator would still be required although positive.  So
this way all he needs is a negative LDO regulator.

Chuck

On Thu, Jun 12, 2025 at 9:03 AM Dmitriy Kiryashov
<[email protected]> wrote:
>
>  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
> >
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