Well, sounds like to me, since this is an industrial product, that the best=
 way to do this is a negative buck regulator.  Just have to find room for i=
t someplace.

Thanks for all the replies

--- On Fri, 2/26/10, Michael Rigby-Jones <Michael.Rigby-Jones@oclaro.com> w=
rote:

> From: Michael Rigby-Jones <Michael.Rigby-Jones@oclaro.com>
> Subject: RE: [EE] poor mans negative rail generation
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>
> Date: Friday, February 26, 2010, 10:17 AM
> =

> =

> > -----Original Message-----
> > From: piclist-bounces@mit.edu
> [mailto:piclist-bounces@mit.edu]
> On
> Behalf
> > Of alan smith
> > Sent: 26 February 2010 15:47
> > To: Microcontroller discussion list - Public.
> > Subject: [EE] poor mans negative rail generation
> > =

> > OK, so I have a part that requires a +/-12V on it, and
> I do have +24V
> > available.=A0 So, a poor mans method would be to
> use a voltage divider
> and
> > generate a +/- 12V and use the centertap for the
> ground reference.
> The
> > 'ground' or virtual ground, would be completely
> isolated from the rest
> of
> > the circuit of course.=A0 The kicker is that there
> are digital signals
> (SPI),
> > referenced back to the board ground, and the part
> requires a digital
> 3.3V
> > for this interface (actually can be 5V).=A0 I could
> generate a local 5V
> from
> > the +12, referenced to the virtual ground again but
> the only way I can
> see
> > to get the SPI is to optically/transformer couple it
> into this
> section.=A0 I
> > believe ADI makes a part that can do this, and its not
> the fact that
> its
> > another part, its also room on the board for another
> part.
> > =

> > Short of this idea, what other methods to generate a
> split supply from
> the
> > +24V ?=A0 I have very little room for much of
> anything, so it has to be
> > small and pretty sure not enough room for a negative
> buck switcher
> unless
> > very tiny.=A0 Total current isnt going to exceed
> 1A
> =

> You need to make absolutely sure that there will be no
> significant
> digital currents introduced into your virtual ground as any
> SPI noise on
> this will come straight out of the device as a voltage or
> current.=A0 =

> =

> You may be able to use current mirrors to do the voltage
> translation;
> they are often very handy for this kind of thing IME.
> =

> Mike =

> =

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