On 28 Jun 2018 at 11:36, Justin Richards wrote:

> Working on a 10cm x 10cm board.
>=20
> Vin is 12V, gets converted to 5v and 3.3v.

Do you have any/much circuitry directly running off the 5V? I.e. if the 5V =
rail is=20
merely a pre-regulated supply for 3.3V linear reg(s) then there is a degree=
 of=20
isolation - and a convenient place to add filtering/suppression.
=20
> Bottom plane is ground and top layer will be 3.3v flood.
>=20
> As always, space is constrained.
>=20
> I have routed the 12v (Vin) 2mm trace on the top layer around half of the
> perimeter.
>=20
> I want to route the regulated 5v 2mm trace directly underneath the 12v
> trace.
>=20
> Is this a bad idea.  Should the 5v trace be routed such that it also runs
> on top of the ground plane.

Not to say it's a bad idea, but on top of each other does intuitively seem =
to be the=20
formula for best possible coupling. The toss up you say is space, and if sp=
ace wins=20
that's fine, as long as effects of coupling are negligible on performance. =
If=20
performance proves to be a factor, then cost is probbaly the next thing to =
look at...=20
e.g. 4 layer board.

> The 12v trace will be supplying an inductive load that gets switched on a=
nd
> off and draws about 500mA.

Nice thing about inductive loads is current ramps up and down slowly. Propb=
lem of=20
course when there is no flyback diode/suppression. If the inductive load is=
 on the=20
circuit board it's somewhat easier to account for. Off board needs a little=
 more=20
thought... long wires pick up noise from "outside" and couple back to your =
5V, etc.

Just my 2c worth.

--=20
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