Need PCB layout advice (adding a switching power supply).

[stevo]

I need to place a switching power supply (TI's PT5049A 12V step up to
20V power module) to a PCB which is used to amplify (100-2000 gains)
and filter (active low pass 5khz cutoff) small signals. The 20V is
used to power an accelerometer. The signal is AC coupled and the rest
of the circuit runs off of 5V.

Can anyone give me some advice on how to layout the PCB so as to
minimize the noise of the switching power supply. The 20V stuff can
be contained on one small portion of the board (i.e. 20v traces do not
run over most of the board). The PCB is a 4 layer (one ground, and
one 5.0V).

My main concern is that the switching power supply does not create
noise in the small signals that I'm trying to amplify and filter.

I am a newbie and ANY advice would be greatly appreciated.

Thanks and God bless you
Stephen

 

[Tim Wescott]

I need to place a switching power supply (TI's PT5049A 12V step up to
20V power module) to a PCB which is used to amplify (100-2000 gains)
and filter (active low pass 5khz cutoff) small signals. The 20V is
used to power an accelerometer. The signal is AC coupled and the rest
of the circuit runs off of 5V.

Can anyone give me some advice on how to layout the PCB so as to
minimize the noise of the switching power supply. The 20V stuff can
be contained on one small portion of the board (i.e. 20v traces do not
run over most of the board). The PCB is a 4 layer (one ground, and
one 5.0V).

My main concern is that the switching power supply does not create
noise in the small signals that I'm trying to amplify and filter.

I am a newbie and ANY advice would be greatly appreciated.

Thanks and God bless you
Stephen

First check to see if TI has any guidelines. If they don't, then both
National and Linear Technology do. Linear Technology also pays a lot of
attention to noise in their switcher designs -- you may want to consider one
of their chips if it's not too late.

Basically what you want to do to isolate a section of board is to build a
moat around it in your ground plane (literally -- you want a gap in the
copper), with one section connected. Make sure that no lines pass over the
moat; they should all pass over the connected section.

Then make sure that your input and output power runs are well filtered.
It's usually a good idea to have an extra LC filter on both the input and
output side. You'll probably want to take your power supply's feedback
before the extra LC filter to keep from screwing up the loop gains.

 

[Guy Macon]

Can anyone give me some advice on how to layout the PCB so as to
minimize the noise of the switching power supply.

Easy answer: many switching power supply ICs come with an
application note with a picture telling you exactly how to
lay out the board. If your IC doesn't, pick one that does.

My main concern is that the switching power supply does not create
noise in the small signals that I'm trying to amplify and filter.

How small? If they are very small, it may be that you simply
can't put a switcher in the same box. Please tell us more about
what you are trying to accomplish. What's the big picture? What
does the system that contains the PWB do?

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