Re[2]: [SI-LIST] : power supply filtering and bypassing

[email protected]
Fri, 06 Mar 98 08:00:13 -0800

Having used card entry LC filters before, there are many pitfalls associated
with them. For sure, the filter should be layed out with a series damping
resistor between the capacitor connections and ground. This resistor value can
be on the order of 1 ohm, and will dampen any filter resonance. If the damping
resistor is left out, it is possible that a transient at the frequency of
L1-(C1+C2+Cxx) could ring the filter, exacerbating whatever peak amplitude
occurs at the input.

The filter can be easily modeled on PSPICE. Measuring the freq response of the
filter is not an easy task - as the filter response will be very sensitive to
source impedance.

Of course, the best solution is a low droput linear regulator - I'm assuming you
don't have the voltage headroom to use a regulator.

Ed Crean

______________________________ Reply Separator _________________________________
Subject: Re: [SI-LIST] : power supply filtering and bypassing
Author: Paul Thompson <[email protected]> at SMTP
Date: 3/5/98 4:14 PM

At 2:41 +0000 3/5/98, Skey, Kevin wrote:
>I'm planning a layout change, separating the GLINK's power and ground
>layer from the rest of the PCB and use a pi filtering arrangement to
>connect the planes, I'm not sure what filter values would be acceptable
>or if this is the right approach at all, any ideas?

Do isolate and filter the power, but think twice before chopping into the
ground plane. This can cause new problems for signals that need to cross
the split, as the return currents won't have a good path which can result
in degraded signal quality and higher EMI. A safer approach is to leave
the ground plane intact, and make sure all traces going over the split are
closer to that plane than the power plane. This will minimize image
currents in the power plane.

As to the type of filtering for the power, you can use either an inductor
or resistor in series with the power (if your average current draw is low
enough so the IR drop is acceptable), and then a variety of caps on the
isolated plane. For instance you might try something like a 47uF Tantalum
and a 2.2uF ceramic (in addition to the .01uF's right on the power pins).
You'll see a dramatic decrease in noise.

If the IR drop is critical, then using a linear regulator from a higher
voltage rail serves as a good filter, albeit at a higher price.

Paul Thompson                                        [email protected] 
System Integrator, Macintosh Desktop Systems         Apple Computer