Ray makes some fine points on this topic.
I have been looking at some simple low pass filters(RC and RLC) for
individual devices with internal PLL's sensitive to low frequencies in the
500 - 900 kHz range. In this application a series 5-10 Ohm should work
well with only one device drawing current through it. The SPICE ideal AC
source does a good job for matching the low source impedance presented by
the power planes.
Has anyone had any luck getting good spice models for surface mount Ferrite
beads?? One vendor gives out a very simple model good for one discrete
frequency, this is pretty useless when sweeping the frequency in SPICE.
Also on the issue of saturation with ferrites and inductors, I haven't
seen any discussion of this in the data sheets. Any ideas out there other
than to stay well below the max current rating? I have seen some
applications with 3 parallel ferrite beads side by side on the board to
avoid saturation, this scares me a little, can there be any self
resonance effects similar to what can happen when bandspreading bypass caps
or does the loss of the ferrites prevent this?
John
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John M. Fisher -- Cisco Systems -- [email protected]
170 W. Tasman Dr., Bldg G, San Jose CA 95134-1706
__ _ _
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Signal Integrity Engineer, Enterprise Line of Business
408-527-9186, Fax 408-526-5504, N6PFN
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At 09:05 AM 3/6/98 -0800, Ray Anderson wrote:
>
> I just wanted to echo Ed's ([email protected]) comments
regarding
> power supply filtering add a couple amplifiying remarks.
>
> In many cases designers have used the small 3 terminal integrated filters
> that combine a couple of capacitors and a ferrite inductor into a single
> unit (typically from TDK, Murata and others) and have been suprised that
> the addition of the filter exacerbates the problem rather than solving it.
>
> If you look at the frequency response curves in the vendor's catalog you
> see a nice, well behaved, low-pass characteristic. The gotcha' is that these
> parts are characterized in a 50 ohm system. Indeed, in a 50 ohm system
they are
> well behaved low pass filters. However, when applied as power supply filters
> this assumption falls apart. In that application they are being driven from
> a very low impedance source (typically on the order of tens of milliohms
or less).
> When driven from a very low impedance then nice low pass response
degrades to
> a very peaky response somewhere near the anticipated roll-off frequency.
>
> I have seen cases with peaking on the order of 10 dB and more. The net
result is
> that any noise energy on the input of the filter that you are trying to
> squash can actually appear on the load side of the filter with a much higher
> amplitude than on the input side.
>
>
> This phenomena can be modeled quite nicely with spice and analyticaly if you
> take the time to work through the transfer function of the LPF.
>
> A typical fix is to make sure a small amount of resistance is in series
> with the input of the filter to effectively raise the driving impedance
> that it sees. One needs to make sure that this doesn't introduce excessive
> DC voltage drop when the load is drawing max current. This typically works
> well for low current loads such as isolated Vcc pins for PLL's and the like.
>
> And as Ed mentioned in an earlier post, this issue is also directly relevant
> to other power supply filtering schemes composed of discrete parts. The same
> caveats apply.
>
> -Ray Anderson
>
> Staff SI Engineer
> Sun Microsystems Inc.
> [email protected]
>
>