[SI-LIST] Re: power planes (4-layer-board)
- From: Daniel Bauer <daniel.bauer16@xxxxxx>
- To: davidjp@xxxxxxx, steve weir <weirsi@xxxxxxxxxx>
- Date: Mon, 16 Feb 2009 22:39:07 +0100
Hi,
do you know any rule of thumb for the maximal size of such an power island
versus the signal frequency / signal rise (fall) time? Only a default example:
power island with one dimension in x direction of 12cm has a bare-board
resonance of approximately 244MHz; which max rise time / signal frequency
should the signal have?
David, could you please tell me the reasons for the restrictions for the
stripline calculation (http://emclab.mst.edu/pcbtlc2/stripline.html)?
W/(H-T) < 0.35; T/H <0.25;
best regards
Daniel
> Daniel, as you propose this will work very well. It avoids injecting
> return current into a high impedance power cavity which can become an
> EMI / SI / PI nightmare. Just be sure to stitch the GND shapes on layer
> 3 well to layer 2 and manage the impedances and isolation as needed of
> your power islands.
>
> Steve.
> Daniel Bauer wrote:
> > Hi,
> >
> > the standard layout for an 4-layer PCB is, using outer layers for the
> > signals and the two inner layers for one ground and one power plane. The
> > power plane seems to have a few advantages as well as disadvantages (using
> > an 4-layer board).
> >
> > If a trace running at the top layer has to change to the bottom layer -
> > also the return current has to change the layer (using stitched
> > capacitors).
> >
> > What do you think about the proposal to use the third layer for ground as
> > well as for power? On every position where an signal trace is running at
> > the bottom layer, the third layer will be a ground plane, connected to the
> > ground plane on the second layer.... and when there`s no trace on the
> > bottom layer I will use an power plane on the third layer. From this it
> > follows that I will get some power islands on the third layer which were
> > connected with traces on the bottom or top layer. The complete return
> > current should use the ground plane (which has an lower impedance) and not
> > one of the small power islands.
> >
> > Would there be an impedance change for the signal traces changing the layer
> > (top to bottom or vica versa) when both inner layers will use an ground
> > plane where the signal is running?
> >
> >
> > x----------| (top layer - signal layer)
> > GND | GND (second layer is a complete ground plane)
> > Power | GND (ground and power islands)
> > |---------x (bottom layer - signal layer)
> >
> > The line will show a signal trace changing from the top to the bottom
> > layer. Both ground planes using at the inner layers, will be connected
> > together by many vias. Could you tell me if this would be a good decision
> > improving signal integrity? Are there any kind of disadvantages?
> >
> >
> > best regards
> > Daniel
> >
> >
> >
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