[SI-LIST] Impedance - Power System - Mother Board and Attachment

Hi,

I may have over-designed a power system on a PCB - and un-necessarily
made it more expensive than it need be. If I have a clear understanding
of the way it should be, I may be able to use fewer PCB layers and less
decoupling capacitors.

I have an attachment (500mm/18 inches away - on a cable - like a mouse)
that plugs into the mother board through a cable. The logic on the
attachment PCB switches at 2-amps. Its power comes from the
motherboard's logic power planes, via a polyswitch fuse, and down the
cable.

I have designed the attachment's "power system impedance" to meet the
2-amp switching currents. So the attachment is fine. 

The motherboard's logic switches at 4-amps. Power from an ATX power
supply goes to its power supply planes. Then power from these planes is
drawn off by the attachment, via the polyswitch fuse. 

I have designed (or overdesigned?) the motherboard's "power system
impedance" to meet 6-amps (4-amps for the motherboard and 2-amps for the
attachment = 6-amps). 

The more I think about it, the more I wonder whether the motherboard's
power system impedance NEED ONLY MEET 4-amps. Possibly 6-amps is
incorrect.

Maybe it only needs to meet 6-amps at DC as each PCB's power system
impedance only needs to cater for its own "fast" switching logic.

Any advice is welcome. 

Thanks

Peter Baxter
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