[SI-LIST] Re: Effect of USB attachment's current drawn through motherboard bypass network

From: John Barnes <jrbarnes@xxxxxxxxx>
To: Peter Baxter <peter.baxter@xxxxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
Date: Fri, 01 Mar 2002 09:19:20 -0500

Peter,
As a rule of thumb, wiring has about 25nH/inch or 1nH/mm inductance.  So
*any* length of cable between two pieces effectively isolates them from
one another at frequencies above a few MHz.  

Design each card to provide the power distribution and bypassing it
needs all by itself.  You will need to design the motherboard to safely
carry the current drawn by the attachment.  If the power supply feeding
the motherboard can provide more than 8 amps, and the attachment is or
could be outside the motherboard's enclosure, fuse *all* the power pins 
going to the attachment cable.  (This mistake cost me a third spin on
the X820e's controller card.)  Put a bulk electrolytic capacitor very
close to the attachment's connector on the motherboard, to keep current
surges on the cable from propagating onto the motherboard.  And both the
motherboard and the attachment should have bulk electrolytic capacitors
close to their power-entry points, to keep them from putting current
surges onto their power cables.

I go further in my designs, and design each circuit as though it was
totally isolated.  This includes all the bypassing needed by each IC,
which I show on that IC's schematic pages.  Then in layout I make sure
that the ceramic bypass capacitors are as close as possible, and never
more than 1/2 inch (12.5mm), from the IC's power and ground pins that I
am trying to protect.  If the manufacturer of the IC doesn't recommend a
bypassing scheme, I use:
*  One 0.1uF SMT ceramic bypass capacitor for each power pin or 
   cluster of power pins (maximum of one non-power pin or ball between
   the pins/balls in a cluster) for a certain supply voltage.
*  One 2.2uF SMT ceramic bypass capacitor for each corner/edge of an
   IC that has that supply voltage, up to four per IC per supply 
   voltage.
*  One additional 220pF SMT ceramic bypass capacitor on each 
   high-frequency power pin/ball-- oscillators, phase-locked    
   loops (PLL's), etc.
*  One bulk electrolytic capacitor:
   -  At the power entry point on the card, for each supply voltage.
   -  At each connector carrying power off the card, for each supply
      voltage.
   -  At each socket for a plug-in card, such as a memory module, for
      each supply voltage.
   -  Per each 4-9 square inches of board area, spread relatively 
      uniformly across the board ("Coffee Cup" Rule-- if you wave a
      coffee cup over the assembled card, it should always cover at 
      least one bulk capacitor).

                                        John Barnes
                                        dBi Corporation
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[SI-LIST] Re: Effect of USB attachment's current drawn through motherboard bypass network

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