RE: [SI-LIST] : Decoupling capacitors (again!)

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From: Zabinski, Patrick J. (zabinski.patrick@mayo.edu)
Date: Tue Aug 15 2000 - 08:40:50 PDT


Martin,

I can't offer much advice, but I can possibly offer some
comfort in that I've had the same problem. For one design
I was recently involved in, I tried to follow the same
approach/theory, and the end result was that I needed
80 decoupling capacitors per ASIC (to maintain 10%
dV), and I had 32 ASICs per board (>2500 caps per board!).
After having others verify
my numbers/calculations, I took close look and realized
the caps would consume more board space than the ASICs.

I could not justify, believe, or afford this, so I
ended up backing down and relying on my old rules of
thumb (BTW: I hate rules of thumb, but I sometimes
use them when I have no better way). The board works
fine with only 12 caps per ASIC.

Looking back, I can see three possible reasons why the approach you
and I took is not quite complete:

        * component packaging effects are not taken to
          account. Not definite on this, but I believe
          a poor package would probably negate any capacitance
          you might have on the board.
        * the board's self-impedance. I believe Larry's
          approach addresses this as effective increase
          in inductance, but the ground/power plane itself
          does offer a low-impedance capacitance. Regardless
          if you have any discrete caps on or not, the planes
          offer some inherent, built-in capacitance.
        * most (all?) dI/dt effects are self-limiting.
          For the calculations you used, they assumed
          dV=0.0. However, if dV>0, then dI/dt will
          be reduced all on its own. I don't have any
          data or theories on how much, but dI/dt
          is likely to be reduced from what you
          predicted (also tied into/related to the
          first issue about packaging).

Sound reasonable? Comments?

Anyway, I sympathize and hope you find a solution. If you
do, please share.

Pat

> -----Original Message-----
> From: Martin J Thompson [mailto:Martin.J.Thompson@trw.com]
> Sent: Tuesday, August 15, 2000 9:49 AM
> To: <"si-list@silab.eng.sun.com"
> Subject: [SI-LIST] : Decoupling capacitors (again!)
>
>
> Hi all, this is my first time posting here, although I've
> been lurking for a while.
>
> My problem is figuring out the decoupling requirements for
> this system:
> FPGA, DSP, 6 SDRAMS, 2 flash, DPRAM, clock frequency is 100MHz.
>
> According to my calculations, my I/O's need to drive a total
> of about 1.5nF of I/O and trace capacitance.
> To achieve the 0.5ns edges that the FPGA will drive (3.3V
> supply) it looks like I need dI=4amps. This is assuming that
> 50% (is this typical?) of the I/O's toggle each cycle. (dI=0.5Cdv/dt)
>
> To achieve a dV of < 0.1V this implies a target impedance of
> around 20mohm, flat up to 1GHz! (Z=dv/di)
>
> This then seems to need around 500-800 decouping caps spread
> around, which is an order of magnitude more than I've ever
> used in the past. This is the first time I have taken a
> 'design' approach to the problem, but the previous boards
> have worked, using various rules of thumb.
>
> Is this sort of number of caps to be expected in this sort of
> system, or can anyone see any sillies in my understanding (or
> even in the sums!)?
>
> Now, if I don't get right out to 1GHz, the edges will suffer,
> but that wouldn't necessarily matter if they stayed below
> 1-1.5ns. Or would this cause the supply to droop elsewhere?
>
> As you might gather from the analysis above I've read Larry
> Smith and co's paper on decoupling design, which states that
> a flat target impedance is indicated. If I can analyse my
> application enough, can I then shape the Ztarget vs frequency
> to make life easier?
>
> Many thanks for your time, any help greatly appreciated,
>
> Martin
>
>
>
> TRW Automotive Advanced Product Development,
> Stratford Road, Solihull, B90 4GW. UK
> Tel: +44 (0)121-627-3569
> mailto:martin.j.thompson@trw.com
>
>
>
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