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

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From: Larry Miller (ldmiller@nortelnetworks.com)
Date: Tue Aug 15 2000 - 08:08:53 PDT


I can hardly believe that you have 1.5 nF of load capacitance. How about 150
pF (even that seems high in such a high speed system). That would reduce
your needs by an order of magnitude......

        Cheers,

        Larry Miller

> -----Original Message-----
> From: Martin J Thompson [SMTP:Martin.J.Thompson@trw.com]
> Sent: Tuesday, August 15, 2000 7:49 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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