[SI-LIST] Re: SSN Vs Load cap
- From: steve weir <weirsi@xxxxxxxxxx>
- To: starsilic@xxxxxxxxx,Ihsan Erdin <erdinih@xxxxxxxxx>
- Date: Tue, 06 Feb 2007 06:36:35 -0800
For my $0.02 anytime I've looked at this increasing C can increase or
decrease SSO depending on several factors not the least of which is
starvation which depends on the driver design and the PDN up to the
driver itself. I think you want to take a cue from Chris Cheng here
and model your power delivery network with your drivers before you
draw conclusions. I would be very hesitant to recommend loading a
driver with unnecessary C as a means of improving package issues.
Regards,
Steve.
At 03:06 AM 2/6/2007, Canes Venatici wrote:
>sso noise = L.di/dt = LCd2v/dt2 implies, for a constant L and once
>we'd reached the
>peak driving current, the increase in cap, reduces the dv/dt. So
>noise should be constant.
>But, since noise is prop. to d2v/dt2, the increase in cap makes,
>d2v/dt2 term to reduce much more than dv/dt.
>So the SSO noise reduces.
>This is simple as i and v are dual to each other.
>Correct me if I'm wrong.
>----- Original Message ----
>From: Ihsan Erdin <erdinih@xxxxxxxxx>
>To: starsilic@xxxxxxxxx
>Cc: sabbu1981@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
>Sent: Friday, February 2, 2007 6:33:17 PM
>Subject: Re: [SI-LIST] Re: SSN Vs Load cap
>
>Canes,
>
>Sabayachi's explanation is plain and accurate. i=Cdv/dt is a lumped
>approximation to Maxwell-ampere eq.; so is v=Ldi/dt to
>Maxwell-Faraday. As such, both have physical connotations. Your
>di/dt=Cdv2/dt2 is mathematically correct but doesn't have any useful
>physical connotations.
>
>Regards
>
>Ihsan
>
>On 2/2/07, Canes Venatici <starsilic@xxxxxxxxx> wrote:
> > Upto the maximum current driving capability of driver, the
> increase in load capacitance causes
> > the current rate to increase and noise increases, so SSO index
> should reduce.
> > Once its reached the maximum drive capability at a particular
> load cap, the increase of cap
> > causes the rate of voltage to fall, as per the max. current spec.
> But since the current remains same (as the driver
> > cant supply more than that), I expect SSO index to be unchanged.
> > Equations :
> > i = c. dv/dt
> > di/dt = c.(d2v/dt2) ....(its d square v by dt square)
> > sso noise = L.di/dt = L.c.(d2v/dt2).
> >
> > Please clarify, if I'm missing anything.
> >
> > Regards
> > Canes
> >
> > ----- Original Message ----
> > From: Sabyasachi Mohapatra <sabbu1981@xxxxxxxxxxx>
> > To: si-list@xxxxxxxxxxxxx
> > Sent: Thursday, February 1, 2007 6:13:12 PM
> > Subject: [SI-LIST] Re: SSN Vs Load cap
> >
> > It is pretty simple.
> > as u increase the load cap with same drive strength of driver,
> > rise or fall slew will degrade.
> > As the buffers will be switching slower now, dI/dt will be less,
> > rate of change of current though package inductor will be less.
> > So less SSO noise.
> >
> > --- In si-list@xxxxxxxxxxxxxxx, Canes Venatici <starsilic@...> wrote:
> > >
> > > Hi all,
> > > I couldn't understand why if load capacitance is increased, SSN is
> > reduced (as the recommendation is increased SSO index, more signal
> > pads can be accommodated by power pads).
> > > As di/dt is directly proportional to load cap, increasing the load
> > causes more drop in parasitic inductance and SSN should increase.
> > > Could anyone clarify.
> > > Thanks,
> > > Canes
> > >
> > >
> > >
> > >
> > >
> > >
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