[SI-LIST] Re: SSO and load capacitance
- From: Larry Smith <LSMITH@xxxxxxxxxx>
- To: steve weir <weirsi@xxxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Tue, 2 Aug 2011 08:18:18 -0700
Jason - the comment is correct: the worst case SSN waveforms will be found with
minimum load capacitance. But some explanation is required.
First, SSN can be broken down into two components: inductive coupling and PDN.
Steve is referring to the PDN part in his response. But usually the greatest
SSN noise amplitude measured at the far end of a signal transmission line comes
from inductive coupling, not PDN.
Inductive coupling is related to mutual inductance between aggressor signals
and the victim signal. It only happens during the rise (fall) time of the
driver because that is when the di/dt takes place. To a first approximation,
the voltage noise that gets launched into a victim transmission line (under the
BGA that makes the SSN) is proportional to m*di/dt where m is the sum of the
mutual inductance from all the aggressors to the victim and i is the current in
the aggressors. Mutual inductance occurs in the wire bonds, package vias,
balls and PCB vias and to a first approximation is proportional to the length
of these structures.
These days, the aggressor rise time is on the order of 200pSec, which is the
time that it takes signals to travel about an inch down a transmission line.
The capacitance load in question is down at the far end of the transmission
line, let's assume 6 inches. The 200pSec rise time aggressors launch an SSN
noise pulse into the victim signal net that is approximately 200pSec wide and
it arrives at the capacitance load about 1000pSec later. The load capacitance
at the far end will have no effect on the SSN event that launches the SSN
glitch into the victim transmission line.
When the SSN glitch arrives at the far end of the transmission line, it often
finds a 50 ohm termination. The noise measured at the far end is identical to
the glitch launched into the near end, assuming lossless lines. Now if there
is any capacitance load at the far end, glitch energy goes into charging up the
load. The measured SSN glitch voltage amplitude will be less with more load
capacitance.
Regards,
Larry Smith
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of steve weir
Sent: Tuesday, August 02, 2011 4:41 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: SSO and load capacitance
Oops the second formula was energy not charge. It should have read
Qload comes from Qbypass = Vdroop*Cbypass.
Steve
On 8/2/2011 4:24 AM, steve weir wrote:
> Jason, there are two possible sources of confusion. The first is
> possible confusion between output load capacitance with die capacitance
> per output driver. Your intuition is correct: If we simplify the PDN /
> driver network to a switched capacitor representation, then we deposit
> Qload = Vdd*Cload on each output line that switches from low to high,
> and remove Qload from each output that switches from high to low. For
> the low to high switching outputs: Qload comes from Qbypass = (Vdd -
> Vdroop)^2/2*Cbypass.
>
> The second source of confusion comes from the fact that any loads that
> remain statically high can draw current from any load capacitance that
> connects to the driver outputs, supporting other outputs that switch
> from low to high.
>
> Steve.
>
>
> On 8/2/2011 3:01 AM, Jason Young wrote:
>> Dear Experts,
>> I have read a couple of documents are from silicon IP vendors discussing the
>> number of power/ground pads needed to meet SSO requirements for a given
>> number of output drivers. These documents mention that worse case conditions
>> for SSO are with the smallest output load capacitance. At first this seems
>> counter intuitive. My initial reasoning would be that a larger capacitance
>> would present a lower impedance load and hence greater dI/dt, greater IR
>> drop and greater supply rail bounce. Could you please help me understand?
>> Regards,
>> Jason
>>
>>
>>
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>
--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV 89501
www.ipblox.com
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