[SI-LIST] Re: SSO and load capacitance
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 02 Aug 2011 04:40:56 -0700
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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>
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Steve Weir
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