[SI-LIST] Re: DC power supply tolerance for target impedance calculation
- From: Feng Wu <wufengthu@xxxxxxxxx>
- To: sij99@xxxxxxxxx
- Date: Fri, 6 Sep 2013 15:46:52 +0800
Hi Jack
I classify the power voltage budget into 3 parts.
1) Self introduced noise: caused by the sink current (static and dynamic)
of the load itself
2) VRM output tolerance.
3) interference: caused by the sink current (static and dynamic) of the
other load in the same power rail.
The voltage variant (dV) used in target impedance calcuation usually does
not include VRM output tolerance.
dV usually comes from the AC part of 1) for self impedance and 3) for
mutual impedance.
when you look at the VRM output tolerance, the following parameter is
important.
1) set point error
2) temperature regulation/aging
3) line regulation
4) load regulation
5) Ripple
6) PCB IR drop from Sense point to the load.--> I usually don't treat it as
VRM output tolerance for most of the cases.
6) transient response --> it is usually not treated as VRM output
tolerance, while it can be used to extract the VRM model.
exactly the VRM spec is based on a specific load and cap configuration. in
your real product, the power engineer will adjust the bulk cap and VRM
internal parameter. then you will see different value with the one in the
spec.
So the best way is to ask your power engineer about the VRM performance.
Thanks
Feng
--
B.R.
Feng Wu (Îâ·ã)
-------------
Signal Integrity Engineer
Cisco Shanghai
-------------
2013/9/5 Jack Si <sij99@xxxxxxxxx>
> Hi experts,
> Usually for my calculation, I take 5% as tolerance for target impedance
> calculation which includes (Power supply tolerance-2%, AC Ripple
> Tolerance-2% and DC drop across the plane-1%). But for a case, my loads
> which require 2% of accuracy. So I can't take a typical value as 2% for
> power supply tolerance. In power regulator data sheet, I found the values
> of load regulation, line regulation and Maximum ripple. I consider the load
> regulation+line regulation value as power supply tolerance. Isn't right? Or
> should I include any other parameter also...
>
> Thanks,
>
> Jack
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