[SI-LIST] Re: Current sense resistors and power integrity
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Mikhail Matusov <matusov@xxxxxxxxxxxx>
- Date: Sat, 23 Aug 2008 00:31:19 -0700
Mikhail, no the output.
VRM => resistor => bulk caps => mlcc caps / loads
This has the least impact on bypass and ringing due to resonance between
the resistor inductance and the bypass network. If mounted, the
resistor exhibits a few nH then for typical ESRs the combination w/ the
bulk caps will be overdamped.
Steve.
Mikhail Matusov wrote:
> Thanks everyone for your help and suggestions. This is actually a
> production board, so the only real choice I have is to replace the
> resistors with a piece of metal or something of that sort. They are by
> the way 2512 Panasonic ERJM1W 1% tolerance type. Inductance is claimed
> to be less than 1 nH. I had a mysterious failure on this board, which
> I could only explain by something being wrong with the power. I was
> about to rent a VNA to try to measure the PDN impedance when I
> discovered that the core voltage was actually set at the low threshold
> of the part's spec, and after the resistor it was a few mV below the
> minimum spec. I increased the voltage and everything works fine now,
> but I am still curious to know whether I should get rid of the
> resistors... Perhaps, they at least should have been in the
> regulator's loop...
>
> Steve, when you are saying to place the resistor back at the VRM, do
> you mean at the input of the VRM?
>
> Thanks,
> /Mikhail
>
>
>
>
>
> ----- Original Message ----- From: "steve weir" <weirsi@xxxxxxxxxx>
> To: "Mikhail Matusov" <matusov@xxxxxxxxxxxx>
> Cc: <si-list@xxxxxxxxxxxxx>
> Sent: Friday, August 22, 2008 4:05 PM
> Subject: Re: [SI-LIST] Current sense resistors and power integrity
>
>
>> Mikhail, what you need to do is maintain a low enough impedance on
>> the load side. The resistor / etch drawbridge or other method will
>> introduce a substantial inductance that you will need to compensate
>> for with a suitably low inductance capacitance on the other side,
>> that also does not resonate with the inductance in your signal
>> frequency range.
>>
>> What I would recommend you do if you can is place your current sense
>> back at the VRM, and then measure with your chip of interest at
>> quiescent versus active. The quiescent on most FPGAs and DSPs can be
>> made pretty small. This way you don't need extra parts in your
>> bypass network, and you won't starve your parts.
>>
>> Steve.
>> Mikhail Matusov wrote:
>>> Hi all,
>>>
>>> Is that a really bad idea to insert a small current sensing
>>> resistor, e.g. 3 mOhm, followed by a bulk capacitor in each power
>>> rail of a let's say a DSP or a FPGA chip to be able to measure its
>>> power consumption?
>>>
>>>
>>> Thanks,
>>> =======================
>>> Mikhail Matusov
>>> Senior Hardware Design Engineer
>>> Square Peg Communications
>>> Tel.: +1 (613) 271-0044 ext.231
>>> Fax: +1 (613) 271-3007
>>> http://www.squarepeg.ca
>>>
>>>
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>>
>>
>> --
>> Steve Weir
>> Teraspeed Consulting Group LLC 121 North River Drive Narragansett, RI
>> 02882
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>
--
Steve Weir
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
California office
(866) 675-4630 Business
(707) 780-1951 Fax
Main office
(401) 284-1827 Business
(401) 284-1840 Fax
Oregon office
(503) 430-1065 Business
(503) 430-1285 Fax
http://www.teraspeed.com
This e-mail contains proprietary and confidential intellectual property of
Teraspeed Consulting Group LLC
------------------------------------------------------------------------------------------------------
Teraspeed(R) is the registered service mark of Teraspeed Consulting Group LLC
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