[SI-LIST] Re: Current sense resistors and power integrity

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
>> California office
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>>
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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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