[SI-LIST] Re: A Question about Target Impedance Threshold computation
- From: steve weir <weirsi@xxxxxxxxxx>
- To: "Smith, Larry" <larrys@xxxxxxxxxxxxxxxx>
- Date: Sun, 30 Jun 2013 22:32:22 -0700
On 6/30/2013 9:50 PM, Smith, Larry wrote:
> Hi all, just catching up on some back email here.. The target impedance
> concept can be applied at many levels of assembly.
> Lots of good comment snipped
> In summary, it is highly probable that all circuits on the PCB will draw DC
> current at the same time and the VRM must be sized accordingly. But as we
> get up into the 1 MHz, 10 MHz, 100 MHz and 1 GHz bands, the probability of
> drawing transient currents in phase at the same frequency greatly diminishes.
> The target impedance at the different chip locations on the PCB should
> reflect this in different frequency bands.
>
> Regards,
> Larry Smith
Not only all those good comments, but as frequency goes up peak currents
become spatially isolated. That's both a good and a bad thing.
Steve
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of steve weir
> Sent: Thursday, June 13, 2013 3:07 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: A Question about Target Impedance Threshold computation
>
> Steven, target impedance applies at the point of use. In some cases,
> interconnect impedance can become significant even at supra-audio frequencies
> in the 100's of kHz, but almost always is an issue by the low Mhz. The
> target Z relation is also just a guideline. It assumes resistive Z which is
> rarely the case over the frequency range any given PDN needs to support.
>
> Steve.
> On 6/13/2013 1:32 PM, Steven J Ohlsen wrote:
>> Hello,
>> I am somewhat new to the world of Power Integrity. I hope this isn't
>> a dumb question, but I can not find any answer in the books I have or
>> online either. I understand that the equation for Target Impedance is
>> (allowed Voltage Variation)/(max change of current). However this
>> seems like a
>> "50,000 ft" rule-of-thumb. If your design meets this criteria then
>> all is well. But if your design has difficulty reaching this
>> impedance target then what. My observation is that when you get down
>> to the board level for example, there can be 30-40 balls on the
>> voltage regulator (and maybe multiple voltage regulators), and 50-100
>> balls on the device under test, and numerous decoupling capacitors
>> scattered around, between what points does the target impedance apply?
>> If you look at the board geometries then the current from Voltage
>> Regulator to Device may travel through a 1-inch (or more) wide path on
>> the power plane. Current is also provided by the decoupling
>> capacitors whose current (probably) has a different path to the device
>> under test . It seems that the current portion of the target
>> impedance equation varies from point to point on the board depending
>> on a host of very complicated relationships. Simply dividing the
>> current portion of the target impedance equation by the number of
>> balls on the voltage regulator or by the number of balls on the device
>> under test also seems incorrect. Is there a more design specific method to
>> compute the target impedance?
>>
>> Thank You all for your responses.
>> Steven Ohlsen
>>
>>
>> ------------------------------------------------------------------
>> To unsubscribe from si-list:
>> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>>
>> or to administer your membership from a web page, go to:
>> //www.freelists.org/webpage/si-list
>>
>> For help:
>> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>>
>>
>> List forum is accessible at:
>> http://tech.groups.yahoo.com/group/si-list
>>
>> List archives are viewable at:
>> //www.freelists.org/archives/si-list
>>
>> Old (prior to June 6, 2001) list archives are viewable at:
>> http://www.qsl.net/wb6tpu
>>
>>
>>
>
> --
> Steve Weir
> IPBLOX, LLC
> 1580 Grand Point Way
> MS 34689
> Reno, NV 89523-9998
> www.ipblox.com
>
> (775) 299-4236 Business
> (866) 675-4630 Toll-free
> (707) 780-1951 Fax
>
> All contents Copyright (c)2013 IPBLOX, LLC. All Rights Reserved.
> This e-mail may contain confidential material.
> If you are not the intended recipient, please destroy all records and notify
> the sender.
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List forum is accessible at:
> http://tech.groups.yahoo.com/group/si-list
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
>
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
--
Steve Weir
IPBLOX, LLC
1580 Grand Point Way
MS 34689
Reno, NV 89523-9998
www.ipblox.com
(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax
All contents Copyright (c)2013 IPBLOX, LLC. All Rights Reserved.
This e-mail may contain confidential material.
If you are not the intended recipient, please destroy all records
and notify the sender.
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List forum is accessible at:
http://tech.groups.yahoo.com/group/si-list
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
