[SI-LIST] Re: Decoupling in DDR3 Applications
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Dan Smith <Dan.Smith@xxxxxxxxx>
- Date: Thu, 18 Feb 2010 08:40:24 -0800
Dan, sufficiently low impedance bypass between VDD and VSS closes the
driver path switching high or switching low. The reason for coupling
VTT 1:1 to VDD is that the standard stipulates VDD as the transmission
path return for address and control signals terminated to VTT.
Steve
Dan Smith wrote:
> Hirshtal,
>
> When the memory device drives high, there is current flow from the VDD rail
> to the VTT rail through the top transistor of the output stage (the bottom
> transistor is turned off). When the memory drives low, there is current flow
> from the VTT rail to GND through the bottom transistor of the output stage
> (the top transistor is turned off). Therefore, in order to meet your
> impedance target (and thus ripple), one must design a low impedance path
> between VTT-VDD and VTT-VSS. In short, follow the path of the current flow
> and then the reason to decouple to both becomes more clear.
>
> Dan
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of steve weir
> Sent: Thursday, February 18, 2010 8:05 AM
> To: Hirshtal Itzhak
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Decoupling in DDR3 Applications
>
> Hirshtal, the standard stipulates transmission return plane for address
> / control as Vdd, and data as Vss. Vtt is used for address / control.
>
> Steve.
>
> Hirshtal Itzhak wrote:
>
>> Hello Experts!
>>
>>
>> I found out that the Unbuffered DIMM Design Specification 21C Page
>> 4.20.19-30 (Revision 1.01 Nov 2009) recommends to decouple VTT by
>> decoupling capacitors to VDD. VREFCA has the same recommendation.
>>
>>
>>
>> On the other hand, on the same page, VREFDQ is recommended to be
>> decoupled to VSS.
>>
>>
>>
>> Also, I couldn't find any specific recommendation for the decoupling
>> voltage in the Unbuffered SO-DIMM Reference Design Specification 21C
>> Page 4.20.18-28 (Revision 1.0 Feb 2009).
>>
>>
>>
>> Does anyone happen to know why decoupling is recommended to be done to
>> VDD rather than VSS in all cases? It seemed to me that VSS was always
>> the preferred voltage to decouple to. And if there's a good reason for
>> not doing it, then why the differences between the various voltages and
>> the two DIMM specs?
>>
>>
>>
>> Also, is there a difference in regard to embedded (non-DIMM)
>> applications?
>>
>>
>>
>> Thanks
>>
>>
>>
>> Itzhak Hirshtal
>>
>>
>>
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>
>
> --
> Steve Weir
> IPBLOX, LLC
> 150 N. Center St. #211
> Reno, NV 89501
> www.ipblox.com
>
> (775) 299-4236 Business
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--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV 89501
www.ipblox.com
(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax
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