[SI-LIST] Re: Power Integrity about Vtt of DDR
- From: Larry Smith <Larry.Smith@xxxxxxx>
- To: Jon Powell <jonpowell@xxxxxxxxxxxx>
- Date: Mon, 17 May 2004 09:48:15 -0700
Jon - Sorry, maybe some of the words need further definition. By
small, we mean the plane area when looking down on top of the board.
The core power planes only need to be big enough to connect the loads,
decaps and VRM.
The thinness of the dielectric between the power and ground planes
determines the spreading inductance of the planes. This becomes
important if you are trying provide low impedance (1 mOhm) power to a
load at 10's of MHz. If we are not careful, the inductance of the
planes can dominate over the parallel combination of all decoupling
capacitors. There is no point in putting more caps on the planes if
the plane inductance dominates.
Note that this discussion pertains to core power where there are no
signals involved. If signal return current is required to jump
between planes, the capacitance between the planes can be very
important. Scott's note below began by saying that it is best to
reference all signals to ground planes. By doing this, you avoid the
issue of signal return current between power and ground planes.
regards,
Larry Smith
Sun Microsystems
Jon Powell wrote:
> Perhaps I am entering into a religious area, but it seems that much recent
> advice is counter to previous threads. Or perhaps people are worried about
> different things or different types of technology.
>
> Would it be possible to give some simple pro con advice on the issue of
> using parallel planes for VCC GND decoupling and storage versus this other
> recommendation of small planes to reduce inductance?
>
> Or is this discussion solely in the area of planes on a connector/DIMM?
>
> jon
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Larry Smith
> Sent: Monday, May 17, 2004 8:27 AM
> To: scott@xxxxxxxxxxxxx
> Cc: Zhangkun; silist
> Subject: [SI-LIST] Re: Power Integrity about Vtt of DDR
>
>
> Scott - your comments are right on the money. There is no reason to
> use large power planes where small ones will do. It is important that
> the dielectric is thin enough so that the spreading inductance of the
> planes does not dominate the parallel inductance of all the caps. The
> thin power plane dielectric is not about capacitance but is all about
> providing a sufficiently low inductance channel for the energy stored
> in the caps to reach the power consumers.
>
> regards,
> Larry Smith
> Sun Microsystems
>
>
> Scott McMorrow wrote:
>
>>Zhangkun,
>>If you keep all the DQ, DQS and DM signals referenced to a ground plane,
>>there will be no power injection into the Vdd power/ground plates at DDR
>>switching frequency. (It is always preferable to have all DDR signals
>>referenced to a ground plane, including Address and Control. But if you
>>have a choice, at least reference the data rate signals, as they have
>>the highest harmonic content.) This reduces high frequency noise on the
>>VDD plane. What is left is the mid-frequency core power noise and
>>address/control switching noise which extends up to a hundred MHz or
>>so. This can be adequeately decoupled by capacitors with the
>>appropriate consideration of ESL. If you use large Vdd/gnd planes, you
>>run the risk of driving the resonant frequency of these planes down to
>>one of the lower harmonics of the core or I/O switching clocks. Then
>>you will need to decouple these resonances with an excess of capacitance
>>to counter the anti-resonance peaks formed between the plane and the
>>capacitors.
>>
>>In general, large planes do little to help matters. You receive the
>>most benefit first from referencing signals to ground and second from
>>thin power/ground pairs. Large planes can oftentimes make matters
>>worse. I realize this sounds like heresy, but it is very true. The
>>problem then becomes in navigating around the plane splits or fill
>>areas, embedding your power fill areas between several ground planes on
>>thick multi-layer boards, and choosing the appropriate position in the
>>board stackup to reduce the overall mounted capacitor inductance for the
>>power layers.
>>
>>For Vtt, in specific, a fill area on the surface of the board, with an
>>adjacent ground plane underneath, which just encompases all of the
>>terminators and capacitors, is extremely efficient. Since one pad of
>>the capacitor is connected directly to the Vtt fill, and the other pad
>>uses vias dropped to the underlying ground plane several mils away, the
>>mounted inductance is extremely low, and approaches the inductance of
>>the capacitor body, itself. You can make the plane large if you want,
>>to lower the Vdd fill area impedance, but do not make it so large that a
>>resonance is excited that will "sing" with the capacitors. You would
>>like to keep the resonant frequency of the plane above at least 3X to 4X
>>the switching freqency. For DDR 400, the switching frequency is 200
>>MHz. As such, keep the plane resonance above 600 to 800 MHz for best
>>performance. This dictates a maximum dimension of around 3.5 to 5
>>inches on FR4 laminates. (Resonant frequency of a 5" cavity in FR4 @170
>>ps/in is 1/(5*170e-12 *2), for the half-wave resonant point.)
>>
>>regards,
>>
>>scott
>>
>>Scott McMorrow
>>Teraspeed Consulting Group LLC
>>2926 SE Yamhill St.
>>Portland, OR 97214
>>(503) 239-5536
>>http://www.teraspeed.com
>>
>>Teraspeed is the registered service mark of
>>Teraspeed Consulting Group LLC
>>
>>Zhangkun wrote:
>>
>>
>>>Dear Scott
>>>
>>>"Both VDD and VTT should be as small a plane or fill area as possible, to
>
> reduce planar resonance effects."
>
>>>I donot agree with this point. When the plane is small, the
>
> plane-capacitance is small. At high frequency domain, the plane-capacitance
> is the most important for decoupling.
>
>>>Best Regards
>>>
>>>Zhangkun
>>>2004.5.16
>>>----- Original Message -----
>>>From: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
>>>To: <zhang_kun@xxxxxxxxxx>
>>>Cc: "si-list" <si-list@xxxxxxxxxxxxx>
>>>Sent: Friday, May 14, 2004 11:37 PM
>>>Subject: [SI-LIST] Re: Power Integrity about Vtt of DDR
>>>
>>>
>>>
>>>
>>>
>>>>Zhangkun
>>>>
>>>>All of your DDR DQS, DQ, and DM signals should be routed overtop of a
>>>>ground plane. This reduces return path effects in the package and
>
> connector.
>
>>>>Both VDD and VTT should be as small a plane or fill area as possible, to
>>>>reduce planar resonance effects. For VTT termination resistors, use 8
>>>>pin 4 resistors R-paks. Place an outer layer Vtt fill area behind the
>>>>resistors and for each Rpak use 1 0603 0.1 uF ceramic capacitor directly
>>>>behind the resistor. This will insure ample decoupling for VTT. For
>>>>Vdd decoupling you will need to compute the worst case instantaneous
>>>>current required by your devices and the acceptable noise level,
>>>>(usually 5%) and from that compute the necessary ESL to accomplish
>>>>this. Based on the ESL you will then select the number of 0.1 uF
>>>>capacitors to meet your ESL needs by calculating (or solving for) the
>>>>mounted inductance of the capacitor, and computing the number in
>>>>parallel necessary to meet your requirement.
>>>>
>>>>regards,
>>>>
>>>>scott
>>>>
>>>>
>>>>Zhangkun wrote:
>>>>
>>>>
>>>>
>>>>
>>>>>Dear all:
>>>>>In DDR, there are two power, VDD and VTT. For example, VDD=2.5V and
>
> VTT=1.25V. VDD should be decoupled very well. How about the decoupling of
> VTT? Is it important? How to arrange the VDD, VTT and GND?
>
>>>>>Best Regards
>>>>>
>>>>>Zhangkun
>>>>>2004.5.14
>>>>>
>>>>>
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>>>>
>>>>--
>>>>
>>>>Scott McMorrow
>>>>Teraspeed Consulting Group LLC
>>>>2926 SE Yamhill St.
>>>>Portland, OR 97214
>>>>(503) 239-5536
>>>>http://www.teraspeed.com
>>>>
>>>>Teraspeed is the registered service mark of
>>>>Teraspeed Consulting Group LLC
>>>>
>>>>
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>>>>
>>>
>>>
>>>
>>--
>>
>>Scott McMorrow
>>Teraspeed Consulting Group LLC
>>2926 SE Yamhill St.
>>Portland, OR 97214
>>(503) 239-5536
>>http://www.teraspeed.com
>>
>>Teraspeed is the registered service mark of
>>Teraspeed Consulting Group LLC
>>
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