[SI-LIST] Re: ddr3 Vtt sso issue
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 17 Jun 2014 04:28:35 -0700
Vtt noise is manageable with reasonable care: Choose a two quadrant VRM
with as wide a bandwidth as possible and then bypass properly. The
usual problems with Vtt noise:
1) Excessive inductance. Fix with stack-up, polygon and bypass planning
/ placement
2) Noise injection due to poor return path planning. Same fixes as 1).
3) Single quadrant VRM. This shouldn't be happening but it still does.
Band aids like giant capacitor banks are a poor solution to a problem
that should never have occurred in the first place.
Steve
On 6/16/2014 11:13 PM, Luping Liu wrote:
> Hi Pete:
> I totally agree with Paul's suggestion,you should use a independent
> regulator and use outer layer P/G plane pair to reduce the inductance of
> the PDN loop in your case, and I have some additional comment for your
> reference.
> Consider the test setup you use,it is unavoidable to have large noise
> on the VTT rail, you can reduce ,but can't eliminate the noise , cause we
> have no such ideal PDN to offer such rush current. What we should really
> care is the eye diagram of the signal,include the add/cmd/dq/clk, it should
> be the final spec to decide if our VTT power design meet the requirement.
> If you find problems in the ADD/CMD eye diagram too,them you can:
>
> Besides N Paul's suggestions, addition 100uF bulk caps may be needed
> to lower the noise.
> Each VTT resistor should have a decap capacitor, 100n/1uF , 0402 size.
> Again, route the VTT plane on top or bottom side( same size with your
> regulator placement), with a GND adjacent.Inductance is important, so try
> to reduce the distance between the voltage regulator and VTT resistor.
> Increase the VTT resistor value, 39 ohm is not a good choice at this
> case.
> Reduce the current of the driver ,of add series resistor, based on
> your simulation.
>
> By the way , do we really need a all 0 to all 1 test pattern for the
> address signal?
>
> Regards,
> LIU Luping
>
>
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