[SI-LIST] Re: Ground, the preferred reference plane

  • From: "Jeremy Plunkett" <jeremy@xxxxxxxxxxxxxxx>
  • To: chris.cheng@xxxxxxxxxxxx
  • Date: Thu, 12 Dec 2002 15:47:27 -0800

Hi Chris,
I think you are mixing up Vref and Vtt...Vref is the threshold reference =
provided to each DDR DIMM and controller ASIC, the only current drawn on =
it is leakage from the input comparators.  Therefore Vref is typically =
generated by a voltage divider off of VDDQ as close as possible to the =
chip it is supplying, and the number of resistors required is quite =
reasonable.

Vtt on the other hand is the termination supply for all the DDR signals =
and must source and sink substantial amounts of current.  This requires =
a DC/DC converter, but still the VTT should be set up to track the VDDQ =
voltage to a tight tolerance by using a voltage divider from =
VDDQ(suitably filtered) as the reference voltage for the regulator.

regards,
Jeremy

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Chris Cheng
Sent: Thursday, December 12, 2002 1:48 PM
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Ground, the preferred reference plane



100% agree but in reality, a lot of PC board doesn't have enough space =
to
fit so many rpack's. Take a look at most of the reference designs for PC
chipsets, most of them call out generating vref with a DC/DC converter =
and
terminate with a single resistor.

-----Original Message-----
From: Vinu Arumugham [mailto:vinu@xxxxxxxxx]
Sent: Wednesday, December 11, 2002 11:55 PM
To: chris.cheng@xxxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: Ground, the preferred reference plane




Chris Cheng wrote:

>Take an example of DDR in most PC design. The vref on receivers is
basically
>the VTT which usually is generated from an external DC/DC converter =
that
>generates VDDQ/2 and is completely isolated from the driver VDDQ.=20
>
That does not sound like a good thing...
The receiver Vref should be driver Vddq/2. We usually generate it with a =

resistor divider from driver Vddq.
This way, Vref will track Vddq better...

Thanks,
Vinu
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