[SI-LIST] RESEND: Stitching CAPs between DDR VDD and VTT Power Rails
- From: "Ingraham, Andrew" <a.ingraham@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 24 Dec 2003 11:39:50 -0500
I'm sending this one again because it also bounced the first time. There
seem to be problems sending mail to freelists.org lately.
----- Original Message -----
From: "Ingraham, Andrew" <a.ingraham@xxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Monday, December 22, 2003 01:05 PM
Subject: Re: [SI-LIST] Stitching CAPs between DDR VDD and VTT Power Rails
> The DDR termination power rail should have sinking capability. Normally
> VTT is generated from the VDD and in switching regulator based solutions
> the upper FET provides the sinking path from VTT to VDD.
Isn't VDD more positive than VTT? If so, then the upper FET in the
regulator provides a *sourcing* path from VDD into VTT, not a sinking path
from VTT. Anyway, the sinking path should be to a more negative voltage,
such as a ground, VSS, or VEE. You would need some other path to sink
current from VTT.
> With the upper condition met, How does an array of stitching capacitors
> (several 0.1uf ceramic caps and few larger tantalum caps) between the
> VDD and VTT helps?
Capacitors provide no DC current path (needed to sink current from VTT).
They (1) help filter the huge amount of ripple from the switching regulator,
and (2) provide a low AC impedance path to AC signals over a wide frequency
range. There are large AC (switching) currents coming from both the
regulator and the signals that terminate to VTT.
Regards,
Andy
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