[SI-LIST] Re: RESEND: Stitching CAPs between DDR VDD and VTT Power Rails
- From: "Harjeet Randhawa" <harjeet.randhawa@xxxxxxxxxxxxxxxxx>
- To: <a.ingraham@xxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 26 Dec 2003 10:39:14 +0530
Hi Andy,
I thought that the Switching regulators have a mechanism for the DDR-VTT
generation where the upper FET provides a sink path to VDDQ during the
boost converter mode.
Please google search for this part "ISL6531" and see section in the
datasheet called as "Current Sinking" There it is shown the current sink
path from VTT to VDDQ back to VTT through he upper switch FET.
Kindly clarify!
With best Regards,
Harjeet.
-----Original Message-----
From: Ingraham, Andrew [mailto:a.ingraham@xxxxxxxx]=20
Sent: Wednesday, December 24, 2003 10:10 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] RESEND: Stitching CAPs between DDR VDD and VTT Power
Rails
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.=20
> Normally VTT is generated from the VDD and in switching regulator=20
> based solutions the upper FET provides the sinking path from VTT to=20
> 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=20
> capacitors (several 0.1uf ceramic caps and few larger tantalum caps)=20
> 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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