[SI-LIST] Re: Stitching CAPs between DDR VDD and VTT Power Rails

Harjeet,
        You need capacitors on the output of your regulator to get the Voltage
ripple in specification (that is the Switched Power supply design side). The
DC current requirement will be based on the worst case DC current draw
through the termination resistors.
        In addition you need high frequency caps for return currents between VDD
and VTT (the driver sources using DDR VDD when driving a logic high and this
AC current needs to see a low impedance return path back to that plane).
These caps usually are placed close to the driver end (VDD to GND) and the
termination end (VTT to GND).

--Prakash.

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir
Sent: Friday, December 19, 2003 7:55 AM
To: harjeet.randhawa@xxxxxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Stitching CAPs between DDR VDD and VTT Power
Rails


Harjeet, you are mixing concepts.  Capacitors set a frequency dependent
Z.  For any PDS, dV = dI * Z.

Steve.
At 06:08 PM 12/19/2003 +0530, Harjeet  Randhawa wrote:
>Hello,
>
>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.
>
>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?
>
>Thanking all in advance,
>Regards,
>Harjeet.
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