The Designer's Guide Community
Forum
Welcome, Guest. Please Login or Register. Please follow the Forum guidelines.
Mar 29th, 2024, 8:06am
Pages: 1
Send Topic Print
Driver impedance, slew rate, loading and signal integrity (Read 625 times)
neoflash
Community Fellow
*****
Offline

Mixed-Signal
Designer

Posts: 397

Driver impedance, slew rate, loading and signal integrity
Dec 22nd, 2008, 6:17am
 
Assume we are designing a CMOS DDR3 driver, user requested different output driver impedance to cover different capacitive loading.

For example, 18 DRAM units may require 20ohm output impedance while 36 units may require 12ohm output impedance.

I'm curious that if whether we should use a faster slew rate for heavy capacitive loading, or use a slower slew rate?

Fast edge rate will incur more reflection, any consideration on this effect? Please recommend some literature if any, thanks.

Back to top
 
 
View Profile   IP Logged
SRF Tech
Community Member
***
Offline



Posts: 59
Arizona
Re: Driver impedance, slew rate, loading and signal integrity
Reply #1 - Jan 2nd, 2009, 8:45pm
 
Hi Neo,
 I would first say that what you are looking for is to ideally have a precise slew rate to meet your timing requirements, that is constant for either large or small capacitive loads.  

I would not try to adjust slew rates, but rather drive currents so that slew rates are consistent over all operating conditions (and that they meet your timing specifications for the DDR3 standard).  That should be your goal.

Let me know if I misunderstand what you are asking

Stephen
Back to top
 
 

Excellence in ESD and IO Design
www.srftechnologies.com
View Profile WWW   IP Logged
shashi.kiran
New Member
*
Offline



Posts: 5
india
Re: Driver impedance, slew rate, loading and signal integrity
Reply #2 - Jul 9th, 2009, 4:26am
 
I completely agree with stephen , since DDR3 driver working around 800mhz it would not be suggestive to slew it and not meet the DDR3 timing spec. However , if ur worrying abu signal integrity its always suggestive to skew the driver and to have better calibration for termination..etc.
Back to top
 
 
View Profile   IP Logged
Pages: 1
Send Topic Print
Copyright 2002-2024 Designer’s Guide Consulting, Inc. Designer’s Guide® is a registered trademark of Designer’s Guide Consulting, Inc. All rights reserved. Send comments or questions to editor@designers-guide.org. Consider submitting a paper or model.