Forwarded for George.. ____________________Reply Separator____________________ Subject: RE: Charge moving from decoupling capacitors Author: george_t...@dell.com List-Post: emc-pstc@listserv.ieee.org Date: 5/23/00 12:35 PM
Let's say that you have a cap placed at 1/4 wavelength away from an IC pwr pin. When the IC draws current from the Pwr/gnd planes, it causes a voltage dip/pwr bounce on the parallel planes. This voltage dip propagates to the cap at 1/4 wavelength away and draws current out of the cap. The current from the cap propagates back to the IC pwr pin at 1/4 wavelength away. The total travel for the incident and reflected wave is 1/4 + 1/4 = 1/2 wavelength. The supply current is 180 degrees out of phase from the IC switching current. Some people say that the cap needs to be closer than 1/2 of rising edge. But you can also calculate the wavelength of the 3rd and 5th harmonic of your pulse to determine your cap placement. Regards, George Tang george_t...@dell.com -----Original Message----- From: Roncone Paolo [mailto:paolo.ronc...@compuprint.it] Sent: Tuesday, May 23, 2000 7:15 AM To: 'george_t...@dell.com '; 'barry...@altavista.com ' Cc: 'emc-p...@ieee.org ' Subject: RE: Charge moving from decoupling capacitors George, can you pls explain your correction ? I supposed your first statement ("At 1/4 wavelength, the charges are 90 degrees out of phase") was the correct one ! Paolo Roncone Compuprint s.p.a. ____________________Reply Separator____________________ Subject: RE: Charge moving from decoupling capacitors Author: george_t...@dell.com List-Post: emc-pstc@listserv.ieee.org Date: 5/22/00 9:14 PM Barry, I need to make a correction. I was rushing to lunch on Thursday, so I did not read over what I wrote. Here is the correction for the 2nd comment below: At 1/4 wavelength, the charges are 180 degrees out of phase, so they are working against the IC current draw. 1/8 wavelength (90 degrees out of phase) is what I consider to be acceptable. Regards, George Tang george_t...@dell.com -----Original Message----- From: Tang, George Sent: Thursday, May 18, 2000 12:31 PM To: 'Barry Ma'; Tang, George Cc: si-l...@silab.eng.sun.com; emc-p...@ieee.org Subject: RE: Charge moving from decoupling capacitors Barry, Thanks for the comments. Here are my comments: Ok, you put caps at a certain distance away from the IC because you only want them to work at 100 MHz. But that distance turns out to be the 1/4 wave distance at 400 MHz, and you placed enough caps at the 1/4 wave distance to cause board resonance. Now what? Do you tell the caps not to work at 400 MHz because it's not their frequency? For your 2nd comment: I used the words "loosely define" for that reason. If you are interested in high frequency decoupling and instantaneous current, you really want to have all your charges moving in phase. At 1/4 wavelength, the charges are 90 degrees out of phase, so they will not do much for your instantaneous current. 1/8 wavelength is what I consider to be acceptable. You can certainly pick a different number. Regards, George Tang george_t...@dell.com ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson: pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson: pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
