AN2586 Freescale Semiconductor / Motorola, AN2586 Datasheet - Page 17

• Manufacturer Part Number: AN2586
• Description: MPC8260 PowerQUICC II Family Power Distribution Trends
• Manufacturer: Freescale Semiconductor / Motorola

Placing a decoupling capacitor on the power distribution network changes the overall impedance profile of
it. When we identify the main peaks of the PDN impedance, we try to add bypass capacitors that are capable
of suppressing those peaks as much as possible. If the capacitor is placed in a board surface opposite to the
chip board surface, a via must be in series with the bypass capacitor, which weakens the efficiency of the
bypass. The reason is that the via it self has an inductance that is approximated as the following (where L is
the inductance of via in nH, H is the height of the inductance, and d is the diameter in inches):
Also vias have capacitance that depends on the via type. Bypass capacitors can be placed as close as possible
to the nodes Vi that exhibit higher |Vi(t)|, implying that we are trying to keep the absolute value of the node
voltage below a certain constant and enabling us to budget for noise that is superimposed over the constant
DC value.
Many techniques automate the bypass capacitors placement process. As in any optimization problem, we
must have an energy function that we are to minimize. Then we have to descend across this energy surface
via changing the bypass capacitor locations. Each time the bypass capacitor location is changed, a new Z
matrix is generated for the PDN. The process continues until the goal of keeping all |Vi(t)| less than a
constant is reached.
3.6 How can the via cause EMI problems?
We must be aware of the effects of vias not from the bypass capacitor only but also from the power feed
point of view. Vias are used to provide current paths to inner layers of the system board. The problem is that
the current at the via must pass normally down the surface of the board. This direction change causes
electromagnetic coupling with other vias, signals, or power, and radiation potential.
The electromagnetic coupling between vias is an important factor of cross-talk between signals that have
vias close to each other. Vias have two types of inductances:
To model the PCB more accurately, vias self- and mutual inductances must be taken into account. The
radiation from Via accrues if the via is carrying a high frequency signal, and the via happens to be close to
two parallel conductors. the emission is towards the PCB edges parallel to the PCB surface.
4 References
MOTOROLA
•
•
1. Howard W. Johnson, Martin Graham. High Speed Digital Design A Handbook Of Black Magic.
2. T.C. Edwards, M.B. STEER. Foundations of Interconnect and Microstrip design, Third edition.
3. Balsha R.Stanisc, Rob A. Rutenbar, L. Richard Carley. Synthesis of power distribution to manage
4. Stephen H. Hall, Garrett W. Hall, James A. McCall. High Speed Digital System Design A
5. Hartmut Grabinski. Interconnects in VLSI Design. Kluwer Acadimic Publishers, 2000.pp 61-147.
6. S. Boyd, L. Vandenberghe, A. El Gamal, S. Yun. Design of Robust Power and Ground Networks.
Self-inductance, a function of the via geometry
Mutual inductance with each neighboring via
Prentice Hall,1993, pp263-292.
Wiley, 2000, pp315-344.
signal integrity in mixed-signal ICs. Kluwer Academic Publishers, 1996.
Handbook of Interconnect theory an Design Practice.Wiley 2000.pp 102-154.
IEEE Trans Adv Pack.2001.
MPC8260 PowerQUICC II Family Power Distribution Trends: A Survey
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
L
=
5.08 h
×
×
ln


4 h
----------- -
×
d
+
1


References
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