Intel 82562EX, Руководство по конструкции

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82540EP/82541(PI/GI/EI) & 82562EZ(EX) Dual Footprint Design Guide
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The following table lists the reference starting values for these capacitors.
The placement of C1 through C6 may also be different for each board design (in other words, not
all of the capacitors may need to be populated). Also, the capacitors may not be needed on both
sides of the magnetics module.
Systems with integrated USB ports on the magnetics module may have difficulty using a chassis
ground configuration due to the need to get DC ground to certain USB pins. One technique is to
fabricate an incomplete ground split (partially open on one end). By connecting the ground planes
around the RJ magnetics module through a slim, high impedance connection, the path of the high
frequency noise can be controlled.
4.1.13 Non-Integrated Magnetics Modules and RJ-45 Connectors
It is possible to employ discrete (non-integrated) magnetics modules and RJ-45 connectors. Similar
rules will apply to design and layout. The differential pairs should be routed to be as short and
symmetrical as possible and the overall lengths of the differential pairs (including the width of the
magnetics module) should not exceed approximately four inches.
Additional design and layout steps will be required to add a dedicated board termination plane
parallel to chassis ground, 75
Ω
termination resistors, and a 1500 pF capacitor. This “Bob Smith”
termination scheme is normally contained inside an integrated magnetics module. The 75
Ω
termination resistors are required to terminate the common mode of the twisted pairs that behave as
a transmission line. These resistors help ensure that emission requirements are met.
In Ethernet designs, it is common practice to terminate unused connections on the RJ-45 connector
and the magnetics module to ground. Depending on overall shielding and grounding design, this
may be done to the chassis ground, signal ground, or a termination plane. Care must be taken when
using various grounding methods to insure that emission requirements are met. In the “Bob Smith”
termination method, a floating termination plane is cut out of a power plane layer. This floating
plane acts as a plate of a capacitor with an adjacent ground plane. The signals can be routed
through 75
Ω
resistors to the plane. Stray energy on unused pins is then carried to the plane.
It is recommended that the termination plane capacitance equal a minimum value of 1500 pF. This
helps reduce the amount of crosstalk on the differential pairs from the unused pairs of the RJ-45
connector. Pads may be placed for an additional capacitance to chassis ground, which may be
required if the termination plane capacitance is not large enough to pass Electrical Fast Transient
(EFT) testing. If a discrete capacitor is used, it should be rated for at least 1000 Vac to meet the
EFT requirements.
Capacitors Value
C3, C4 4.7
µ
F or 10
µ
F
C1, C2, C5, C6 470 pF to 0.1
µ
F