LXT970ATC datasheet(43/74 Pages) INTEL

43 / 74 page

Dual-Speed Fast Ethernet Transceiver — LXT970A

Datasheet

The recommended implementation is to divide the VCC plane into two sections. The digital

section supplies power to the digital VCC pin, VCCIO pin, and to the external components. The

analog section supplies power to VCCA, VCCT, and VCCR pins of the LXT970A. The break

between the two planes should run under the device. In designs with more than one LXT970A, a

single continuous analog VCC plane can be used to supply them all.

The digital and analog VCC planes should be joined at one or more points by ferrite beads. The

beads should produce at least a 100

Ω impedance at 100 MHz. The beads should be placed so that

current flow is evenly distributed. The maximum current rating of the beads should be at least

150% of the current that is actually expected to flow through them. A bulk cap (2.2 -10

µF) should

be placed on each side of each ferrite bead to stop switching noise from traveling through the

ferrite.

In addition, a high-frequency bypass cap (.01

µf) should be placed near each analog VCC pin.

3.3.3

Ground Noise

The best approach to minimize ground noise is strict use of good general design guidelines and by

filtering the VCC plane.

3.3.4

Power and Ground Plane Layout Considerations

Great care needs to be taken when laying out the power and ground planes. The following

guidelines are recommended:

• Follow the guidelines in the LXT970 Layout Guide for locating the split between the digital

and analog VCC planes.

• Keep the digital VCC plane away from the TPOP/N and TPIP/N signals, away from the

magnetics, and away from the RJ-45 connectors.

• The ground plane should be one continuous, unbroken plane.

• Place the layers so that the TPOP/N and TPIP/N signals can be routed near or next to the

ground plane. It is more important to shield TPOP/N than TPIP/N for EMI reasons.

3.3.5

Interfaces for Twisted-Pair /Fiber

3.3.5.1

Twisted-Pair

Figure 21 on page 46 shows the recommended termination circuits for the fiber and twisted-pair

interfaces. The twisted-pair interface consists of magnetics and a 100

Ω termination resistance in

parallel on the device-side of the winding. On the transmit pair, a common-mode bypass capacitor

to ground is strongly recommended. A similar technique on the receive side can improve

performance in some cases, however the results are highly application specific and must be

verified. Route TREF nearby, but not in-between TPOP and TPON. A “Bob Smith” termination is

often provided for the unused signal pairs.

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