LXT975BHC datasheet(41/74 Pages) INTEL

41 / 74 page

Fast Ethernet 10/100 Quad Transceivers — LXT974/LXT975

Datasheet

circuitry inside the LXT974/975, which helps line performance. Second, if the VCC planes are

laid out correctly, it keeps digital switching noise away from external connectors, reducing EMI

problems.

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

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

analog section supplies power to VCCH, VCCT, and VCCR pins of the LXT974/975. The break

between the two planes should run under the device. In designs with more than one LXT974/975,

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. Each LXT974/975 draws a

maximum of 500 mA from the analog supply so beads rated at 750 mA should be used. 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.

3.1.2.1

Ground Noise

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

filtering the VCC plane.

3.1.3

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 LXT974/975 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.

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

ground plane. For EMI reasons, it is more important to shield TPOP/N and TPIP/N.

3.1.3.1

Chassis Ground

For ESD reasons, it is a good design practice to create a separate chassis ground that encircles the

board and is isolated via moats and keep-out areas from all circuit-ground planes and active

signals. Chassis ground should extend from the RJ-45 connectors to the magnetics, and can be

used to terminate unused signal pairs (‘Bob Smith’ termination). In single-point grounding

applications, provide a single connection between chassis and circuit grounds with a 2kV isolation

capacitor. In multi-point grounding schemes (chassis and circuit grounds joined at multiple

points), provide 2kV isolation to the Bob Smith termination.

3.1.4

MII Terminations

Series termination resistors are not required on the MII signals driven by the LXT974/975.

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