LXT9782 datasheet(46/78 Pages) INTEL

46 / 78 page

LXT9762/9782 — Fast Ethernet 10/100 Multi-Port Transceiver with Serial MII

Datasheet

3.0

Application Information

3.1

Design Recommendations

The LXT97x2 complies with IEEE requirements and provides outstanding receive Bit Error Rate

(BER) and long-line-length performance. Obtaining maximum performance from the LXT97x2

requires attention to detail and good design practices. Refer to the LXT97x2 Design and Layout

Guide for detailed design and layout information.

3.1.1

General Design Guidelines

Adhering to generally accepted design practices minimizes noise levels on power and ground

planes. Up to 50 mV of noise is considered acceptable. 50 to 80 mV of noise is considered

marginal. High-frequency switching noise can be reduced, and its effects can be eliminated, by

following these simple guidelines throughout the design:

• Fill in unused areas of the signal planes with solid copper and attach them with vias to a VCC

or ground plane that is not located adjacent to the signal layer.

• Use ample bulk and decoupling capacitors throughout the design (a value of .01

µF is

recommended for decoupling caps).

• Provide ample power and ground planes.

• Provide termination on all high-speed switching signals and clock lines.

• Provide impedance matching on long traces to prevent reflections.

• Route high-speed signals next to a continuous, unbroken ground plane.

• Filter and shield DC-DC converters, oscillators, etc.

• Do not route any digital signals between the LXT97x2 and the RJ-45 connectors at the edge of

the board.

• Do not extend any circuit power and ground plane past the center of the magnetics or to the

edge of the board. Use this area for chassis ground, or leave it void.

3.1.2

Power Supply Filtering

Power supply ripple and digital switching noise on the VCC plane can cause EMI problems and

degrade line performance. Minimize ground noise as much as possible using good general

techniques and by filtering the VCC plane. Predicting the noise performance of any design is

difficult, although certain factors greatly increase the risk of noise problems:

• Poorly-regulated or over-burdened power supplies

• Wide data busses (32-bits+) running at a high clock rate

• DC-to-DC converters

Intel recommends filtering the power supply to the analog VCC pins of the LXT97x2. This has

two benefits. First, prevents digital switching noise from affecting the analog circuitry inside the

LXT97x2, 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.

LXT9782 Datasheet(PDF) 46 Page - Intel Corporation

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