lxt9781 Intel Corporation, lxt9781 Datasheet - Page 45

• Manufacturer Part Number: lxt9781
• Description: Fast Ethernet 10/100 Multi-port Transceiver With Rmii
• Manufacturer: Intel Corporation

3.1.3
3.1.3.1
3.1.4
3.1.5
Datasheet
Intel recommends filtering the power supply to the analog VCC pins of the LXT97x1. This has
two benefits. First, it keeps digital switching noise out of the analog circuitry inside the LXT97x1,
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 break the VCC plane into two sections. The digital section
supplies power to the VCCD and VCCIO pins of the LXT97x1. The analog section supplies power
to the VCCA pins. The break between the two planes should run underneath the device. In
designs with more than one LXT97x1, 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 impedance at 100 MHz. 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 uF) should
be place on each side of each bead.
In addition, a high-frequency bypass cap (.01uf) should be placed near each analog VCC pin.
Power and Ground Plane Layout Considerations
Great care needs to be taken when laying out the power and ground planes.
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.
RMII Terminations
Series termination resistors are not typically required on the RMII signals driven by the LXT97x1.
The RBIAS Pin
The LXT97x1 requires a 22.1 k
ground. Place the RBIAS resistor as close to the RBIAS pin as possible. Run an etch directly from
the pin to the resistor, and sink the other side of the resistor to a filtered ground. Surround the
RBIAS trace with a filtered ground; do not run high-speed signals next to RBIAS.
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Follow the guidelines in the LXT97x1 Design and Layout Guide for locating the split between
the digital and analog VCC planes.
Keep the digital VCC plane away from the TPFOP/N and TPFIP/N signals, away from the
magnetics, and away from the RJ45 connectors.
Place the layers so that the TPFOP/N and TFPIP/N signals can be routed near or next to the
ground plane. For EMI reasons, it is more important to shield TPFOP/N than TPFIP/N.
Fast Ethernet 10/100 Multi-Port Transceiver with RMII — LXT9761/9781
1% resistor directly connected between the RBIAS pin and
45