
LXT9785 and LXT9785E Advanced 8-Port 10/100 Mbps PHY Transceivers
Datasheet 
Document Number: 249241
Revision Number: 007
Revision Date: August 28, 2003
165
Intel recommends filtering the power supply to the analog VCC pins of the LXT9785/LXT9785E. 
This has two benefits. First, it keeps digital switching noise out of the analog circuitry inside the 
LXT9785/LXT9785E, helping with line performance. Second, if the VCC planes are laid out 
correctly, digital switching noise is kept 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 LXT9785/LXT9785E. 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 the LXT9785/LXT9785E, 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
μ
F) should 
be placed on each side of each bead.
In addition, a high-frequency bypass cap (0.01 
μ
F) should be placed near each analog VCC pin.
5.2.2
Power and Ground Plane Layout Considerations
Great care needs to be taken when laying out the power and ground planes. 
 Follow the guidelines in the 
LXT9785 Design and Layout Guide (formerly Application Note 
151)
 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, the magnetics, and 
the RJ-45 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.
5.2.2.1
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 2 kV isolation capacitor. In 
multi-point grounding schemes (chassis and circuit grounds joined at multiple points), provide 
2 kV isolation to the Bob Smith termination.
5.2.3
MII Terminations
Series termination resistors are required on all the SS-SMII output signals driven by the LXT9785/
LXT9785E. Special trace layout consideration should be used when using the SMII interface. Keep 
all traces orthogonal and as short as possible. Whenever possible, route the clock and sync traces 
evenly between the longest and shortest data routes. This minimizes round-trip, clock-to-data 
delays and allows a larger margin to the setup and hold requirements.
5.2.4
Twisted-Pair Interface
Use the following standard guidelines for a twisted-pair interface: