Texas Instruments TIDA-010046, Руководство по конструкции

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System Overview
19 TIDUES1A – October 2019 – Revised February 2020
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EMC Compliant 10/100-Mbps Ethernet PHY Reference Design With IEEE
802.3at Type-1 ( ≤ 12.95 W) PoE-PD
2.5 Board Design Guideline
This section provides approaches for optimal board design.
2.5.1 MDI Connections
2.5.1.1 MDI Connections Between Ethernet PHY and Isolation Transformer
The MDI connection is via transmit (TD+/TD–) and receive (RD+/RD–) differential pair pins. These
differential pairs connect to a termination network, ESD protection devices, then to 1:1 magnetics
(transformer) as
Figure 15 shows.
Figure 15. MDI Connections Between Ethernet PHY and Isolation Transformer
General MDI layout guidelines and key design recommendations for reduced EMI:
• Depending on the PHY device, parallel termination might be needed for each of the MDI differential
signal pairs. The termination impedance is typically 100
Ω
differentially.
– Newer devices like the DP83825I include integrated MDI termination so no external termination
resistors are required.
– For the DP83822 device, place the 49.9 Ω , 1% resistors, and 0.1- μ F decoupling capacitor, near the
TD+/TD– and RD+/RD– pins and via directly to the AVDD plane as Figure 16 shows.
– To reduce the crosstalk interference on signals, keep differential MDI network signals physically
separated from single-ended digital signals. Place digital signals far away from the analog traces to
help maintain signal integrity. A good general rule to follow is that no digital signal should be
located within 300 mils (7.5 mm) of the differential pairs.
– MDI signals are differential. Within the pairs (for example, TD+/TD– or RD+/RD–) the trace lengths
should run parallel to each other and matched in length. Matched lengths minimize delay
differences, avoiding an increase in common-mode noise and increased EMI. To ensure data
integrity, match the trace length within ±50 mils, see
Table 7 through Table 9 .
– The separation between the TD+/TD– and the RD+/RD– differential pairs must be at least 0.5 mm.
It is best to separate them with a ground plane.
– Route single-ended traces with 50-
Ω
impedance to ground, whereas, the differential traces are
routed with 100- Ω controlled impedance. Many online tools are available to calculate controlled
impedance for micro strip and stripline traces. For more details, see the AN-1469 PHYTER Design
and Layout Guide application report.