Does Not Maintain Parallelism
Avoid
Stubs
Ground or Power Plane
Do NOT cross Power
Or Ground Planes
Ground or Power Plane
System Overview
21
TIDUES1A – October 2019 – Revised February 2020
Copyright © 2019–2020, Texas Instruments Incorporated
EMC Compliant 10/100-Mbps Ethernet PHY Reference Design With IEEE
802.3at Type-1 (
≤
12.95 W) PoE-PD
Table 8. Differential Signal Trace Length from DP83825I Ethernet PHY to
Magnetics on PSE Side Board
MDI DIFFERENTIAL PAIR NAME
TRACE LENGTHS FROM DP83825I ETHERNET PHY TO
MAGNETICS ON PSE SIDE BOARD
TD_OUT_P
600.481 mils
TD_OUT_M
607.136 mils
RD_OUT_P
594.252 mils
RD_OUT_M
594.170 mils
Table 9. Differential Signal Trace Length from DP83825I Ethernet PHY to
Magnetics on PD Side Board
MDI DIFFERENTIAL PAIR NAME
TRACE LENGTHS FROM DP83825I ETHERNET PHY TO
MAGNETICS ON PD SIDE BOARD
TD_P
646.238 mils
TD_N
650.510 mils
RD_P
646. 203 mils
RD_N
644.074 mils
Figure 17. Differential Signal Pair and Plane Crossing
Figure 18. Differential Signal Traces
2.5.1.2
MDI Connections Between Isolation Transformer and RJ45 Connector
shows the MDI connections between magnetics and the RJ45 connectors.
For non-PoE ports like J9 on the PSE side board, the center tap of the isolated winding has Bob-Smith
termination through a 75-
Ω
resistor and a 1000-pF capacitor to chassis ground. The termination capacitor
should be rated to a voltage of at least 2-kV. The “Bob Smith” termination is used to reduce noise resulting
from common-mode current flows. The initial ESD, EFT transient hits circulate through Bob-Smith
terminations to chassis ground.
PoE enables ports like J8 on the PSE side board and J2 on the PD side board, the Bob-Smith termination
network should be modified to block the DC common-mode voltage between pairs from creating a large
current through the termination resistors, which would blow up the termination resistors. This is done by
inserting 10-nF blocking capacitors in series with the termination resistors.
