5.6.2 Cable Transient Event and PHY Protection
Cable transient events are + and - DC surges that are induced across the transformer onto the PHY side of the TX+/- and RX+/- signals
as shown in the figure below. The PHY side of the transformer should not contain any DC component other than the typical 3.3V pull-up
on the center tap of the transformer for analog signal biasing. Especially in POE applications, there are two main reasons why cable
transient events occur, negative rail PSE switching, and hot unplug/plug-in events.
Figure 10
Transient observer on the PHY side of Eth Magnetics
Scale X = 1uS/div
Scale Y = 5V/div
Note:
for further details about Cable transient events refer to file AN1718 of SMSC
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