A Note About PoE Input Potentials
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5
A Note About PoE Input Potentials
PoE applications are typically -48V systems, in which the notations GND and -48V normally refer to the
high and low input potentials, respectively. However, for easy readability, the LM5072 datasheet was
written in the positive voltage convention with positive input potentials referenced to the VEE pin of the
LM5073. Therefore, when testing the evaluation board with a bench power supply, the negative terminal of
the power supply is equivalent to the PoE system’s -48V potential, and the positive terminal is equivalent
to the PoE system ground. To prevent confusion between the datasheet and this application note, the
same positive voltage convention is used.
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Connection and Custom Configurations
This section contains information about the setup and configuration of the LM5073 evaluation board.
Figure 1
shows the evaluation board PCB layout.
The following are the connections:
•
J1, RJ45 connector for PoE input and data link.
•
J2, PJ102A power jack for the front Aux (FAUX) power input. The center pin of J2 is the high potential
pin.
•
J3, PJ102A power jack for the rear Aux (RAUX) power input. The center pin of J3 is the high potential
pin.
•
J4, RJ45 connector interface for data link to PHY circuit.
•
J5 and J6, 3.3V output port of the onboard buck regulator. J5 is the high potential pin.
•
P1 and P2, a pair of pins for quick PoE input connection from a bench power supply to the center taps
of the Ethernet RX and TX magnetics. Pin polarity reversible.
•
P3 and P4, a pair of pins for quick PoE input connection from a bench power supply to the nodes of
the spare pairs. Pin polarity reversible.
•
P5 and P6, a pair of pins for quick FAUX power input connection to a bench power supply. P5 is the
high potential pin.
•
P7 and P8, a pair of pins for quick RAUX power input connection to a bench power supply. P7 is the
high potential pin.
•
P9 and P10, PD interface power output port to an external DC-DC converter. P9 is the high potential
pin.
•
P11, active high shutdown signal pin to control an external DC-DC converter.
•
P12, active low shutdown signal pin to control an external DC-DC converter.
•
P13, bias voltage (Vcc) to or from an external DC-DC converter, limited from 9V to 14V.
•
P14, chassis ground pin.
•
P15, 3.3V from external DC-DC converter to bias the secondary windings’ center taps of the Ethernet
RX and TX transformers.
The evaluation board is designed with multi function features. Jumpers are used for easy reconfiguration
of the evaluation board to meet various application requirements. The jumpers are listed in
Table 1
.
2
AN-1574 LM5073 Evaluation Board
SNVA214A – April 2007 – Revised April 2013
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