LTC4292 datasheet(20/39 Pages) AD

20 / 39 page

Data Sheet

LTC9101-2B/LTC9102

APPLICATIONS INFORMATION

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Rev. 0 | 20 of 39

POE BASICS

Common Ethernet data connections consist of two or four twisted

pairs of copper wire (commonly known as Ethernet cable), trans-

former-coupled at each end to avoid ground loops. PoE systems

take advantage of this coupling arrangement by applying voltage

between the center taps of the data transformers to transmit power

from the PSE to the PD without affecting data transmission. Figure

36 shows a high level PoE system schematic.

To avoid damaging legacy data equipment that does not expect

to see DC voltage, the PoE standard defines a protocol that deter-

mines when the PSE may apply and remove power. Valid PDs are

required to have a specific 25k common-mode resistance at their

input. When such a PD is connected to the cable, the PSE detects

this signature resistance and applies power. When the PD is later

disconnected, the PSE senses the open circuit and removes power.

The PSE also removes power in the event of a current fault or short

circuit.

When a PD is detected, the PSE looks for a classification signature

that tells the PSE the maximum power the PD will draw. The PSE

can use this information to allocate power among several ports, to

police the power consumption of the PD, or to reject a PD that will

draw more power than the PSE has available.

Figure 36. 802.3bt System Diagram—2-Pair PSE and Single-Signature PD

LTC4292 Datasheet(PDF) 20 Page - Analog Devices