LM5072 datasheet(11/24 Pages) NSC

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Modes of Operation

Per the IEEE 802.3af specification, when a PD is connected

to a PoE system it transitions through several operating

modes in sequence including detection, classification (op-

tional), turn on, normal operation, and power removal. Each

operating mode corresponds to a specific PoE voltage range

fed through the Ethernet cable. Figure 3 shows the IEEE

802.3af specified sequence of operating modes and the

corresponding PD input voltages.

Current steering diode-bridges are required for the PD inter-

face to accept all allowable connections and polarities of

PoE voltage from the RJ-45 connector (see the example

application circuits in Figures 18, 19, 20 and 21). The bridge

will cause some reduction of the input voltage sensed by the

LM5072. To guarantee full compliance to the specification in

all operating modes, the LM5072 takes into account the

voltage drop across the bridge diodes and responds appro-

priately to the voltage received from the PoE cable. Table 1

presents the response in each operating mode to voltages

across the VIN and VEE pins.

TABLE 1. Operating Modes With Respect To Input Voltage

Mode of Operation

Voltage from PoE Cable per

IEEE 802.3af

LM5072 Input Voltage

IN pin to VEE pin)

Detection (Signature)

2.7V to 10.0V

1.5V to 10.0V

Classification

14.5V to 20.5V

12V to 23.5V

Startup

42V max

38V (UVLO Release, V

IN Rising)

Normal Operation

57V to 36V

75V to 32V (UVLO, V

IN Falling)

20184617

FIGURE 3. Sequence of PoE Operating Modes

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