VEE

12V

LM5072

3

7

VIN

24.5k

PoE Line

Input

PoE Line

Return

2.7V

10V

14.5V

Voltage

Discovery

Classification

Classification

Time

42V

30V

Full Power Application

Power Removal

LM5072

SNVS437D – MARCH 2006 – REVISED APRIL 2013

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Figure 16. Sequence of PoE Operating Modes

Table 1. Operating Modes With Respect To Input Voltage

Mode of Operation

Voltage from PoE Cable per

LM5072 Input Voltage

IEEE 802.3af

(VIN 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, VIN Rising)

Normal Operation

57V to 36V

70V to 32V (UVLO, VIN Falling)

Detection Signature

During detection mode, a PD must present a signature resistance between 23.75 k

Ω and 26.25 kΩ to the PoE

power sourcing equipment. This signature impedance distinguishes the PD from non-PoE capable equipment to

protect the latter from being accidentally damaged by inadvertent application of PoE voltage levels. To simplify

the circuit implementation, the LM5072 integrates the 24.5 k

Ω signature resistor, as shown in Figure 17.

Figure 17. Detection Circuit With Integrated PD Signature Resistor

During detection mode, the voltage across the VIN and VEE pins is less than 10V. Once signature mode is

complete, the LM5072 will disengage the signature resistor to reduce power loss in all other modes.

Classification

Classification is an optional feature of the IEEE802.3af specification. It is primarily used to identify the power

requirements of a particular PD device. This feature will allow the PSE to allocate the appropriate available

power to each device on the network. Classification is performed by measuring the current flowing into the PD

during this mode. IEEE 802.3af specifies five power classes, each corresponding to a unique range of

classification current, as presented in Table 2. The LM5072 simplifies the classification implementation by

requiring a single external resistor connected between the RCLASS and VEE pins to program the classification

current. The resistor value required for each class is also given in Table 2.

12

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