LM5072 datasheet(12/24 Pages) NSC

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Detection Signature

During detection mode, a PD must present a signature re-

sistance 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 4.

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 re-

quirements 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 mea-

suring 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 implemen-

tation by requiring a single external resistor connected be-

tween the RCLASS and VEE pins to program the classifica-

tion current. The resistor value required for each class is also

given in Table 2.

TABLE 2. Classification Levels and Required External Resistor Value

Class

PD Max Power Level

ICLASS Range

LM5072

RCLASS Value

From

0 (Default)

0.44W

12.95W

0 mA

4 mA

Open

0.44W

3.84W

9 mA

12 mA

130

Ω

3.84W

6.49W

17 mA

20 mA

71.5

Ω

6.49W

12.95W

26 mA

30 mA

46.4

Ω

Reserved

36 mA

44 mA

31.6

Ω

Figure 5 shows the LM5072’s implementation of PD classi-

fication using an external resistor connected to the RCLASS

pin. During classification, the voltage across the VIN and

VEE pins is between 13V and 23.5V. In this voltage range,

the class resistor RCLASS is engaged by enabling the 1.25V

buffer amplifier and MOSFET. After classification is com-

plete, the voltage from the PSE will increase to the normal

operating voltage of the PoE system (48V nominal). When

rises above 23.5V, the LM5072 will disengage the

RCLASS resistor to reduce on-chip power dissipation.

The classification feature is disabled when either the front or

rear auxiliary power options are selected, as the classifica-

tion function is not required when power is supplied from an

auxiliary source. The classification function is also disabled

when the LM5072 reaches the thermal shutdown tempera-

ture threshold (nominally 165˚C). This may occur if the

20184618

FIGURE 4. Detection Circuit With Integrated PD

Signature Resistor

20184619

FIGURE 5. PD Classification – Fulfilled With a Single External Resistor

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