3

VIN

4

RCLASS

EN

+

-

7

+

-

Threshold

VEE

to UVLO

PoE Line

Input

PoE Line

Return

VIN = 23.5V

LM5072

RCLASS

1.25V

1.25V

front_aux

rear_aux

thermal_limit

LM5072

www.ti.com

SNVS437D – MARCH 2006 – REVISED APRIL 2013

Table 2. Classification Levels and Required External Resistor Value

Class

PD Max Power Level

ICLASS Range

LM5072

RCLASS Value

From

To

From

To

0 (Default)

0.44W

12.95W

0 mA

4 mA

Open

1

0.44W

3.84W

9 mA

12 mA

130

Ω

2

3.84W

6.49W

17 mA

20 mA

71.5

Ω

3

6.49W

12.95W

26 mA

30 mA

46.4

Ω

4

Reserved

Reserved

36 mA

44 mA

31.6

Ω

Figure 18. PD Classification – Fulfilled With a Single External Resistor

Figure 18 shows the LM5072’s implementation of PD classification 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 complete, the voltage from the PSE will increase to the normal operating voltage of the PoE

system (48V nominal). When VIN 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

classification 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 temperature threshold (nominally 165°C). This

may occur if the LM5072 is operated at elevated ambient temperatures and the classification time exceeds the

IEEE802.3af limit of 75 ms.

When the classification option is not required, simply leave the RCLASS pin open to set the PD to the default

Class 0 state. Class 0 requires that the PSE allocate the maximum IEEE802.3af specified power of 15.4 W

(12.95 W at the PD input terminals) to the PD.

Copyright © 2006–2013, Texas Instruments Incorporated

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