Signature

Resistor

+

-

3

VIN

7

VEE

LM 5073

PoE Line

Input

PoE Line

Return

12V

Time

Full Power Application

Classification

Discovery

Power Removal

42V

36V

20.5V

14.5V

10V

2.7V

Voltage

LM5073

SNVS490C – MARCH 2007 – REVISED APRIL 2013

www.ti.com

Figure 8. Sequence of PoE Operating Modes

Table 1. Operating Modes With Respect To Input Voltage

Voltage at PD Input Connector per

LM5073 Input Voltage

Mode of Operation

IEEE 802.3af

(VIN pin to VEE pin)

Detection (Signature)

2.7V to 10.1V

1.5V to 10.0V

Classification

14.5V to 20.5V

12V to 23.5V

Startup Threshold

42V max

38V (UVLO Release, VIN Rising)

Normal Operation

36V to 57V

65V to 32V (UVLO, VIN Falling)

Detection Signature

In the 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 equipment to protect

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

implementation, the LM5073 integrates the signature resistor, as shown in Figure 9.

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

complete, the LM5073 will disengage the signature resistor to reduce power loss in all other modes.

Figure 9. Detection Circuit With Integrated PD Signature Resistor

8

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