Datasheet

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 www.ti.com
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 (V
IN
pin to V
EE
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 70V to 32V (UVLO, V
IN
Falling)
Detection Signature
During detection mode, a PD must present a signature resistance between 23.75 kΩ 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 kΩ 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.
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