AN2783 datasheet(18/47 Pages) STMICROELECTRONICS

18 / 47 page

PoE section

AN2783

18/47

With the SMAJ58A the standoff is 58 V and the clamping voltage for a standard 10/1000 µs

transient is 93 V. PM8800 is able to withstand transient voltage up to 100 V without any

damage.

PoE section

8.1

Signature

Signature is the first phase in the PoE standard and allows a PSE (power source equipment)

to recognize the presence of a PD (powered device) that can accept power on the Ethernet

cable.

The PM8800 integrates a 24.5 k

Ω resistance to simplify a standard PoE PD interface

design. Its value has been chosen to take into account the voltage drop across the diode

bridge and its effect on the effective resistor value presented at the RJ45 connector input.

This resistor is disconnected for input voltages higher than 11.5 V.

The required signature capacitance is obtained with C1= 100 nF, 100 V.

8.2

Classification

Classification is the second phase in the PoE standard and allows the PSE to allocate the

right amount of power for the PD connected on a single port.

The IEEE802.3af standard defines 4 power classes.

PM8800 has a dedicated pin for the classification resistor. The reference board has R35 left

open, corresponding to CLASS 0.

To select a different class please refer to the table below:

Table 3.

RCLASS resistor value

To provide a constant current during the classification phase, PM8800 has an internal

voltage regulator that maintains 1.4 V typ. across the classification resistor.

The value of R35 is calculated taking into account the power consumption of PM8800 during

the classification phase, which is about 1.5 mA.

802.3af

classification current (mA)

CLASS

PD power(W)

RCLASS(Ω)min.

max

0.44 -12.95

Open

0.44 - 3.84

158

3.84 - 6.49

82.5

6.49 - 12.95

52.3

Reserved

36.5

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