AN2487 datasheet(11/32 Pages) STMICROELECTRONICS

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AN2487

Circuit descriptions

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3.1.2

Transient protection

A 58 V Zener diode (D3) is placed across the DC bus so that it clamps all inputs to slightly

more than that value. ESD and connection transients then cannot harm downstream

circuitry.

3.1.3

Input capacitor

The 802.3af specification requires at least 0.05 µF of capacitance for the maintain power

signature. 0.1 µF is installed at C1. This capacitor must be rated for at least 90 volts.

3.2

STHS4257 powered device interface controller

3.2.1

Sequence of operation

During startup (connection of PD or power-up of a PSE output), a reduced input voltage is

used to identify the loads requiring power. Between -2.8 V and -10 V, an incremental load

slope of 25 k indicates that the load is requesting power. This is called the PoE signature

resistance.

3.2.2

POE Signature resistance

The STHS4257 powered device interface controller generates the 25 k

Ω signature

resistance slope internally. No external resistor is required. Note that there is a Signature

Disable function on the chip, which is not used in this design. The signature disable function

can be used to disable power from the PSE if the powered device is receiving its power from

another source.

3.2.3

Classification

Between -15.5 and -20 V the load current indicates a power classification. Optionally, the

Powered Device can inform the power sourcing equipment (PSE) to expect loading in a

defined range. A precision resistor could be added at the R1 location to set Classification

Current. The resistor was not installed in the demo board, so the board reports Class 0

(zero), informing the PSE that power may be required over any part of the full 0.44 W to

12.95 W load range. Table 3 in the STHS4257 datasheet and Tables 33-3 and 33-4 in the

IEEE 803.2af specification show the defined ranges and resistor values. Table 1 below

combines the information.

Table 1.

Power classes and class identification

Class

Power range

Notes

Classification current

R1 value

0.44 to 12.95 W

Default

0 to 4 mA

Open

0.44 to 3.84 W

Optional

9 to 12 mA

124

Ω

3.84 to 6.49 W

Optional

17 to 20 mA

68.1

Ω

6.49 to 12.95 W

Optional

26 to 30 mA

45.3

Ω

Reserved

36 to 44 mA

30.9

Ω

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