AN2487 datasheet(5/32 Pages) STMICROELECTRONICS

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AN2487

Overview

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Overview

The IEEE 802.3af specification is intended to greatly simplify the wiring to remote

equipment by providing power over the same medium that carries the signal to/from the

equipment. Applications that particularly benefit from this concept are security cameras,

wireless access points, and internet telephones. Power wiring, with its associated conduit

installation and licensed electrician fees, is eliminated. The wiring voltage is kept to a safe

level, less than 60 volts DC. This voltage is not even applied to a connector until the

downstream equipment is determined to be connected and asking for power. Lower voltage

signaling is used to determine the need for power.

The cable voltage was selected so that existing 48 V telecom equipment could be used to

supply the power without requiring DC-DC conversion. Careful consideration was given to

existing communications wiring. Category 5 and 6 wiring and connectors can be used

unmodified to carry power, if the current is limited to values that cannot cause overheating.

Category 5 and 6 cables contain extra wire pairs that are unused in the present Ethernet

structure. These can be used to carry power (PSE(a) Pinout Alternative B), or the signal-

carrying twisted pairs themselves can carry power if center-tapped transformers are used to

inject and recover DC current (PSE Pinout Alternative A).

For further safety, the powered device (PD) must present a “maintain power” signature. If the

load is disconnected, the source cuts off power to the cable.

Isolation is required between the power source and the PD ground, if it exists. In some

applications this is not an issue, such as in plastic-cased standalone Ethernet devices

without external connections.

If external connections are required, the powered device's accessible metal must have a

local ground to avoid hazards caused by lightning-induced ground jumps.

1.1

Limitations on available power

Voltage must be kept below 60 V to meet IEC60950 and other specifications' safety limits.

The standard telecom battery voltage range is 44 V to 57 V.

Steady-state current must be kept under 350 mA to prevent excessive temperature rise in

the wire and connectors. (Higher current, 400 mA for 50ms max, can be used for startup.)

The 802.3af specification permits only one power scheme per channel, either unused pair or

simplex leads. Both schemes cannot be used together to increase available power.

Consequently, at low input voltage, a minimum power of only 15.4 W is available for any load

(350 mA at 44 V).

The specification also requires operation with 20 ohms of cable and connector resistance,

further limiting the minimum power available at the powered device input to 12.94 W.

The specification also requires that power of either polarity be accepted. A bridge rectifier

with its forward drop loss further reduces input power. At 350 mA, a 1N4004 diode drops 0.7

volts. Loss in a bridge built of parts similar to these is 0.49 W, leaving only 12.45 W at the

PD power supply input. At about 82% efficiency, the power converter can deliver no more

than approximately 10 W.

PSE: power sourcing equipment

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