PM8805 datasheet(17/45 Pages) STMICROELECTRONICS

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DS12813 Rev 1

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PM8805

PM8805 overview

The PM8805 integrates a dual MOSFET bridge and a PD-PoE single-signature interface,

specifically designed to support high power, 4-pair (4P) applications, as being specified in

the new IEEE802.3bt standard, but also capable of working with high efficiency, 2P

applications.

Typically, after the data transformer a diode bridge is used to set a defined polarity to the

input voltage since such polarity on the Ethernet cable may vary.

Considering the maximum power levels defined by the IEEE802.3-2015 standard (25 W for

a Type2 PD) the diode bridges power losses may still be acceptable, but with new power

hungry applications like UPOE, with more than 50 W at PD end, the power losses of a

standard diode bridge become too high.

The same consideration is valid for standard applications that need to squeeze the

maximum available power from the PSE. For example, comparing a diode bridge with Vd=

0.8 V with bridged 100 mΩ Ron MOSFETs, the gain at 0.5 A, 50 V is about 0.75 W out of 25

W, i.e. about 3% on overall efficiency.

4.1

Detection and DET pin

In Power over Ethernet (PoE) systems, the Power Sourcing Equipment (PSE) senses the

Ethernet connection to detect whether a Powered Device (PD) is plugged into the cable

termination physical interface (PI). This is done by applying a small voltage (2.7 V to 10 V)

on one of the two couples of pairs that constitute the Ethernet cable and measuring the

equivalent resistance in at least two consecutive steps.

During this phase, called Detection, the PD must present a signature resistance between

23.75 kΩ and 26.25 kΩ on each pair-set.

If a single PD is connected to the PI, it may share the same detection resistor for both pair-

sets. Such a PD is called Single-Signature PD.

On the PM8805, the signature resistor must be connected between DET and VOB pins.

This resistor is in series to a pass transistor (see Figure 4) enabled only during the detection

phase. No current is flowing through the signature resistor for the rest of the operative

phases (Classification and Power On).

The value of the detection resistance has to be selected also taking into account for the

additional current flowing into any external circuitry put under the input voltage, e.g. the

active bridge. For this reason the typical value that can be used in most cases is 26.1 kΩ.

During Detection, most of the circuitry inside the PM8805, as well as the active bridge, is

disabled to minimize the offset current. The current drawn by the IC in this phase is typically

15 uA at 8 V with resistive behavior, so the residual offset current is much less than 10 uA.

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