PM8805 datasheet(33/45 Pages) STMICROELECTRONICS

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DS12813 Rev 1

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PM8805

PM8805 overview

If the length of the first classification finger is less than Tcls_typ3 (81 ms) the MPS current

profile has a 27% duty cycle pulse with a period of Toff_mps27 + Ton_mps27 (305 ms typ.).

This is the standard behavior when the PD is connected to Type1 or Type2 PSEs.

Type 3 and Type4 PSEs instead generate a long first Class Event, (longer than Tcls_typ3).

In that case the PM8805 sets the MPS duty cycle to 3%, with a period of

Ton_mps3+Toff_mps3 (285 ms typ.) allowing to minimize standby power consumption.

If a 4-pair PSE is detected, the MPS current pulse is set to Imps4P (20 mA typ.) instead of

Imps2P (11.5 mA typ.) and is shared by the two sets of pairs.

The PM8805 standby mode does not just activate MPS current, but also configures the

device in order to minimize the quiescent current consumption, and to avoid the MPS

current pulses to be drawn by the capacitance connected to VOUT.

For this purpose, the high sides of the bridges are turned off, as well as the charge pump,

and the hot swap MOSFET is operated in linear mode, close to saturation. This

implementation introduces a drop voltage across the hot swap MOSFET of about 4 V,

causing the MPS current of MPS to be drawn from the PSE instead of the DC/DC capacitors

connected to VOUT, thus facilitating the PSE reading the MPS current pulses even if the

pulses duty cycle is close to the lower limits defined by the standard.

This feature is very important for applications which are sensitive to the standby current

consumption.

When the PM8805 is powered by an auxiliary source, typically the MPS current is not

activated, unless it is required to maintain the PoE line alive even if the internal bridge is

open: see the following section for more details.

In case of thermal shutdown, MPS current is turned off to ensure that the current

consumption is below the minimum threshold, thus allowing the PSE to disconnect the PD.

Table 13. MPS current profile

First class event = long

First class event = short

2-pair power

I=11.5mA; D=3%

I=11.5mA; D=27%

4-pair power

I=20mA; D=3%

I=20mA; D=27%

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