PM8805 datasheet(35/45 Pages) STMICROELECTRONICS

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DS12813 Rev 1

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PM8805

PM8805 overview

If the auxiliary source voltage is higher than the PoE voltage (for example, PoE at 42 V and

Aux at 48 V), even if rear aux with MPS mode is selected, the PoE to rear aux transition

implies the loss of the link with the PSE, basically because the MPS current is drawn from

the auxiliary (through the body diode of the hot swap MOSFET) instead of the PSE (through

the input bridges).

Latched shutdown

Pulling both FAUX and RAUX input pins high at the same time, the PM8805 goes into a

Latched Shutdown condition as described in Table 12, with a filter of 3.3 ms before

command actuation.

In this condition, the PM8805 goes into a very low consumption mode where the hot swap

MOSFET is opened with the PGD de-asserted, the MPS current is turned off, and the active

bridges are also off. The overall current consumption stays below Idd_off (2 mA max.), so a

PSE monitoring DC MPS removes and recycle power.

Note that while Type1 and Type2 PSEs may monitor AC MPS, DC MPS or both, Type3 and

Type 4 PSEs are required to monitor only DC MPS, so power recycle is always assured

when the PD is powered by Type3 and Type 4 PSEs.

After power recycle the system reboots completely, performing a new PoE Detection,

Classification and Power On.

Sleep / Wake up

Pulling RAUX, FAUX, STBY pins high together, after a 3.3 ms filter delay, the PM8805

toggles between SLEEP and WAKE UP modes.

The sleep mode, as described in Table 12, puts the PM8805 hot swap MOSFET in off

mode, the PGD signal goes low, while the MPS circuitry is turned on. So, the PD is able to

maintain active the connection with the PSE even if the system is turned off and the overall

current consumption is much lower than 10 mA.

While the system is in sleep mode, if the same input pins are pulled high again for more than

3.3 ms, the hot swap MOSFET is closed raising the output voltage, then the PGD signal is

asserted in order to re-boot the system afterwards.

This operational mode allows the system to go into a deep sleep mode while the PoE line is

maintained active and therefore with a WAKE UP the PD system starts sooner than waiting

for new Detection and Classification cycles.

The main difference with the standby mode is the absence of output voltage, which may be

needed in some cases to further minimize the current consumption of the PD system in this

operational mode.

PM8805 датащи(PDF) 35 Page - STMicroelectronics