PM8805 datasheet(30/48 Pages) STMICROELECTRONICS

30 / 48 page

PM8805 overview

PM8805

30/48

DS12813 Rev 2

Figure 8. Simplified front aux connections

Connection of the auxiliary source prior to the internal hot swap MOSFET is preferred if

used as an alternative (or a backup) to PoE.

The main reason for this is because the priority of the front auxiliary over PoE cannot be

guaranteed in every condition, since it depends on the insertion sequence and the voltage

of the PoE line with respect to the auxiliary source.

In more detail, the front auxiliary takes priority over the PoE if:

1.The front aux voltage is higher than PoE voltage, or,

2.The front aux connector is plugged when PoE is not (or not yet) present.

If a PSE is already powering the PD, and the auxiliary voltage is lower than PoE, it doesn‘t

provide power until the PoE power is removed, acting as a backup power. The minimum

operating voltage for the front auxiliary connection is 15 V.

The switch from PoE line to front aux is a one way operation and to back to PoE is needed

to remove the front aux voltage and wait for a new detection and classification of PD on the

PoE line.

Using a front aux configuration, the charge of the output capacitors is controlled by the

internal current source, which is activated when the input voltage is above the UVLO_R

threshold or if the voltage on the FAUX pin is above FAUX_thr (1.25 V typ.). With FAUX=1,

the PGD signal is asserted - and the hot swap MOSFET activated - as soon as the inrush

phase is completed (VDS_good and VGS_good conditions met) with no PGD_delay.

This configuration has the advantage of limiting the inrush current when plugging the DC

jack, and the disadvantage of the power losses of the hot swap MOSFET.

PM8805 Datasheet(PDF) 30 Page - STMicroelectronics