Auxiliary sources

PM8800A

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The minimum external voltage to be used in this configuration is about 16 V.

Option I connection is activated when the voltage at the relevant pin is higher than 2 V and a

The AUXI_IRL pin can be connected to the auxiliary source through a diode being the

current internally limited to about 300

µA.

It is important to highlight that UVLO override and inrush current limit are mutually exclusive,

and when UVLO is overridden, default inrush current limit is provided. The PSE will not

detect the presence of a valid PD, since the PoE diode bridges are reversed biased.

When the auxiliary source is connected after the inrush phase, two possibilities arise

depending on the voltage of the external power supply. If the voltage is lower than the one

from PoE, the diode is reverse biased and PSE will keep on supplying the PD. If the auxiliary

source has a voltage greater than the PoE, the PoE diode bridge will be reverse biased and

auxiliary source will supply the PD. In case the voltages are comparable, current sharing

occurs between the sources according to their respective source resistance.

The Option II configuration bypasses the PD interface and connects directly the external

source to the PWM section. This type of connection is not limited by the internal current limit

circuits and therefore is more suitable for lower auxiliary voltages.

Internal under voltage lock out threshold will be lowered by connecting the auxiliary source

to the AUXII pin. The minimum external voltage to be used in this configuration is about

12 V.

Note that inrush current in this case is not limited and an external solution must be found:

the simplest solution is to put a low value resistor in series, but this lower the converter

efficiency. More efficient solution is the use of a MOSFET as power switch, able to limit the

current during the charging phase, and to add only few milliohm in series during normal

operation.

Main advantage of this connection is the "prevalence" over PoE voltage, that can be

than 100

µA (see following equation). In this case the PD interface is switched off regardless

the presence of PoE voltage.

AUXII can be connected directly to the auxiliary voltage through a diode. The current in this

case is internally limited to about 250

µA.

If a prevalent AUXII is detected the PD interface is switched off, the hot swap MOSFET is

opened regardless the PoE voltage level.

I

AUXI

V

AUXI

V

D

–

2

–

R

AUXI

---------------------------------------

70

µA

≥

=

I

AUXII

V

–

1.1

·

–

R

AUXII

-------------------------------------------

100

µA

≥

=