PM8800ATR datasheet(29/35 Pages) STMICROELECTRONICS

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PM8800A

Auxiliary sources

29/35

Auxiliary sources

The majority of powered devices is designed to work with power from either PoE network or

auxiliary sources. Even though having both sources simultaneously connected is not the

normal operating case, the presence of an auxiliary supply allows PDs to be used also when

the PoE is not available or not sufficient. This is particularly true for appliances requiring

more than 12.95 W connected to 802.3af compliant PSE.

High degree of flexibility is granted by the PM8800A for designs based on external sources.

As depicted in the following figure there are two available options for connecting auxiliary

sources to the PM8800A. One possibility consists in diode O-ring the auxiliary source into

the PM8800A PoE pins (VIN and VSS) referred to "Option I ". In the second alternative,

referred to "Option II" the external supply voltage is diode ORed into the PWM section of the

device.

Figure 19.

Alternatives for auxiliary source connection

The two options differ for the in-rush and DC current protection, for the allowed auxiliary

source voltage range, power sequence and prevalence of the external source with respect

to the PoE one.

Option I configuration has been designed to take advantage of the inherent in-rush and DC

current protection also when working with voltages from external power supplies.

When the auxiliary source is connected prior to PD detection by the PSE and has a voltage

above the VUVLO_R (38-57 V), the internal UVLO is released and the MOSFET is switched

on, starting the inrush phase. Inrush and DC current limit can be programmed by respective

external resistors see Section 5.4 and Section 5.5.

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

to the AUXI_IRL pin. Wall adapter with supply voltage lower than 38 V can be used, still

profiting from the inrush and DC current limit provided by the hot-swap MOSFET.

PM8800

VIN

VSS

RCLASS

DCCL

AUXI_IRL

VFB

nPGD

GND

AGND

COMP

VCC

AUXII

Option I

Option II

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