PM8803

Auxiliary sources

Doc ID 018559 Rev 2

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Auxiliary sources

The majority of Powered Devices is designed to work with power from either a 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.

Different alternatives are available for connecting auxiliary sources to the PoE section of a

PD device. Auxiliary sources can be connected prior to the hot-swap MOSFET, after the hot-

swap MOSFET or even at the output of the DC/DC converter.

All the above-mentioned methods are available with the PM8803.

Both

Figure 1 and Figure 2 show simplified application schematics where auxiliary sources

can be connected either Prior (Front) or After (Rear) the internal hot-swap MOSFET (VDD

and RTN) making use of a resistor divider between the external source and respectively SP

or SA pins.

Connection of the wall adapter prior to the internal hot-swap MOSFET has a limitation on

the voltage of the adapter itself, since it is seen as an alternative of the PoE line and the

embedded DC/DC section would be activated only when its value is above the UVLO_R

threshold. If the voltage on the SP pin is above 1.1 V, the internal UVLO threshold is

bypassed and the PM8803 operates with voltage as low as 15 V typical. The current flowing

into the hot-swap MOSFET will be limited by a dual threshold: typically140 mA during the

inrush phase, and a user programmable value (DCCL pin) for the rest of the phases. Priority

of one source over another cannot be guaranteed by design, since it depends on timings of

insertion and the value of the PoE line with respect to the auxiliary. If, for example, the PoE

connection is already established, the auxiliary source cannot prevail unless its value is

higher than that of the PoE after the diode bridge.

Please note that with the use of a low-voltage adapter applied before the hot swap frontal

connection, the max power drawn could be evaluated as ( Vin - Vd ) x Imax; in case of a

15 V adapter the input power will be limited to about (15 - 0.5) x 1 A = 14.5 W, much lower

than the available power from the PoE connection.

High-power systems must use high-voltage power adapters, with voltage at 48 V, and rear

connections, not to be limited by the internal DC current limitation.

Both

Figure 1 and Figure 2 show simplified application schematics where auxiliary sources

are also connected after the internal hot-swap MOSFET (VDD and RTN). This connection,

together with the resistor divider on the SA pin, allows priority of the external source with

respect to the PoE. Indeed, if the voltage of this pin is above the value of 1.20 V, the PM8803

will disable its PD interface section and enable the DC/DC section only. The internal hot-

swap MOSFET will be opened.

Depending on the value of the auxiliary source, the resistor divider must be dimensioned in

order to have voltage on the SA and SP pins above their thresholds but still below their

maximum operative value of 3.3 V specified in

Table 3.

Figure 15 depicts a smooth transition between a PoE and a wall adapter whose voltage is

5 V higher than the PoE voltage.