Auxiliary sources

PM8801

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DocID018560 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 PM8801.

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 is activated only when its value is above the UVLO_R threshold.

Default inrush and DC current limitation are considered in this case.

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 source, for

example, the PoE connection is already established, the auxiliary source cannot prevail

unless its value is higher than the one from the PoE after the diode bridge.

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

are 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 typ., the

PM8801 will disable its PD interface section and enable only the DC/DC section. 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 pin above the threshold 1.20 V (see Table 6) but still below

its maximum operative value of 3.3 V, as reported on Table 3.

Figure 17. Smooth transition from POE to auxiliary source and vice-versa

Ch1:SA, Ch2: VSS-RTN, Ch3: Iinput, Ch4: 5Vout

The minimum operative voltage for auxiliary sources connected on the SA pin is 13 V, thus

allowing the use of a 15 V typ

±10% power adapter.