Auxiliary sources

PM8803

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Doc ID 018559 Rev 2

Figure 15.

Smooth transition from POE to auxiliary source

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

The minimum operative voltage for auxiliary sources is 13 V, thus allowing the use of a 15 V

typ. +/-5% power adapter. The internal logic will enable operations of the PWM controller

only if the input voltage is over the signature threshold (10.8 V typ, 10.3 V -11.3 V range).

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 lowers the converter’s

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

the current during the charging phase, and to add only a few m

Ω in-series during normal

operation.

No DC current limit is foreseen for the rear connection, since the current will not be flowing

through the hot-swap MOSFET. Cycle-by-cycle, overcurrent protection and thermal

protection are instead always active, as well as when the voltage on the SA pin is above

1.20 V.

The T2P signal will remain high (de-asserted) if a rear auxiliary source is connected.

If the T2P signal was asserted when the auxiliary source was connected, it will be turned

off; its status is stored unless the input voltage drops. So if the PSE stays connected until

the auxiliary source is removed, the T2P indication turns on again when the wall adapter is

disconnected.

The removal of the external auxiliary source such as a wall adapter usually is followed by a

system reboot because the PSE needs some time to re-detect the PD.

If a rear auxiliary connection (using the SA pin) is foreseen in the converter design, it is

suggested to move the 0.1 µF capacitor required by the IEEE802.3at standard from the

input (VDD to VSS ) to the the internal hot-swap MOSFET (VSS to GND) . Alternatively, split

the 0.1 µF in two capacitors of 47 nF: one placed at the input terminal, the second one

across the hot-swap MOSFET.

AM045105v1