KX-NT321RU (serv.man2) - Panasonic PBX Service Manual (repair manual). Page 12

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This Power Supply Circuit supports the power supply through an Ethernet line called PoE (Power over Ethernet) as well as the
input from a regular AC adapter. It converts the power supply from the both to 5.0V and is supplied to a 3.3V and 1.5V regulator
on the MAIN board.
The power supply circuit consists of the three sections below.

When the hub or router supporting 802.3af is connected to LAN port (JK2), the authentication voltage and the classification volt-
age are supplied between Pin 1, 2 and 3, 6 or Pin 4, 5 and 7, 8 of JK2. 
When this voltage is detected automatically by IC9, it applies the corresponding current and is identified on the hub side, so that
the hub starts to supply 48V.
In the case of both power sources being available, power from LAN port (JK2) and the power from DC jack(JK3) by D3 and D5,
the power from LAN port is supplied to the switching regulator IC8 preferentially.

This switches the power from the DC jack or LAN port by IC8 to generate 5.0V. The switching frequency is 520kHz.

If the voltage supplied to the Switching Regulator Circuit goes under (4.4V), pin1 of IC5 then goes 0V and informs the
decreased voltage for VoIP (IC1).