Figure 5-2. External Power Input Connected to PD70210A Output

5.2.1

External Source Connected to PD Device Output

The PD70224 WA_EN pin will be used for protecting the PSE when an external adapter is connected.

In this mode, the risk to PSE side exists when a higher voltage external adapter is hot connected to the system.

When the WA_EN input voltage is higher than its threshold level, PD70224 internal FETs are disabled, converting the

device into standard diode bridge.

An optional “slow start” circuit prevents adapter jack contact arcing when an adapter is “hot plugged” by limiting its

surge current. For the detailed circuit, see AN3472, Implementing Aux Power in PoE.

The PD70210A also has a specific input pin, to disable the isolation switch, when an external adapter is connected.

In this case, WA_EN resistors divider depends on the “turn off” threshold of the PD70210A and PD70224.

Zooming into the resistors to be selected in external adapter connection.

Figure 5-3. External Power Input Resistors Dividers

R1 and R2 sets a rough threshold for PFET Q1 enable to detect whether external adapter exists or not. It should be

set to be lower threshold than PD70224 and PD70210A disable levels.

R3, R4, and R5 set PD70210A disable threshold and PD70224 disable threshold.

PD70210A disable threshold should be set so that it will always be lower than PD70224 disable threshold.

1 V is a good choice for the margin between the two.

So, in case of 44 V–57 V external adapter, the disable setting can be selected as follows:

PFET enable threshold = 35 V

PD70224 disable threshold = 43 V

R1 and R2 setting should be so that the value of Q1 VGS < 20 V at max voltage condition of external adapter.

While external adapter voltage is above 35 V, Q1 will be above its VGSth value.

PD70224

Application Information

Datasheet

DS00003575C-page 17