AN2783 datasheet(19/47 Pages) STMICROELECTRONICS

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AN2783

PoE section

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R35 is disconnected at the end of the classification phase, when the input voltage rises

above 23 V.

8.3

UVLO and power-on

Power-on is the final state after successful detection and classification. The input voltage is

increased and an internal switch is closed to connect the PD load. The inrush current is

actively limited by the PM8800 itself.

The PM8800 is fully compliant with UVLO thresholds and inrush current limits defined in the

IEEE802.3af standard.

8.4

Inrush current limit

The inrush current in PM8800 has a three-step limit depending on the voltage across the

hot-swap MOSFET. The first two steps are fixed at 140 mA and 250 mA respectively, the

last step has a default value of 440 mA and it is programmable.

The external resistor to select the desired inrush current is found with the following formula:

Equation 3

The PM8800 useful programming range for the inrush current limitation is between140 and

440 mA. The practical resistor value ranges between 25 k

Ω and 82 kΩ.

Depending on the application, attention must be given to the choice of the inrush current

limit to avoid that the voltage drop on the external Ethernet cable causes UVLO conditions

during the charging phase of the bulk capacitor.

It is recommended to select this voltage drop (can be estimated as max: 20

Ω x I inrush) to

be lower than the UVLO hysteresis (7 Vmin) in order to avoid hiccup turn-on.

The inrush current is set to the default three-step values when the AUXI pin is pulled up over

the 2 V internal threshold by an auxiliary voltage.

Figure 8.

Inrush current limit vs. RIRL

IRL kΩ

[]

11200

IRL mA

[]

------------------------

100 150 200 250 300 350 400 450 500

IIRL [mA]

AN2783 Datasheet(PDF) 19 Page - STMicroelectronics