MP8020 – IEEE 802.3af/at/bt, PoE, POWERED DEVICE INTERFACE

MP8020 Rev. 1.0

MonolithicPower.com

22

2/17/2022

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The MP8020 also has an inrush current limit

function after start-up, which is about 1/7 of the

steady state current limit.

Selecting the PoE Power MOSFET

The MP8020 internal hot-swap MOSFET has a

maximum 1.6A current limit. For type 1, 2, and

3 PDs (≤51W), the internal hot-swap MOSFET

is sufficient. For type 4 PDs (>51W), an external

MOSFET must be placed in parallel with the

internal hot-swap MOSFET. The GATE1 pin

can drive one external N-channel MOSFET.

The MOSFET should be selected to meet the

following specifications:

 The voltage rating should be at least 100V

for high-voltage surge environments.

 The on resistance (RDS(ON)) should be below

200mΩ for power dissipation and for light-

load shutdown functions. A 100mΩ RDS(ON)

is typically recommended.

 The gate charge (QG) should be as small as

possible to provide faster response times

under overload conditions. It is typically

recommended for QG to be below 15nC

(VGATE = 4.5V).

 The gate threshold voltage should be below

4V to fully drive the GATE1 and GATE2

voltages.

SENSE Pin Setting

The external parallel MOSFET current limit is

configured by a resistor (RSENSE) between the

VDD

and

SENSE

pins.

It

is

typically

recommended for RSENSE to be 18mΩ. The total

current limit (IIN_LIM) can be calculated using

Equation (1):

IN _ LIM

INNER

SENSE

26mV

II

R



(1)

Where IIN_LIM is the PD current limit, and IINNER is

the MP8020 internal hot-swap MOSFET current

limit.

When the external MOSFET is connected, it is

recommended to set IINNER to 0.9A.

Selecting the Adapter MOSFET

The GATE2 pin is designed to drive one

external N-channel MOSFET for the adapter

supply. Select the adapter MOSFET using the

same guidelines as the PoE power MOSFET.

Design Example

Table 6 shows a design example following the

application guidelines for the specifications

below.

Table 6: Design Example

VDD

Class Level

VADP

54V

Class 8

24V

For the detailed application schematic, see

Figure 8 on page 24. For the typical

performance and circuit waveforms, see the

Typical Performance Characteristics section on

page 12. For more device applications, refer to

the related evaluation board datasheet.