Classification Mode (12.6V

In the classification mode, the PSE classifies the PD

based on the power consumption required by the PD.

This allows the PSE to efficiently manage power distri-

bution. The IEEE 802.3af standard defines five different

current.

The PSE determines the class of a PD by applying a volt-

age at the PD input and measures the current sourced

out of the PSE. When the PSE applies a voltage between

12.6V and 20V, the MAX5940_ exhibit a current charac-

teristic with values indicated in Table 2. The PSE uses the

classification current information to classify the power

requirement of the PD. The classification current includes

the current drawn by the 25.5k

Ω detection signature

resistor and the supply current of the MAX5940_ so the

total current drawn by the PD is within the IEEE 802.3af

standard figures. The classification current is turned off

whenever the device is in power mode.

Power Mode

gradually turn on the internal N-channel MOSFET Q1

(see Figure 2). The MAX5940_ charge the gate of Q1

with a constant current source (10µA, typ). The drain-

to-gate capacitance of Q1 limits the voltage rise rate at

the drain of the MOSFET, thereby limiting the inrush

current. To reduce the inrush current, add external

drain-to-gate capacitance (see the Inrush Current Limit

section). When the drain of Q1 is within 1.2V of its

source voltage and its gate-to-source voltage is above

5V, the MAX5940_ asserts the PGOOD/PGOOD out-

puts. The MAX5940_ have a wide UVLO hysteresis and

turn-off deglitch time to compensate for the high

impedance of the twisted-pair cable.

Undervoltage Lockout

The MAX5940_ operate up to a 67V supply voltage with a

(MAX5940A/MAX5940C) or 39V (MAX5940B/MAX5940D)

MAX5940B/MAX5940D have an adjustable UVLO thresh-

old using a resistor-divider connected to UVLO (see

Figure 3). When the input voltage is above the UVLO

threshold, the IC is in power mode and the MOSFET is

on. When the input voltage goes below the UVLO thresh-

IEEE 802.3af PD Interface Controller

For Power-Over-Ethernet

_______________________________________________________________________________________

7

CLASS

USAGE

Ω)

MAXIMUM POWER USED BY PD (W)

0

Default

10k

0.44 to 12.95

1

Optional

732

0.44 to 3.84

2

Optional

392

3.84 to 6.49

3

Optional

255

6.49 to 12.95

4

Not Allowed

178

Reserved*

*Class 4 reserved for future use.

IEEE 802.3af PD CLASSIFICATION

CURRENT SPECIFICATION (mA)

CLASS

MIN

MAX

MIN

MAX

0

10k

12.6 to 20

02

04

1

732

12.6 to 20

9.17

11.83

9

12

2

392

12.6 to 20

17.29

19.71

17

20

3

255

12.6 to 20

26.45

29.55

26

30

4

178

12.6 to 20

36.6

41.4

36

44

*VIN is measured across the MAX5940 input pins, which does not include the diode bridge voltage drop.

Table 2. Setting Classification Current