IEEE 802.3af/at-Compliant, Powered Device Interface

Controllers with Integrated Power MOSFET

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Detailed Description

Operating Modes

MAX5981 operates in four different modes: PD detec-

tion, PD classification, mark event, and PD power. The

devices enter PD detection mode when the input voltage

is between 1.4V and 10.1V. The device enters PD clas-

sification mode when the input voltage is between 12.6V

and 20V. The device enters PD power mode once the

In detection mode, the power source equipment (PSE)

range (1V step minimum) and then records the current

measurements at the two points. The PSE then computes

DV/DI to ensure the presence of the 24.9kI signature

to DET for proper signature detection. The MAX5981

pulls DET low in detection mode. DET goes high imped-

ance when the input voltage exceeds 12.5V. In detection

mode, most of the MAX5981 internal circuitry is off and

the offset current is less than 10µA.

If the voltage applied to the PD is reversed, install pro-

tection diodes at the input terminal to prevent internal

damage to the MAX5981 (see the Typical Application

Circuit). Since the PSE uses a slope technique (DV/DI) to

calculate the signature resistance, the DC offset due to

the protection diodes is subtracted and does not affect

the detection process.

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 distribu-

tion. Class 0–5 is defined as shown in Table 1. (The IEEE

802.3af/at standard defines only Class 0–4 and Class 5

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

age at the PD input and measuring the current sourced

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

12.6V and 20V, the MAX5981A/MAX5981B exhibit a cur-

rent characteristic with a value shown in Table 1. The

PSE uses the classification current information to classify

the power requirement of the PD. The classification cur-

current of the MAX5981A/MAX5981B so the total current

drawn by the PD is within the IEEE 802.3af/at standard

figures. The classification current is turned off whenever

the device is in power mode.

2-Event Classification and Detection

During 2-Event classification, a Type 2 PSE probes PD

for classification twice. In the first classification event,

the PSE presents an input voltage between 12.6V and

20.5V and the MAX5981A/MAX5981B present the pro-

voltage below the mark event threshold of 10.1V and

the MAX5981A/MAX5981B present the mark current

When the MAX5981A/MAX5981B are powered by a Type

2 PSE, the 2-Event identification output 2EC asserts low

after the internal isolation n-channel MOSFET is fully

Table 1. Setting Classification Current

CLASS

MAXIMUM

POWER USED

BY PD

(W)

(I)

(V)

CLASS CURRENT SEEN AT

IEEE 802.3at PD

CLASSIFICATION CURRENT

SPECIFICATION (mA)

MIN

MAX

MIN

MAX

0

0.44 to 12.95

615

12.6 to 20

0

4

0

5

1

0.44 to 3.84

117

12.6 to 20

9

12

8

13

2

3.84 to 6.49

66.5

12.6 to 20

17

20

16

21

3

6.49 to 12.95

43.7

12.6 to 20

26

30

25

31

4

12.95 to 25.5

30.9

12.6 to 20

36

44

35

45

5

> 25.5

21.3

12.6 to 20

54

64

51

68