MAX5972AETE+T【PDF 8/13 页】IC CONTROLLER INTERFACE 16TQFN

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IEEE 802.3af/at-Compliant, Powered Device Interface

Controller with Integrated Power MOSFET

8 ______________________________________________________________________________________

Typical Operating Circuit

Detailed Description

Operating Modes

Depending on the input voltage (V

= V

- V

), the

MAX5972A operates in four different modes: PD detec-

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

device enters 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

input voltage exceeds V

Detection Mode (1.4V d V

d 10.1V)

In detection mode, the power source equipment (PSE)

applies two voltages on V

in the range of 1.4V to 10.1V

(1V step minimum) and then records the current measure-

ments at the two points. The PSE then computes DV/DI

to ensure the presence of the 24.9k?signature resistor.

Connect the signature resistor (R

DET

) from V

to DET for

proper signature detection. The MAX5972A pulls DET low

in detection mode. DET goes high impedance when the

input voltage exceeds 12.5V. In detection mode, most of

the MAX5972A internal circuitry is off and the offset cur-

rent is less than 10礎.

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

tection diodes at the input terminal to prevent internal

damage to the MAX5972A (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.

Classification Mode (12.6V d V

d 20V)

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 distribution.

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

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

for any special requirement.) An external resistor (R

CLS

)

connected from CLS to V

sets the classification current.

2-EVENT

CLASSIFICATION

DETECTION

-54V

MAX5972A

RTN

WAD

2EC/WAD

2EC

24V/48V

BATTERY

1.5mA

68nF

RJ-45

AND

BRIDGE

RECTIFIER

GND

SMAJ58A

DET

24.9kI

CLS

DET

CLS

ENABLE

DC-DC

CONVERTER

IN+

IN-

GND

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