IEEE 802.3af Power-Over-Ethernet

Interface/PWM Controller for Power Devices

______________________________________________________________________________________

11

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.

Detailed Description

The MAX5942A/MAX5942B integrate a complete power

IC for powered devices (PD) in a power-over-ethernet

(PoE) system. The MAX5942A/MAX5942B provide PD

Interface and a compact DC-DC PWM controller suitable

for flyback and forward converters in either isolated or

nonisolated designs.

The MAX5942A/MAX5942B PD interface complies with

the IEEE 802.3af standard, providing the PD with a

detection signature, a classification signature, and an

integrated isolation switch with programmable inrush

current control. These devices also feature power-mode

undervoltage lockout (UVLO) with wide hysteresis, and

power-good status outputs.

An integrated MOSFET provides PD isolation during

detection and classification. The MAX5942A/MAX5942B

guarantee a leakage current offset of less than 10µA dur-

ing the detection phase. A programmable current limit

prevents high inrush current during power-on. The

devices feature power-mode UVLO with wide hysteresis

and long deglitch time to compensate for twisted-pair

cable resistive drop and to ensure glitch-free transition

between detection, classification, and power-on/off

phases. The MAX5942A/MAX5942B provide both active-

high (PGOOD) and active-low (PGOOD) outputs. Both

devices offer an adjustable UVLO threshold with a

default value compliant to the IEEE 802.3af standard.

The MAX5942A/MAX5942B are designed to work with or

without an external diode bridge in front of the PD.

Use the MAX5942A/MAX5942B PWM current-mode con-

trollers to design flyback- or forward-mode power sup-

plies. Current-mode operation simplifies control-loop

design while enhancing loop stability. An internal high-

voltage startup regulator allows the device to connect

directly to the input supply without an external startup

resistor. Current from the internal regulator starts the con-

troller. Once the tertiary winding voltage is established,

the internal regulator is switched off and bias current

for running the PWM controller is derived from the

tertiary winding. The internal oscillator is set to 275kHz

and trimmed to ±10%. This permits the use of small

magnetic components to minimize board space. Both the

MAX5942A and MAX5942B can be used in power sup-

plies providing multiple output voltages. A functional dia-

gram of the PWM controller is shown in Figure 4. Typical

application circuits for forward and flyback topologies are

shown in Figure 5 and Figure 6, respectively.

Powered Device Interface

Operating Modes

The PD front-end section of the MAX5942A/MAX5942B

operates in three different modes: PD detection signa-

ture, PD classification, and PD power, depending on its

are designed to operate with or without the optional

diode bridge while still complying with the IEEE 802.3af

standard (see Application Circuit 1).

Detection Mode (1.4V

In detection mode, the power source equipment (PSE)

10.1V (1V step minimum), and then records the current

measurements at the two points. The PSE then com-

putes

∆V/∆I to ensure the presence of the 25.5kΩ sig-

nature

resistor.

In

this

mode,

most

of

the

MAX5942A/MAX5942B 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 on the input terminal to prevent internal

damage to the MAX5942A/MAX5942B (see Figures 8

and 9). Since the PSE uses a slope technique (

∆V/∆I) 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

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. The IEEE 802.3af standard defines five different