MAX5941BCSE(2003) データシートの表示（PDF） - Maxim Integrated

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MAX5941BCSE
(Rev.:2003)

IEEE 802.3af-Compliant Power-Over-Ethernet Interface/PWM Controller for Power Devices

Maxim Integrated 

IEEE 802.3af-Compliant Power-Over-Ethernet

Interface/PWM Controller for Power Devices

Table 1. PD Power Classification/RCL Selection

CLASS

USAGE

0

Default

1

Optional

2

Optional

3

Optional

4

Not allowed

*Class 4 reserved for future use.

RCL (Ω)

10k

732

392

255

178

MAXIMUM POWER USED BY PD (W)

0.44 to 12.95

0.44 to 3.84

3.84 to 6.49

6.49 to 12.95

Reserved*

Detailed Description

The MAX5941A/MAX5941B integrate a complete power

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

(PoE) system. The MAX5941A/MAX5941B provide PD

interface and a compact DC-DC PWM controller suitable

for flyback and forward converters in either isolated or

nonisolated designs.

The MAX5941A/MAX5941B powered device (PD) inter-

face complies with the IEEE 802.3af standard, providing

the PD with a detection signature, a classification signa-

ture, and an integrated isolation switch with programma-

ble inrush current control. These devices also feature

power-mode undervoltage lockout (UVLO) with wide hys-

teresis, and power-good status outputs.

An integrated MOSFET provides PD isolation during

detection and classification. The MAX5941A/MAX5941B

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 phas-

es. The MAX5941A/MAX5941B 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 MAX5941A/MAX5941B are designed to work with or

without an external diode bridge in front of the PD.

Use the MAX5941A/MAX5941B 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

MAX5941A and MAX5941B 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

applications circuits for forward and flyback topologies

are shown in Figure 5 and Figure 6, respectively.

Powered Device Interface

Operating Modes

The powered device (PD) front-end section of the

MAX5941A/MAX5941B operates in three different modes:

PD detection signature, PD classification, and PD power,

depending on its input voltage (VIN = GND - VEE). All

voltage thresholds are designed to operate with or with-

out the optional diode bridge while still complying with

the IEEE 802.3af standard (see Application Circuit 1).

Detection Mode (1.4V ≤ VIN ≤ 10.1V)

In detection mode, the power source equipment (PSE)

applies two voltages on VIN in the range of 1.4V to

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 MAX5941A/

MAX5941B 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 MAX5941A/MAX5941B (see Figure 7).

Since the PSE uses a slope technique (∆V/∆I) to calcu-

late the signature resistance, the DC offset due to the

protection diodes is subtracted and does not affect the

detection process.

Classification Mode (12.6V ≤ VIN ≤ 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 distribu-

tion. The IEEE 802.3af standard defines five different

classes as shown in Table 1. An external resistor (RCL)

connected from RCL to VEE sets the classification current.

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