MAX5940A-MAX5940B Ver la hoja de datos (PDF) - Maxim Integrated

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MAX5940A-MAX5940B

IEEE 802.3af PD Interface Controller For Power-Over-Ethernet

Maxim Integrated 

IEEE 802.3af PD Interface Controller

For Power-Over-Ethernet

Pin Description

PIN

MAX5940A MAX5940B

1, 7

—

NAME

N.C.

FUNCTION

No Connection. Not internally connected.

Undervoltage Lockout Programming Input for Power Mode. When UVLO is above its

threshold, the device enters power mode. Connect UVLO to VEE to use the default

—

1

UVLO

undervoltage lockout threshold. Connect UVLO to an external resistor-divider to define a

threshold externally. The series resistance value of the external resistors must add to 25.5kΩ

(±1%) and replaces the detection resistor. To keep the device in undervoltage lockout, pull

UVLO to between VTH,G,UVLO and VREF,UVLO.

2

2

RCLASS Classification Setting. Add a resistor from RCLASS to VEE to set a PD class (see Tables 1

and 2).

Gate of Internal N-Channel Power MOSFET. GATE sources 10µA when the device enters

3

3

GATE

power mode. Connect an external 100V ceramic capacitor (CGATE) from GATE to OUT to

program the inrush current. Pull GATE to VEE to turn off the internal MOSFET. The detection

and classification functions operate normally when GATE is pulled to VEE.

4

4

VEE

Negative Input Power. Source of the integrated isolation N-channel power MOSFET. Connect

VEE to -48V.

5

5

OUT Output Voltage. Drain of the integrated isolation N-channel power MOSFET.

Power-Good Indicator Output, Active-High, Open-Drain. PGOOD is referenced to OUT.

6

6

PGOOD

PGOOD goes high impedance when VOUT is within 1.2V of VEE and when GATE is 5V above

VEE. Otherwise, PGOOD is pulled to OUT (given that VOUT is at least 5V below GND).

Connect PGOOD to the ON pin of a downstream DC-DC converter.

Power-Good Indicator Output, Active-Low, Open-Drain. PGOOD is referenced to VEE.

—

7

PGOOD

PGOOD is pulled to VEE when VOUT is within 1.2V of VEE and when GATE is 5V above VEE.

Otherwise, PGOOD goes high impedance. Connect PGOOD to the ON pin of a downstream

DC-DC converter.

8

8

GND Ground. GND is the positive input terminal.

Detailed Description

Operating Modes

The PD front-end section of the MAX5940A/MAX5940B

operates in 3 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 without the optional diode

bridge while still complying with the IEEE 802.3af stan-

dard (see Figure 4).

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 MAX5940A/

MAX5940B 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 MAX5940A/MAX5940B (see the Typical

Application Circuits). Since the PSE uses a slope tech-

nique (∆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 ≤ 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 distrib-

ution. The IEEE 802.3af standard defines five different

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

connected from RCLASS to VEE sets the classification

current.

6 _______________________________________________________________________________________

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