MAX5940A(2003) Ver la hoja de datos (PDF) - Maxim Integrated

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Fabricante

MAX5940A
(Rev.:2003)

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

Maxim Integrated 

IEEE 802.3af PD Interface Controller

For Power-Over-Ethernet

Inrush Current Limit

The MAX5940A/MAX5940B charge the gate of the

internal MOSFET with a constant current source (10µA,

typ). The drain-to-gate capacitance of the MOSFET lim-

its the voltage rise rate at the drain, thereby limiting the

inrush current. Add an external capacitor from GATE to

OUT to further reduce the inrush current. Use the fol-

lowing equation to calculate the inrush current:

IINRUSH

=

IG

x

COUT

CGATE

PGOOD/PGOOD Outputs

(MAX5940A/MAX5940B)

PGOOD is an open-drain, active-high logic output.

PGOOD goes high impedance when VOUT is within 1.2V

of VEE and when GATE is 5V above VEE. Otherwise,

PGOOD is pulled to VOUT (given that VOUT is at least 5V

below GND). Connect PGOOD to the ON pin of a down-

stream DC-DC converter. Connect a 100kΩ pullup resis-

tor from PGOOD to GND if needed.

(MAX5940B only)

PGOOD is an open-drain, active-low logic output.

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. Connect a 100kΩ

pullup resistor from PGOOD to GND if needed.

Thermal Dissipation

During classification mode, if the PSE applies the maxi-

mum DC voltage, the maximum voltage drop from GND

to VRCLASS will be 13V. If the maximum classification

current of 42mA flows through the MAX5940A/

MAX5940B, then the maximum DC power dissipation will

be 546mW, which is slightly higher than the maximum

DC power dissipation of the IC at maximum operating

temperature. However, according to the IEEE 802.3af

standard, the duration of the classification mode is limit-

ed to 75ms (max). The MAX5940A/MAX5940B handle

the maximum classification power dissipation for the

maximum duration time without sustaining any internal

damage. If the PSE violates the IEEE 802.3af standard

by exceeding the 75ms maximum classification duration,

it may cause internal damage to the IC.

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