Inrush Current Limit

The MAX5940_ charge the gate of the internal MOSFET

with a constant current source (10µA, typ). The drain-

to-gate capacitance of the MOSFET limits 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 following equation to

calculate the inrush current:

PGOOD/

PGOOD Outputs

(MAX5940A/MAX5940C only)

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

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

stream DC-DC converter. Connect a 100k

Ω pullup resis-

tor from PGOOD to GND if needed.

(MAX5940B/MAX5940D only)

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

goes high impedance. Connect PGOOD to the ON pin of

a downstream DC-DC converter. Connect a 100k

Ω

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

rent of 42mA flows through the MAX5940_, then the maxi-

mum 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 limited to 75ms (max). The

MAX5940_ handle the maximum classification power dis-

sipation 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.

II

x

C

C

INRUSH

G

OUT

GATE

=

IEEE 802.3af PD Interface Controller

For Power-Over-Ethernet

_______________________________________________________________________________________

9