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:

PGOOD/PGOOD Outputs

(MAX5940A/MAX5940B)

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 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

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.

II

x

C

C

INRUSH

G

OUT

GATE

=

IEEE 802.3af PD Interface Controller

For Power-Over-Ethernet

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