IEEE 802.3af PD Interface Controller
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 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.
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.
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 cur-
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.