Rainbow Electronics MAX5940B User Manual
Page 9
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.
PGOOD goes high impedance when V
OUT
is within 1.2V
of V
EE
and when GATE is 5V above V
EE
. Otherwise,
PGOOD is pulled to V
OUT
(given that V
OUT
is at least 5V
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.
PGOOD is pulled to V
EE
when V
OUT
is within 1.2V of V
EE
and when GATE is 5V above V
EE
. Otherwise, PGOOD
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
to V
RCLASS
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.
I
I
x
C
C
INRUSH
G
OUT
GATE
=
MAX5940A/MAX5940B
IEEE 802.3af PD Interface Controller
For Power-Over-Ethernet
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