LT4293
12
Rev. 0
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Overtemperature Protection
The IEEE 802.3 specification requires a PD to withstand
any applied voltage from 0V to 57V indefinitely. During
classification, however, the power dissipation in the
LT4293 may be as high as 1.5W. The LT4293 can easily
tolerate this power for the maximum IEEE classification
timing but overheats if this condition persists abnormally.
The LT4293 includes an overtemperature protection
feature which is intended to protect the device during
momentary overload conditions. If the junction tempera-
ture exceeds the overtemperature threshold, the LT4293
pulls down HSGATE pin, and disables classification.
EXTERNAL INTERFACE AND COMPONENT SELECTION
PoE Input Bridge
A PD is required to polarity-correct its input voltage. There
are several different options available for bridge rectifiers;
silicon diodes, Schottky diodes, and ideal diodes. When
silicon or Schottky diode bridges are used, the diode for-
ward voltage drops affect the voltage at the VPORT pin.
The LT4293 is designed to tolerate these voltage drops.
Note, the voltage parameters shown in the Electrical
Characteristics are specified at the LT4293 package pins.
A silicon diode bridge consumes up to 4% of the avail-
able power. In addition, silicon diode bridges exhibit poor
pairset-to-pairset unbalance performance. Each branch of
a silicon diode bridge shares source/return current, and
thermal runaway can cause large, non-compliant current
unbalances between pairsets.
While using Schottky diodes can help reduce the power
loss with a lower forward voltage, the Schottky bridge
may not be suitable for high temperature PD applications.
Schottky diode bridges exhibit temperature induced leak-
age currents. The leakage current has a voltage depen-
dency that can invalidate the measured detection signa-
ture. In addition, these leakage currents can back-feed
through the unpowered branch and the unused bridge,
violating IEEE 802.3 specifications.
For high efficiency applications, the LT4293 supports an
LT4321-based PoE ideal diode bridge that reduces the
forward voltage drop from 0.7V to 20mV per diode while
maintaining IEEE 802.3 compliance. The LT4321 simpli-
fies thermal design, eliminates costly heatsinks, and can
operate in space-constrained applications.
Auxiliary Input Bridge
Some PDs are required to receive AC or DC power from an
auxiliary power source. A diode bridge is typically required
to handle the voltage rectification and polaritycorrection.
In high efficiency applications, or in low auxiliary input
voltage applications, the voltage drop across the rectifier
cannot be tolerated. The LT4293 can be configured with
an LT4320-based ideal diode bridge to recover the diode
voltage drop and ease thermal design.
For applications with auxiliary input voltages below
10V, the LT4293 must be configured with an LT4320-
based ideal diode bridge to recover the voltage drop and
guarantee the minimum VPORT voltage is within the
VPORT AUX Mode Range as specified in the Electrical
Characteristicstable.
Input Capacitor
A 0.1μF capacitor is needed from VPORT to GND to meet
the input impedance requirement in IEEE 802.3 and to
properly bypass the LT4293. When operating with the
LT4321, locally bypass each with a 0.047μF capacitor, thus
keeping the total port capacitance within specification.
Transient Voltage Suppressor
The LT4293 specifies an absolute maximum voltage of
100V and is designed to tolerate brief overvoltage events
due to Ethernet cable surges. To protect the LT4293 from
an overvoltage event, install a unidirectional transient
voltage suppressor (TVS) such as an SMAJ58A between
the VPORT and GND pins. For PD applications that require
APPLICATIONS INFORMATION