LTC4263 LINER [Linear Technology], LTC4263 Datasheet - Page 18

• Manufacturer Part Number: LTC4263
• Description: Single IEEE 802.3af Compliant PSE Controller with Internal Switch
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC4263
Common Mode Chokes
Both non-powered and powered Ethernet connections
achieve best performance for data transfer and EMI when a
common mode choke is used on each port. For cost reduc-
tion reasons, some designs share a common mode choke
between two adjacent ports. This is not recommended.
Sharing a common mode choke between two ports couples
start-up, disconnect and fault transients from one port
to the other. The end result can range from momentary
noncompliance with IEEE 802.3af to intermittent behavior
and even to excessive voltages that may damage circuitry
in both the PSE and PD connected to the port.
Transient Suppressor Diode
IEEE 802.3af Power over Ethernet is a challenging Hot
Swap™ application because it must survive unintentional
abuse by repeated plugging in and out of devices at the
port. Ethernet cables could potentially be cut or shorted
together. Consequently, the PSE must be designed to
handle these events without damage.
The most severe of these events is a sudden short on a
powered port. What the PSE sees depends on how much
CAT-5 cable is between it and the short. If the short oc-
curs on the far end of a long cable, the cable inductance
will prevent the current in the cable from increasing too
quickly and the LTC4263 built-in short-circuit protection
will control the current and turn off the port. However,
the high current along with the cable inductance causes
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a large fl yback voltage to appear across the port when the
MOSFET is turned off. In the case of a short occurring
with a minimum length cable, the instantaneous current
can be extremely high due to the lower inductance. The
LTC4263 has a high speed fault current limit circuit that
shuts down the port in 20μs (typ). In this case, there is
lower inductance but higher current so the event is still
severe. A transient suppressor is required to clamp the port
voltage and prevent damage to the LTC4263. An SMAJ58A
or equivalent device works well to maintain port voltages
within a safe range. A bidirectional transient suppressor
should not be used. Good board layout places the transient
suppressor between the port and the LTC4263 to enhance
the protective function.
If the port voltage reverses polarity and goes positive, the
OUT pin can be overstressed because this voltage is stacked
on top of the 48V supply. In this case, the transient sup-
pressor must clamp the voltage to a small positive value
to protect the LTC4263 and the PSE capacitor.
Component leakages across the port can have an adverse
affect on AC disconnect and even affect DC disconnect if
the leakage becomes severe. The SMAJ58A is rated at less
than 5μA leakage at 58V and works well in this applica-
tion. There is a potential for stress induced leakage, so
suffi cient margins should be used when selecting transient
suppressors for these applications.
Hot Swap is a trademark of Linear Technology Corporation.
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