LTC4263
18
4263fe
APPLICATIONS INFORMATION
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 reduction 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
occurs 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
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
suppressor 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
application. There is a potential for stress induced leakage,
so suffi cient margins should be used when selecting
transient suppressors for these applications.
Capacitors
Sizing of both the C
DET
and C
PSE
capacitors is critical for
proper operation of the LTC4263 AC disconnect sensing.
See the AC Disconnect section for more information. Note
that many ceramic capacitors have dramatic DC voltage
and temperature coeffi cients. Use 100V or higher rated
X7R capacitors for C
DET
and C
PSE
, as these have reduced
voltage dependence while also being relatively small and
inexpensive. Bypass the 48V supply with a 0.1μF, 100V
capacitor located close to the LTC4263. The V
DD5
supply
also requires a 0.1μF bypass capacitor.
