Datasheet

LTC4263-1
17
42631fa
APPLICATIONS INFORMATION
Figure 9. 4-Pair High Power PoE Gigabit Ethernet System Diagram
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 intermittent
behavior to excessive voltages that may damage circuitry
in both the PSE and PD connected to the port.
Transient Suppressor Diode
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 LTC4263-1 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
LTC4263-1 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 LTC4263-1. An
SMAJ58A or equivalent device works well to maintain
port voltages within a safe range. A bidirectional transient
suppressor should not be used.
Good layout practices place the transient voltage sup-
pressor close to the LTC4263-1, before the common
mode choke (if used) and data magnetics to enhance the
protective function.
SMAJ58A
58V
0.1μF
0.1μF
+
–
Tx
Rx
56V
+
–
56V
Rx
Tx
DATA PAIR
DATA PAIR
1
2
1
2
3
6
3
6
PSE
0.1μF
V
SS
OUT
V
DD48
V
DD5
LTC4263-1
SMAJ58A
58V
0.1μF
0.1μF
Tx
Rx
Rx
Tx
DATA PAIR
DATA PAIR
4
5
4
5
7
8
7
8
0.1μF
V
SS
OUT
V
DD48
V
DD5
LTC4263-1
42631
F
SMAJ58A
8s B2100
PD
R
CLASS
V
IN
PWRGD
V
OUT
LTC4264
GND
DC/DC
CONVERTER
5μF
MIN
+
+
+
–
–
–
V
OU
T
0.1μF
SMAJ58A
R
CLASS
V
IN
PWRGD
V
OUT
LTC4264
GND
DC/DC
CONVERTER
5μF
MIN
0.1μF