Datasheet

LTC4278
19
4278fc
An input diode bridge must be rated above the maximum
current the PD application will encounter at the tempera-
ture the PD will operate. Diode bridge vendors typically
call out the operating current at room temperature, but
derate the maximum current with increasing temperature.
Consult the diode bridge vendors for the operating current
derating curve.
A silicon diode bridge can consume over 4% of the available
power in some PD applications. Using Schottky diodes can
help reduce the power loss with a lower forward voltage.
A Schottky bridge may not be suitable for some high
temperature PD application. The leakage current has a
voltage dependency that can reduce the perceived signature
resistance. In addition, the IEEE 802.3af/at specification
mandates the leakage back-feeding through the unused
bridge cannot generate more than 2.8V across a 100k
resistor when a PD is powered with 57V.
Sharing Input Diode Bridges
At higher temperatures, a PD design may be forced to
consider larger bridges in a bigger package because the
maximum operating current for the input diode bridge is
drastically derated. The larger package may not be accept-
able in some space-limited environments.
One solution to consider is to reconnect the diode bridges
so that only one of the four diodes conducts current in
each package. This configuration extends the maximum
operating current while maintaining a smaller package
profile. Figure 7 shows how to reconnect the two diode
bridges. Consult the diode bridge vendors for the derating
curve when only one of four diodes is in operation.
Input Capacitor
The IEEE 802.3af/at standard includes an impedance
requirement in order to implement the AC disconnect
function. A 0.1µF capacitor (C14 in Figure 7) is used to
meet this AC impedance requirement.
Transient Voltage Suppressor
The LTC4278 specifies an absolute maximum voltage of
100V and is designed to tolerate brief overvoltage events.
However, the pins that interface to the outside world can
routinely see excessive peak voltages. To protect the
LTC4278, install a transient voltage suppressor (D3) be-
tween the input diode bridge and the LTC4278 as shown
in Figure 7.
APPLICATIONS INFORMATION
Figure 7. PD Front-End with Isolation Transformer, Diode Bridges,
Capacitors, and a Transient Voltage Suppressor (TVS)
14
13
12
1
2
3
RX
–
6
RX
+
3
TX
–
2
TX
+
RJ45
T1
COILCRAFT
ETHI - 230LD
4278 F07
1
7
8
5
4
10
9
11
5
6
4
D3
SMAJ58A
TVS
B1100
BR1
HD01
BR2
HD01
TO PHY
V
PORTP
LTC4278
C1
V
PORTN
V
NEG
SPARE
–
SPARE
+
C14
0.1µF
100V