LTC4269-1 Linear Technology Corporation, LTC4269-1 Datasheet - Page 19

• Manufacturer Part Number: LTC4269-1
• Description: IEEE 802.3 At PD And Synchronous No-Opto Flyback Controller
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
Input Diode Bridge
Figure 2 shows how two diode bridges are typically con-
nected in a PD application. One bridge is dedicated to the
data pair while the other bridge is dedicated to the spare
pair. The LTC4269-1 supports the use of either silicon or
Schottky input diode bridges. However, there are tradeoffs
in the choice of diode bridges.
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 specifi cation
mandates the leakage back-feeding through the unused
RJ45
1
2
3
6
4
5
7
8
TX
TX
RX
RX
+
–
+
–
Figure 7. PD Front-End with Isolation Transformer, Diode Bridges,
Capacitors, and a Transient Voltage Suppressor (TVS).
SPARE
SPARE
+
–
ETHI - 230LD
14
12
13
10
11
COILCRAFT
9
T1
1
3
2
5
4
6
BR2
HD01
TO PHY
C14
0.1μF
100V
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 confi guration extends the maximum
operating current while maintaining a smaller package
profi le. 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.
SMAJ58A
BR1
HD01
TVS
D3
V
PORTN
LTC4269-1
V
PORTP
V
42691 F07
NEG
C1
LTC4269-1
19
42691f