Datasheet

LTC4267-1
17
42671fa
For more information www.linear.com/4267-1
applicaTions inForMaTion
Diode Bridge
IEEE 802.3af allows power wiring in either of two configu-
rations: on the TX/RX wires or via the spare wire pairs in
the RJ45 connector. The PD is required to accept power in
either polarity on either the main or spare inputs; therefore
it is common to install diode bridges on both inputs in
order to accommodate the different wiring configurations.
Figure 9 demonstrates an implementation of these diode
bridges. The IEEE 802.3af specification also mandates
that the leakage back through the unused bridge be less
than 28µA when the PD is powered with 57V.
The LTC4267-1 has several different modes of operation
based on the voltage present between V
PORTN
and V
PORTP
pins. The forward voltage drop of the input diodes in a
PD design subtracts from the input voltage and will af-
fect the transition point between modes. When using the
LTC4267-1, it is necessary to pay close attention to this
forward voltage drop. Selection of oversized diodes will help
keep the PD thresholds from exceeding IEEE specifications.
The input diode bridge of a PD can consume over 4% of
the available power in some applications. It may be de-
sirable to use Schottky
diodes in order to reduce power
loss. However, if the standard diode bridge is replaced
with a Schottky bridge, the transition points between the
modes will be affected. Figure 10 shows a technique for
using Schottky diodes while maintaining proper threshold
points to meet IEEE 802.3af compliance. D13 is added
to compensate for the change in UVLO turn-on voltage
caused by the Schottky diodes and consumes little power.
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 9) is used to
meet the AC impedance requirement.
Input Series Resistance
Linear Technology has seen the customer community cable
discharge requirements increase by nearly 500,000 times
the original test levels. The PD must survive and operate
reliably not only when an initially charged cable connects
and dissipates the energy through the PD front end, but
also when the electrical power system grounds are subject
to very high energy events (e.g. lightning strikes).
In these high energy events, adding 10Ω series resistance
into the V
PORTP
pin greatly improves the robustness of
the LTC4267-1 based PD. (See Figure 9.) The TVS limits
the voltage across the port while the 10Ω
and
0.1µF ca-
pacitance reduces the edge rate the LT4267-1 encounters
across its pin. The added 10Ω series resistance does not
operationally affect the LTC4267-1 PD Interface nor does
it affect its compliance with the IEEE802.3 standard.
Figure 9. PD Front End with Isolation Transformer, Diode Bridges and Capacitor
16
14
15
1
3
2
RX
–
6
RX
+
3
TX
–
2
TX
+
RJ45
T1
PULSE H2019
42671 F09
1
7
8
5
4
11
9
10
6
8
7
D3
SMAJ58A
TVS
BR1
HD01
BR2
HD01
TO PHY
SPARE
–
SPARE
+
C14
0.1µF
100V
LTC4267-1
10Ω
V
PORTN
V
PORTP