LTC4264
16
4264f
Diode Bridge
IEEE 802.3af allows power wiring in either of two confi gu-
rations on the TX/RX wires, and power can be applied to
the PD via the spare wire pair in the RJ45 connector. The
PD is required to accept power in either polarity on both
the data and spare inputs; therefore it is common to install
diode bridges on both inputs in order to accommodate
the different wiring confi gurations. Figure 9 demonstrates
an implementation of the diode bridges. The IEEE 802.3af
specifi cation also mandates that the leakage back through
the unused bridge be less than 28µA when the PD is
powered with 57V.
The PD may be confi gured to handle 2-pair or 4-pair
power delivery over the Ethernet cable. In a 2-pair power
delivery system, one of the two pairs is delivering power
to the PD—either the main pair or the spare pair, but not
both. In a 4-pair system, both the main and spare pairs
deliver power to the PD simultaneously (see Figures 1
and 2). In either case, a diode bridge is needed on the
front end to accept power in either polarity. Contact LTC
applications for more information about implementing a
4-pair PoE system.
The IEEE standard includes an AC impedance requirement
in order to implement the AC disconnect function. Capaci-
tor C14 in Figure 9 is used to meet this AC impedance
requirement. A 0.1µF capacitor is recommended for this
application.
The LTC4264 has several different modes of operation
based on the voltage present between the V
IN
and GND
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
LTC4264, 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 802.3af
specifi cations.
The input diode bridge of a PD can consume over 4% of
the available power in some applications. Schottky diodes
can be used in order to reduce power loss. The LTC4264 is
designed to work with both standard and Schottky diode
APPLICATIONS INFORMATION
bridges while maintaining proper threshold points for IEEE
802.3af compliance.
Figure 4 shows how two diode bridges are typically con-
nected in a PD application. One bridge is dedicated to the
data pair while the second bridge is dedicated to the spare
pair. For high power applications, a diode bridge typically
used in an IEEE 802.3af system cannot handle the higher
currents because the operating current is derated at the
upper temperature range. To solve this problem, the PD
application can utilize larger diode bridges, use discrete
diodes or consider the following alternative option.
Realizing that the two diode bridges do not need to be
exclusive to the data and spare pairs, the bridges may
be reconnected so that current is shared between them.
The new confi guration extends the maximum operating
current while maintaining the smaller package profi les.
Figure 9 shows an example of how the two diode bridges
may be reconnected. Consult the diode bridge vendors for
operating current derating curves when only one of four
diodes is in operation.
Auxiliary Power Source
In some applications, it may be necessary to power the PD
from an auxiliary power source such as a wall adapter. The
auxiliary power can be injected into the PD at several loca-
tions and various trade-offs exist. Figure 10 demonstrates
four methods of connecting external power to a PD.
Option 1 in Figure 10 inserts power before the LTC4264
interface controller. In this confi guration, it is necessary
for the wall adapter to exceed the LTC4264 UVLO turn-
on requirement. This option provides input current limit
for the adapter, provides a valid power good signal and
simplifi es power priority issues. As long as the adapter
applies power to the PD before the PSE, it will take priority
and the PSE will not power up the PD because the external
power source will corrupt the 25k signature. If the PSE
is already powering the PD, the adapter power will be in
parallel with the PSE. In this case, priority will be given to
the higher supply voltage. If the adapter voltage is higher,
the PSE should remove the port voltage since no current
