ltc4268cdkd-1-trpbf Linear Technology Corporation, ltc4268cdkd-1-trpbf Datasheet - Page 23

• Manufacturer Part Number: ltc4268cdkd-1-trpbf
• Description: High Power Pd With Synchronous Noopto Flyback Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
isolation transformer and proper termination methods.
These vendors have transformers specifi cally designed
for use in high power PD applications.
IEEE 802.3af allows power wiring in either of two
confi gurations 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
to minimize heating. 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 Figure 1). 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. Capacitor
C14 in Figure 9 is used to meet this AC impedance
requirement. A 0.1μF capacitor is recommended for this
application.
The LTC4268-1 has several different modes of operation
based on the voltage present between the V
V
in a PD design subtracts from the input voltage and will
affect the transition point between modes.
PORTP
pins. The forward voltage drop of the input diodes
PORTN
and
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 LTC4268-1
is designed to work with both standard and Schottky
diode bridges while maintaining proper threshold points
for IEEE 802.3af compliance.
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 LTC4268-1
interface controller. In this confi guration, it is necessary
for the wall adapter to exceed the LTC4268-1 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 may remove the port voltage since no current will
be drawn from the PSE. On the other hand, if the adapter
voltage is lower, the PSE will continue to supply power to
the PD and the adapter will not be used. Proper operation
will occur in either scenario.
Option 2 applies power directly to the DC/DC converter.
In this confi guration the adapter voltage does not need to
exceed the LTC4268-1 turn-on UVLO requirement and can
be selected based solely on the PD load requirements. It
LTC4268-1
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