ltc4264cde-trpbf Linear Technology Corporation, ltc4264cde-trpbf Datasheet - Page 18

• Manufacturer Part Number: ltc4264cde-trpbf
• Description: High Power Pd Interface Controller With 750ma Current Limit
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
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 LTC4264 turn-on UVLO requirement and can
be selected based solely on the PD load requirements. It
is necessary to include diode D9 to prevent the adapter
from applying power to the LTC4264. Power priority is-
sues require more intervention. If the adapter voltage is
below the PSE voltage, then the priority will be given to the
PSE power. The PD will draw power from the PSE while
the adapter will remain unused. This confi guration is ac-
ceptable in a typical PoE system. However, if the adapter
voltage is higher than the PSE voltage, the PD will draw
power from the adapter. In this situation, it is necessary
to address the issue of power cycling that may occur if
a PSE is present. The PSE will detect the PD and apply
power. If the PD is being powered by the adapter, then
the PD will not meet the minimum load requirement and
the PSE may subsequently remove power. The PSE will
again detect the PD and power cycling will start. With an
adapter voltage above the PSE voltage, it is necessary to
either disable the signature as shown in option 2, or install
a minimum load on the output of the LTC4264 to prevent
power cycling. A 3k, 1W resistor connected between GND
and V
Option 3 applies power directly to the DC/DC converter
bypassing the LTC4264 and omitting diode D9. With the
diode omitted, the adapter voltage is applied to the LTC4264
in addition to the DC/DC converter. For this reason, it is
necessary to ensure that the adapter maintain the voltage
between 42V and 57V to keep the LTC4264 in its normal
operating range. The third option has the advantage of
corrupting the 25k signature resistance when the external
voltage exceeds the PSE voltage and thereby solving the
power priority issue.
LTC4264
18
OUT
will present the required minimum load.
Option 4 bypasses the entire PD interface and injects
power at the output of the low voltage power supply. If
the adapter output is below the low voltage output there
are no power priority issues. However, if the adapter is
above the internal supply, then option 4 suffers from the
same power priority issues as option 2 and the signature
should be disabled or a minimum load should be installed.
Shown in option 4 is one method to disable to the signature
while maintaining isolation.
If employing options 1 through 3, it is necessary to ensure
that the end-user cannot access the terminals of the aux-
iliary power jack on the PD since this would compromise
IEEE 802.3af isolation requirements and may violate local
safety codes. Using option 4 along with an isolated power
supply addresses the isolation issue and it is no longer
necessary to protect the end-user from the power jack.
The above power cycling scenarios have assumed the
PSE is using DC disconnect methods. For a PSE using
AC disconnect, a PD with less than minimum load will
continue to be powered.
Wall adapters have been known to generate voltage spikes
outside their expected operating range. Care should be
taken to ensure no damage occurs to the LTC4264 or any
support circuitry from extraneous spikes at the auxiliary
power interface.
Classifi cation Resistor Selection (R
The IEEE 802.3af specifi cation allows classifying PDs
into four distinct classes with class 4 being reserved
for future use (Table 2). The LTC4264 supports all IEEE
classes and implements an additional Class 5 for use in
custom PoE applications. An external resistor connected
from R
current. The designer should determine which class the
PD is to advertise and then select the appropriate value of
CLASS
to V
IN
(Figure 6) sets the value of the load
CLASS
)
4264f