ltc4263cde-trpbf Linear Technology Corporation, ltc4263cde-trpbf Datasheet - Page 19

• Manufacturer Part Number: ltc4263cde-trpbf
• Description: Single Ieee 802.3af Compliant Pse Controller With Internal Switch
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Capacitors
Sizing of both the C
proper operation of the LTC4263 AC disconnect sensing.
See the AC Disconnect section for more information. Note
that many ceramic capacitors have dramatic DC voltage
and temperature coeffi cients. Use 100V or higher rated
X7R capacitors for C
voltage dependence while also being relatively small and
inexpensive. Bypass the 48V supply with a 0.1μF , 100V
capacitor located close to the LTC4263. The V
also requires a 0.1μF bypass capacitor.
Fuse
While the LTC4263 does not require a fuse for proper
operation or for compliance with IEEE 802.3af, some
safety requirements state that the output current must be
limited to less than 2A in less than 60 seconds if any one
component fails or is shorted. Since the LTC4263 is the
primary current limiter, its failure could result in excess
current to the port. To meet these safety requirements, a
fuse can be placed in the positive leg of the port. The fuse
must be large enough that it will pass at least 450mA when
derated for high temperature but small enough that it will
fuse at less than 2A at cold temperature. This requirement
can usually be satisfi ed with a 1A fuse or PTC. Placing the
fuse between the RJ-45 connector and the LTC4263 and
its associated circuitry provides additional protection for
this circuitry. Consult a safety requirements expert for the
application specifi c requirements.
DET
DET
and C
and C
PSE
PSE
capacitors is critical for
, as these have reduced
DD5
supply
Power Supply
Poor regulation on the 48V supply can lead to noncompli-
ance. The IEEE specifi cation requires a PSE output voltage
between 44V and 57V. When the LTC4263 begins powering
an Ethernet port, it controls the current through the port to
minimize disturbances on V
is underdamped or otherwise unstable, its voltage could
go outside of the IEEE-specifi ed limits, causing the PSE
to be noncompliant. This scenario can be even worse
when a PD is unplugged because the current can drop
immediately to zero. In both cases the port voltage must
always stay between 44V and 57V. Beyond this, the IEEE
802.3af specifi cation places specifi c ripple, noise and load
regulation requirements on the PSE. Disturbances on V
can also adversely affect detection, classifi cation and AC
disconnect sensing. For these reasons, proper bypassing
and stability of the V
Another problem that can affect the V
fi cient power, leading to the supply voltage dropping out
of the specifi ed range. The 802.3af specifi cation states
that if a PSE powers a PD it must be able to provide the
maximum power level requested by the PD based on the
PD’s classifi cation. The specifi cation does allow a PSE
to choose not to power a port, typically because the PD
requires more power than the PSE has available to deliver.
If a PSE is built with a V
ing full power to all ports, it is recommended to use the
LTC4263 power management feature to prevent ports
from being turned on when there is insuffi cient power.
Because the specifi cation also requires the PSE to sup-
ply an inrush current of 400mA at up to a 5% duty cycle,
the V
higher than the maximum total power the PSE needs to
supply to the PDs.
SS
supply capability should be at least a few percent
SS
supply is important.
SS
supply not capable of deliver-
SS
. However, if the V
SS
LTC4263
supply is insuf-
SS
19
supply
4263fc
SS