LTC4263CS-PBF datasheet(19/24 Pages) LINER

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LTC4263

4263fd

APPLICATIONS INFORMATION

Capacitors

Sizing of both the CDET and CPSE capacitors is critical for

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 coefﬁcients. Use 100V or higher rated

X7R capacitors for CDET and CPSE, as these have reduced

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 VDD5 supply

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 satisﬁed 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 speciﬁc requirements.

Power Supply

Poor regulation on the 48V supply can lead to noncompli-

ance. The IEEE speciﬁcation 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 VSS. However, if the VSS supply

is underdamped or otherwise unstable, its voltage could

go outside of the IEEE-speciﬁed 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 speciﬁcation places speciﬁc ripple, noise and load

regulation requirements on the PSE. Disturbances on VSS

can also adversely affect detection, classiﬁcation and AC

disconnect sensing. For these reasons, proper bypassing

and stability of the VSS supply is important.

Another problem that can affect the VSS supply is insuf-

ﬁcient power, leading to the supply voltage dropping out

of the speciﬁed range. The 802.3af speciﬁcation 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 classiﬁcation. The speciﬁcation 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 VSS supply not capable of deliver-

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 insufﬁcient power.

Because the speciﬁcation also requires the PSE to sup-

ply an inrush current of 400mA at up to a 5% duty cycle,

the VSS supply capability should be at least a few percent

higher than the maximum total power the PSE needs to

supply to the PDs.

LTC4263CS-PBF Datasheet(PDF) 19 Page - Linear Technology

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