LTC4263CS datasheet(19/24 Pages) LINER

19 / 24 page

LTC4263

4263fe

APPLICATIONS INFORMATION

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

noncompliance. 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

insufﬁcient 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

delivering 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 supply 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.

Isolation

The IEEE 802.3af standard requires Ethernet ports to be

electrically isolated from all other conductors that are

user accessible. This includes the metal chassis, other

connectors, and the AC power line. Environment A isolation

is the most common and applies to wiring within a single

building serviced by a single AC power system. For this

type of application, the PSE isolation requirement can be

met with the use of a single, isolated 48V supply powering

several LTC4263 ports. Environment B, the stricter isolation

requirement, is for networks that cross an AC power

distribution boundary. In this case, electrical isolation

must be maintained between each port in the PSE. The

LTC4263 can be used to build a multi-port Environment B

PSE by powering each LTC4263 from a separate, isolated

48V supply. In all PSE applications, there should be no

user accessible connections to the LTC4263 other than

the RJ-45 port.

LTC4263CS Datasheet(PDF) 19 Page - Linear Technology

LTC4263CS Datasheet(HTML) 19 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL