LTC4263
19
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 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.
Power Supply
Poor regulation on the 48V supply can lead to
noncompliance. 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
SS
. However,
if the V
SS
supply 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
SS
can also adversely affect detection, classifi cation
and AC disconnect sensing. For these reasons, proper
bypassing and stability of the V
SS
supply is important.
Another problem that can affect the V
SS
supply is
insuffi 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
SS
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 insuffi cient
power. Because the specifi cation also requires the PSE
to supply an inrush current of 400mA at up to a 5% duty
cycle, the V
SS
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.
