LTC4258
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APPLICATIO S I FOR ATIO
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Common Mode Chokes
Both nonpowered and powered Ethernet connections
achieve best performance (for data transfer, power trans-
fer and EMI) when a common mode choke is used on each
port. In the name of cost reduction, some designs share a
common mode choke between two adjacent ports. Even
for nonpowered Ethernet, sharing a choke is not recom-
mended. With two ports passing through the choke, it
cannot limit the common mode current of either port.
Instead, the choke only controls the sum of both ports’
common mode current. Because cabling from the ports
generally connects to different devices up to 200m apart,
a current loop can form. In such a loop, common mode
current flows in one port and out the other, and the choke
will not prevent this because the sum of the currents is
zero. Another way to view this interaction between the
paired ports is that the choke acts as a transformer
coupling the ports’ common modes together. In
nonpowered Ethernet, common mode current results
from nonidealities like ground loops; it is not part of
normal operation. However, Power over Ethernet sends
power and hence significant current through the ports;
common mode current is a byproduct of normal opera-
tion. As described in the Choosing External MOSFETs
section and under the Power Supplies heading below,
large transients can occur when a port’s power is turned
on or off. When a powered port is shorted (see Surge
Suppressors and Circuit Protection), a port’s common
mode current may be excessive. Sharing a common mode
choke between two ports couples start-up, disconnect
and fault transients from one port to the other. The end
result can range from momentary noncompliance with
802.3af to intermittent behavior and even to excessive
voltages that may damage circuitry (in both the PSE and
PD) connected to the ports.
Detect Pin Diodes
During detection and classification, the LTC4258 senses
the port voltage through the detect diodes D
DET
. Excessive
voltage drop across D
DET
will corrupt the LTC4258’s
detect and classification results. Select a diode for D
DET
that will have less than 0.7V of forward drop at 0.4mA and
less than 0.9V of forward drop at 50mA.
Power Supplies
The LTC4258 must be supplied with 3.3V (V
DD
) and
–48V (V
EE
). Poor regulation on either of these supplies
can lead to noncompliance. The IEEE requires a PSE
output voltage between 44V and 57V. When the LTC4258
begins powering an Ethernet port, it controls the current
through the port to minimize disturbances on V
EE
. How-
ever, if the V
EE
supply is underdamped or otherwise
unstable, its voltage could go outside of the IEEE specified
limits, causing all ports in 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. In addition, the 802.3af specification places
specific ripple, noise and load regulation requirements on
the PSE. Among other things, disturbances on either V
DD
or V
EE
can adversely affect detection and classification
sensing. Proper bypassing and stability of the V
DD
and V
EE
supplies is important.
Another problem that can affect the V
EE
supply is insuffi-
cient power, leading to the supply voltage drooping 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 because the PD requires more
power than the PSE has left to deliver. If a PSE is built with
a V
EE
supply capable of less than 15.4W • (number of
PSE’s Ethernet ports), it must implement a power alloca-
tion algorithm that prevents ports from being powered
when there is insufficient power. Because the specifica-
tion also requires the PSE to supply 400mA at up to a 5%
duty cycle, the V
EE
supply capability should be at least a
few percent more than the maximum total power the PSE
will supply to PDs. Finally, the LTC4258s draw current
from V
EE
. If the V
DD
supply is generated from V
EE
, that
power divided by the switcher efficiency must also be
added to the V
EE
supply’s capability.
Fast VEE transients can damage the LTC4258. Limit the
VEE slew rate to 50mV/µs. In most applications, existing
VEE bypass capacitors will cause the VEE supply to slew
much slower than 50mV/µs.
