LTC4258 LINER [Linear Technology], LTC4258 Datasheet - Page 26

• Manufacturer Part Number: LTC4258
• Description: Quad IEEE 802.3af Power over Ethernet Controller with Integrated Detection
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC4258
Power Supplies
The LTC4258 must be supplied with 3.3V (V
–48V (V
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
ever, if the V
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
or V
AC disconnection sensing. Proper bypassing and stability
of the V
26
EE
can adversely affect detection, classification and the
DD
EE
). Poor regulation on either of these supplies
and V
EE
EE
supply is underdamped or otherwise
U
supplies is important.
U
W
U
EE
DD
. How-
) and
DD
Another problem that can affect the V
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
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
few percent more than the maximum total power the PSE
will supply to PDs. Finally, the LTC4258s draw current
from V
power divided by the switcher efficiency must also be
added to the V
EE
supply capable of less than 15.4W • (number of
EE
. If the V
EE
EE
supply’s capability.
DD
supply capability should be at least a
supply is generated from V
EE
supply is insuffi-
EE
, that
4258p