LTC4258 datasheet(26/28 Pages) LINER

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LTC4258

4258p

Power Supplies

The LTC4258 must be supplied with 3.3V (VDD) and

–48V (VEE). 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 VEE. How-

ever, if the VEE 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 VDD

or VEE can adversely affect detection, classification and the

AC disconnection sensing. Proper bypassing and stability

of the VDD and VEE supplies is important.

APPLICATIO S I FOR ATIO

Another problem that can affect the VEE 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 VEE 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 VEE 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 VEE. If the VDD supply is generated from VEE, that

power divided by the switcher efficiency must also be

added to the VEE supply’s capability.

LTC4258 Datasheet(PDF) 26 Page - Linear Technology