LTC4259A datasheet(30/32 Pages) LINER

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LTC4259A

4259Af

APPLICATIO S I FOR ATIO

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 LTC4259As 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.

OSCIN Input

AC disconnect also relies on an oscillating signal applied

to the OSCIN pin. Requirements for this signal are pro-

vided in the OSCIN Input and Oscillator Requirements

section. Out-of-band noise on the OSCIN pin will disrupt

the LTC4259A’s ability to sense the absence of a PD. Any

noise present at the OSCIN pin is amplified by the LTC4259A

and driven out of the DETECT pins (of powered ports with

AC disconnect enabled). Due to the amount of capacitance

connected to the DETECT pins, driving this noise can

easily require more than IACDMIN, tripping the DETECT pin

current sense and keeping the port powered. During

circuit board layout, keep wiring from the oscillator to the

OSCIN pin away from noise sources like digital clock and

data lines. A single-stage RC lowpass filter (shown in

Figure 20) will attenuate out-of-band noise.

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