LTC4268IDKD-1#PBF【PDF 16/46 页】IC PD HIGH POWER W/CNTRL 32-DFN

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LTC4268-1

42681fc

applicaTions inForMaTion

classification for use in closed systems and is not defined

or supported by the IEEE 802.3af. With the extended clas-

sification range available in the LTC4268-1, it is possible

for system designers to define multiple classes using load

currents between 40mA and 75mA.

During classification, the PSE presents a fixed voltage

between 15.5V and 20.5V to the PD (Figure 5). With the

input voltage in this range, the LTC4268-1 asserts a load

current from the V

PORTP

pin through the R

CLASS

resistor.

The magnitude of the load current is set with the selection

of the R

CLASS

resistor. The resistor value associated with

each class is shown in Table 2.

A substantial amount of power is dissipated in the

LTC4268-1 during classification. The IEEE 802.3af specifi-

cation limits the classification time to 75ms in order avoid

excessive heating. The LTC4268-1 is designed to handle

the power dissipation during the probe period. If the PSE

probing exceeds 75ms, the LTC4268-1 may overheat. In

this situation, the thermal protection circuit will engage

and disable the classification current source, protecting

the LTC4268-1 from damage. When the die cools, clas-

sification is automatically resumed.

Classification presents a challenging stability problem

for the

PSE due to the wide range of loads possible. The

LTC4268-1 has been designed to avoid PSE interoperability

problems by maintaining a positive I-V slope throughout

the signature and classification ranges up to UVLO turn

on as shown in Figure 6. The positive I-V slope avoids

areas of negative resistance and helps prevent the PSE

from power cycling or getting stuck during signature

or classification probing. In the event a PSE overshoots

beyond the classification voltage range, the available load

current aids in returning the PD back into the classification

voltage range. (The PD input may otherwise be trapped

by a reverse-biased diode bridge and the voltage held by

the 0.1礔 capacitor.) By gently ramping the classification

current on and maintaining a positive I-V slope until UVLO

turn-on, the LTC4268-1 provides a well behaved load,

assuring interoperability with any PSE.

PORTP

CLASS

PORTN

LTC4268-1

CONSTANT

LOAD

CURRENT

INTERNAL

TO LTC4268-1

42681 F05

CLASS

CURRENT PATH

PSE

PSE CURRENT MONITOR

PSE

PROBING

VOLTAGE

SOURCE

15.5V TO 20.5V

PORT

(V)

42681 F06

Figure 5. PSE Probing PD During Classification

Figure 6. LTC4268-1 Positive I-V Slope

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