LTC4265 datasheet(21/32 Pages) LINER

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LTC4270/LTC4271

42701fb

To aid in diagnosis, the Port Status register will always

report the results of the last class pulse, so an invalid

Class 4–Class 2 combination would report a second class

pulse was run in the High Power Status register (which

implies that the first cycle found class 4), and Class 2 in

the Port Status register.

POWER CONTROL

The primary function of the LTC4270/LTC4271 is to

control the delivery of power to the PSE port. It does this

by controlling the gate drive voltage of an external power

MOSFETwhilemonitoringthecurrentviaanexternalsense

resistorandtheoutputvoltageattheOUTpin.Thiscircuitry

serves to couple the raw VEE input supply to the port in

a controlled manner that satisfies the PDs power needs

while minimizing both power dissipation in the MOSFET

and disturbances on the VEE backplane.

Inrush Control

Once the command has been given to turn on a port, the

LTC4270/LTC4271 ramps up the GATE pin of that port’s

external MOSFET in a controlled manner. Under normal

power-up circumstances, the MOSFET gate will rise until

the port current reaches the inrush current limit level

(typically 425mA), at which point the GATE pin will be

servoed to maintain the specified IINRUSH current. During

this inrush period, a timer (tSTART) runs. When output

chargingiscomplete,theportcurrentwillfallandtheGATE

pin will be allowed to continue rising to fully enhance the

MOSFET and minimize its on-resistance. The final VGS is

nominally 12V. The inrush period is maintained until the

tSTART timer expires. At this time if the inrush current limit

level is still exceeded, the port will be turned back off and

a tSTART fault reported.

Current Limit

Each LTC4270/LTC4271 port includes two current limit-

ing thresholds (ICUT and ILIM), each with a corresponding

timer (tCUT and tLIM). Setting the ICUT and ILIM thresholds

ApplicAtions inForMAtion

depends on several factors: the class of the PD, the volt-

age of the main supply (VEE), the type of PSE (Type 1 or

Type 2), the sense resistor (0.5Ω or 0.25Ω), the SOA of

the MOSFET, and whether or not the system is required

to enforce class current levels.

Per the IEEE specification, the LTC4270/LTC4271 will al-

low the port current to exceed ICUT for a limited period of

time before removing power from the port, whereas it will

actively control the MOSFET gate drive to keep the port

current below ILIM. The port does not take any action to

limit the current when only the ICUT threshold is exceeded,

but does start the tCUT timer. If the current drops below

the ICUT current threshold before its timer expires, the

tCUT timer counts back down, but at 1/16 the rate that it

counts up. If the tCUT timer reaches 60ms (typical) the

port is turned off and the port tCUT fault is set. This allows

the current limit circuitry to tolerate intermittent overload

signalswithdutycyclesbelowabout6%;longerdutycycle

overloads will turn the port off.

The ILIM current limiting circuit is always enabled and ac-

tively limiting port current. The tLIM timer is enabled only

when the tLIM Enable bit is set. This allows tLIM to be set

to a shorter value than tCUT to provide more aggressive

MOSFET protection and turn off a port before MOSFET

damage can occur. The tLIM timer starts when the ILIM

threshold is exceeded. When the tLIM timer reaches 12ms

(typical) the port is turned off and the port tLIM fault is

set. When the tLIM Enable bit is disabled tLIM behaviors

are tracked by the tCUT timer, which counts up during both

ILIM and ICUT events.

ICUT is typically set to a lower value than ILIM to allow the

port to tolerate minor faults without current limiting.

Per the IEEE specification, the LTC4270/LTC4271 will

automatically set ILIM to 425mA (shown in bold in Table 6)

during inrush at port turn-on, and then switch to the

programmed ILIM setting once inrush has completed. To

maintain IEEE compliance, ILIM should be kept at 425mA

for all Type 1 PDs, and 850mA if a Type 2 PD is detected.

ILIM is automatically reset to 425mA when a port turns off.

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