LTC4292 datasheet(27/40 Pages) AD

27 / 40 page

LTC4291-1/LTC4292

Rev 0

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APPLICATIONS INFORMATION

assigned Class, the main supply voltage (VEE), the PSE

Type (Type 3 or 4), and the MOSFET SOA.

A single set of programmable port ICUT-2P and ILIM-2P

thresholds is shared by both channels. The thresholds

should be set based on the classification result as shown

in Table 4 and Table 5. For a dual-signature PD assigned

unequal Classes, the highest Class is used to set the

thresholds. For example, a dual-signature PD assigned

Class 1 and Class 5 would enforce ICUT-2P and ILIM-2P

based on Class 5.

Per the IEEE specification, the LTC4291-1/LTC4292 will

allow the channel current to exceed ICUT-2P for a limited

period of time before removing power from the port, or

channel, as appropriate whereas it will actively control

the MOSFET gate drive to keep the channel current below

ILIM-2P. The channel does not take any action to limit the

current when only the ICUT-2P threshold is exceeded, but

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

ICUT-2P 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 59ms (typical), the port or channel,

as appropriate, is turned off and the corresponding tCUT

faults are set. This allows the channel to tolerate intermit-

tent overload signals with duty cycles below about 6%;

longer duty cycle overloads will remove power from the

port or channel, as appropriate.

The ILIM-2P current limiting circuit is always enabled and

actively limiting channel current. The tLIM timer is enabled

only when the tLIM Timer Configuration field is set to a

non-zero value. 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-2P threshold is exceeded.

When the tLIM timer reaches 1.7ms (typical) times the

value in the tLIM Timer Configuration field, the port or

channel, as appropriate, is turned off and the appropriate

tLIM faults are set. When the tLIM Timer Configuration field

is set to 0, tLIM behaviors are tracked by the tCUT timer,

which counts up during both ILIM-2P and ICUT-2P events.

To maintain IEEE compliance, the programmed tLIM Timer

Configuration field should be set as shown in the LTC4291

Software Programming documentation.

ICUT-2P is typically set to a lower value than ILIM-2P,

allowing the port to tolerate minor faults without current

limiting.

To maintain IEEE compliance, the programmed ILIM-2P

should be set as shown in Tables 4 and 5. The pro-

grammed ILIM-2P setting is automatically applied follow-

ing the completion of inrush.

The tCUT and tLIM timers are maintained on a per channel

basis. When a tCUT or tLIM fault occurs a determination is

made to turn off one or both channels. See the Port Fault

vs Channel Fault section for details.

ILIM-2P Foldback

The LTC4291-1/LTC4292 ILIM-2P threshold is imple-

mented as a two-stage foldback circuit that reduces the

channel current if the channel voltage falls below the nor-

mal operating voltage. This keeps MOSFET power dissipa-

tion at safe levels. Current limit and foldback behavior are

programmable on a per-port basis.

The LTC4291-1/LTC4292 supports current levels well

beyond the maximum values in the 802.3bt specifica-

tion. Large values of ILIM-2P may require larger external

MOSFETs, additional heat sinking, and setting the tLIM

Timer Configuration field to a lower value.

MOSFET Fault Detection

LTC4291-1/LTC4292 PSE ports are designed to tolerate

significant levels of abuse, but in extreme cases it is pos-

sible for an external MOSFET to be damaged. A failed

MOSFET may short source to drain, which will make the

port appear to be on when it should be off; this condition

may also cause the sense resistor to fuse open, turning

off the port but causing SENSE to rise to an abnormally

high voltage. A failed MOSFET may also short from gate to

drain, causing GATE to rise to an abnormally high voltage.

OUT, SENSE and GATE are designed to tolerate up to 80V

faults without damage.

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