LTC4292 datasheet(28/40 Pages) AD

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LTC4291-1/LTC4292

Rev 0

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APPLICATIONS INFORMATION

If the LTC4291-1/LTC4292 sees a power good condition

on either channel of an unpowered port (neither channel

powered), it disables all port functionality, reduces the

gate drive pull-down current for the port and reports a FET

Bad fault. This is typically a permanent fault, but the host

can attempt to recover by resetting the port, or by reset-

ting the entire chip if a port reset fails to clear the fault.

If the MOSFET is in fact bad, the fault will quickly return,

and the port will disable itself again. The remaining ports

of the LTC4291-1/LTC4292 are unaffected.

An open or missing MOSFET will not trigger a FET Bad

fault, but will cause a tSTART fault if the LTC4291-1/

LTC4292 attempts to turn on the port.

Disconnect

The LTC4291-1/LTC4292 monitors powered channels to

ensure the PD continues to draw the minimum speci-

fied current. The IHOLD-2P threshold, monitored as the

VHOLD-2P threshold across the 0.15Ω sense resistor, is

used to determine if a PD has been disconnected.

The IHOLD-2P threshold is set automatically in auto mode

and is set by the user in semi-auto and manual modes.

When powering a single-signature PD assigned Class 0 to

Class 4 over a single channel, set the IHOLD-2P threshold

to 7.5mA (typ) via the Disconnect Configuration bit. In all

other cases, set the IHOLD-2P threshold to 3.5mA (typ).

A disconnect timer (tDIS) counts up whenever channel

current is below the IHOLD-2P threshold, indicating that the

PD has been disconnected. If the appropriate tDIS timer(s)

expire, the port or channel (Table 11) will be turned off

and the corresponding tDIS faults are set. If the current

increases above IHOLD-2P before the tDIS timer expires, the

timer(s) reset. As long as the PD exceeds the minimum

current level before tDIS expires, it will remain powered.

Although not recommended, the DC disconnect fea-

ture can be disabled by clearing the corresponding DC

Disconnect Enable bits. Disabling the DC disconnect fea-

ture forces the LTC4291-1/LTC4292 out of compliance

with the IEEE standard. A powered port will stay powered

after the PD is removed; the still-powered port may be

subsequently connected to a non-PoE data device, poten-

tially causing damage.

The LTC4291-1/LTC4292 does not include AC discon-

nect circuitry. AC disconnect is not a supported feature

of 802.3bt.

Port Fault vs Channel Fault

The tCUT, tLIM and tDIS timers are maintained on a per-

channel basis. When any channel timer expires, a deter-

mination is made to remove power from both, one, or

neither channel of the port.

Optional behavior is allowed by the 802.3bt standard

when faults occur on single-signature PDs. This option

allows a single-signature PD to remain powered on pair-

set X, even if a fault occurs on pairset Y. The FAULT2Pn bit,

when set, enables this optional behavior. This behavior is

not recommended for normal operation, as a fault in the

PD or cabling is indicative of imminent PD or cable failure.

Table 11. Channel Fault Effect on Port/Channel State

PD CON-

FIGURATION

FAULT2Pn

FAULT RESULT:

TURN OFF PORT OR CHANNEL

tCUT**

tLIM

tDIS

Single

Port

Port*

Channel

Dual

Channel

*If tDIS Expires on Both Channels

**Port power policing (PCUT-4P) raises a tCUT event. When enabled, port

power policing removes power from the port regardless of FAULT2Pn

configuration.

Fault Telemetry

As discussed in the preceding sections, faults may occur

on one or both channels, resulting in power removal on

one or both channels. The fault event registers have tra-

ditionally been implemented at the port level. In order to

trace faults to the offending channel, a second layer of fault

registers have been added to the LTC4291-1/LTC4292:

the Fault Telemetry registers. See the LTC4291 Software

Programming documentation for additional information.

LTC4292 Datasheet(PDF) 28 Page - Analog Devices

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