LT4295 datasheet(32/50 Pages) AD

32 / 50 page

LTC9101-1/

LTC9102/LTC9103

Rev. 0

For more information www.analog.com

dissipation at safe levels. Current limit and foldback

behavior are programmable on a per-port basis.

The LTC9101-1/LTC9102/LTC9103 supports current

levels well beyond the maximum values in the 802.3bt

specification. 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

LTC9101-1/LTC9102/LTC9103 PSE ports are designed to

tolerate significant levels of abuse, but in extreme cases

it is possible 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 abnor-

mally 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.

If the LTC9101-1/LTC9102/LTC9103 detects any of these

conditions for more than 3.8ms, it disables all port func-

tionality, 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 resetting 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 LTC9101-1/LTC9102/

LTC9103 are unaffected.

An open or missing MOSFET will not trigger a FET Bad

fault, but will cause a tSTART fault if the LTC9101-1/

LTC9102/LTC9103 attempts to turn on the port.

Disconnect

The LTC9101-1/LTC9102/LTC9103 monitors powered

channels to ensure the PD continues to draw the mini-

mum specified current. The IHOLD-2P threshold, monitored

as the VHOLD-2P threshold across the 0.1Ω sense resistor,

is used to determine if a PD has been disconnected.

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 65ms (typical),

the port or channel, as appropriate, is turned off and the

corresponding tCUT faults are set. This allows the channel

to tolerate intermittent 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 1.9ms (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 LTC9101-1 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, ILIM-2P should be set as

shown in Table 6 and Table 7. The programmed ILIM-2P

setting is automatically applied following 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 LTC9101-1/LTC9102/LTC9103 ILIM-2P threshold is

implemented as a two-stage foldback circuit that reduces

the channel current if the channel voltage falls below the

normal operating voltage. This keeps MOSFET power

APPLICATIONS INFORMATION

LT4295 Arkusz danych(PDF) 32 Page - Analog Devices

LT4295 Arkusz danych(HTML) 32 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

Arkusz danych Pobierz

Link URL