LTC4258 datasheet(18/28 Pages) LINER

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LTC4258

4258fb

APPLICATIO S I FOR ATIO

Foldback

Foldback is designed to limit power dissipation in the

MOSFET during power-up and momentary short-circuit

conditions. At low port output voltages, the voltage across

the MOSFET is high, and power dissipation will be large if

significant current is flowing. Foldback monitors the port

output voltage and reduces the VLIM current limit level

linearly from its full value (212.5mV typ) at a port voltage

of 18V to approximately 1/7th of the full value (30mV typ)

at a port voltage of 0V. With 0.5

Ω sense resistors, this

limits the short-circuit current to 60mA (typ) instead of the

full 425mA (typ) current limit. When the LTC4258 is in

foldback, the tICUT timer is active.

Short-Circuit Protection

If a port is suddenly shorted out, the MOSFET power

dissipation can rise to very high levels, jeopardizing the

MOSFET even before the normal current limit circuit can

respond. A separate short-circuit current limit circuit

watches for significant overcurrent events (VSENSE

>275mV, >550mA with a 0.5

Ω sense resistor) and pulls

the GATE pin down immediately if such an event occurs,

shutting off the MOSFET in less than 1

µs (with no external

capacitor on GATE). Approximately 100

µs later, GATE is

allowed to rise back up and the normal current limit circuit

will take over, allowing ILIM current to flow and causing the

tICUT timer to count up. During a short circuit, ILIM will be

reduced by the foldback feature to 1/7th of the nominal

value.

Figures 14 and 15 show the LTC4258 controlling port

current during short circuits. In Figure 14, the MOSFET is

turned off 0.5

µs after the port is shorted with 1Ω. The

spike in port voltage and current at the moment the

MOSFET turns off is the response of inductance in the

system, such as the magnetics and the Ethernet cable; see

Surge Suppressors and Circuit Protection for further

details. The 0.1

µF port bypass cap (see Figure 1) provides

some port current for 0.25

µs after the MOSFET is off. In

Figure 15, the LTC4258 quickly turns the port off and the

spike above ground is again due to inductance. It then

ramps the MOSFET gate up, similar to applying power

after a PD is detected, bringing the port into a controlled

425mA (typ) ILIM current limit. When the short is removed,

the port current no longer needs to be limited and LTC4258

ramps up its GATE pin to fully enhance the MOSFET.

Short-circuit protection quickly stops excessive current

and limits the energy delivered to a short or faulty PD. Yet

the LTC4258 only stops current briefly, so momentary

faults typically do not cause the PD to lose power and PDs

receive at least 50ms of 400mA to 450mA peak current as

required by the 802.3af standard.

Figure 14. Rapid Response to 1

Ω Short

Figure 15. Rapid Response to Momentary 100

Ω Short

250ns/DIV

4258 G04

GND

PORT

VOLTAGE

20V/DIV

GATE

VOLTAGE

10V/DIV

PORT

CURRENT

20A/DIV

VEE

+15V

VDD = 3.3V

VEE = –48V

VEE

0mA

SHORT APPLIED

FET OFF

FAST PULL-DOWN

ACTIVATED

100

µs/DIV

4258 G05

PORT

VOLTAGE

20V/DIV

GATE

VOLTAGE

10V/DIV

PORT

CURRENT

500mA/DIV

VDD = 3.3V

VEE = –48V

VEE

GND

0mA

CURRENT LIMIT

SHORT APPLIED

SHORT REMOVED

FAST PULL-DOWN

VEE

+15V

LTC4258 Datasheet(PDF) 18 Page - Linear Technology

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