LTC4258 datasheet(18/28 Pages) LINER

18 / 28 page

LTC4258

4258p

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.

Choosing External MOSFETs

Power delivery to the ports is regulated with external

power MOSFETs. These MOSFETs are controlled as previ-

ously described to meet the IEEE 802.3af specification.

Under normal operation, once the port is powered and the

PD’s bypass capacitor is charged to the port voltage, the

external MOSFET dissipates very little power. This sug-

gests that a small MOSFET is adequate for the job. Unfor-

tunately, other requirements of the IEEE 802.3af mandate

a MOSFET capable of dissipating significant power. When

the port is being powered up, the port voltage must reach

30V or more before the PD turns on. The port voltage can

then drop to 0V as the PD’s bypass capacitor is charged.

According to the IEEE, the PD can directly connect a 180

µF

capacitor to the port and the PSE must charge that

capacitor with a current limit of 400mA to 450mA for at

least 50ms.

An even more extreme example is a noncompliant PD that

provides the proper signature during detection but then

behaves like a low valued resistor, say 50

Ω,inparallelwith

a 1

µF capacitor. When the PSE has charged this

noncompliant PD up to 20V, the 50

Ω resistor will draw

400mA (the minimum IEEE prescribed ILIM current limit)

keeping the port voltage at 20V for the remainder of tSTART.

The external MOSFET sees 24V to 37V VDS at 400mA to

450mA, dissipating 9.6W to 16.7W for 60ms (typ).

The LTC4258 implements foldback to reduce the current

limit when the MOSFET VDS is high; see the Foldback

section. Without foldback, the MOSFET could see as much

as 25.7W for 60ms (typ) when powering a shorted or a

noncompliant PD with only a few ohms of resistance. With

foldback, the MOSFET sees a maximum of 18W for the

duration of tSTART.

The LTC4258’s duty cycle protection enforces 15 times

longer off time than on time, preventing successive at-

tempts to power a defective PD from damaging the

MOSFET. System software can enforce even longer wait

times. When the LTC4258 is operated in semiauto or

manual mode—described in more detail under Operating

Modes—it will not power on a port until commanded to do

so by the host controller. By keeping track of tSTART and

tICUT faults, the host controller can delay turning on the

port again after one of these faults even if the LTC4258

reports a Detect Good. In this way the host controller

implements a MOSFET cooling off period which may be

programmed to protect smaller MOSFETs from repeated

thermal cycling. The LTC4258 has built-in duty cycle

protection for tICUT and tSTART (see tICUT Timing and tSTART

Timing sections) that is sufficient to protect the MOSFETs

shown in Figure 1.

Before designing a MOSFET into your system, carefully

compare its safe operating area (SOA) with the worst case

conditions (like powering up a defective PD) the device will

LTC4258 Datasheet(PDF) 18 Page - Linear Technology

LTC4258 Datasheet(HTML) 18 Page - Linear Technology

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

Datasheet Download

Link URL