LTC4263 datasheet(20/28 Pages) LINER

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LTC4274

4274fa

APPLICATIONS INFORMATION

current reaches the inrush current limit level (typically

450mA), at which point the GATE pin will be servoed to

maintain the specified IINRUSH current. During this inrush

period, a timer (tSTART) runs. When output charging is

complete, the port current will fall and the GATE pin will

be allowed to continue rising to fully enhance the MOSFET

and minimize its on-resistance. The final VGS is nominally

13V. If the tSTART timer expires before the inrush period

completes, the port will be turned back off and a tSTART

fault reported.

Current Limit

Each LTC4274 port includes two current limiting thresholds

(ICUT and ILIM), each with a corresponding timer (tCUT

and tLIM). Setting the ICUT and ILIM thresholds depends

on several factors: the class of the PD, the voltage of the

main supply (VEE), the type of PSE (1 or 2), the sense

resistor (0.5Ω or 0.25Ω), the SOA of the MOSFET, and

whether or not the system is required to implement class

enforcement.

Per the IEEE spec, the LTC4274 will allow the port cur-

rent to exceed ICUT for a limited period of time before

removing power from the port, whereas it will actively

control the MOSFET gate drive to keep the port current

below ILIM. The port does not take any action to limit the

current when only the ICUT threshold is exceeded, but

does start the tCUT timer. The tLIM timer starts when the

ILIM threshold is exceeded and current limit is active. If

the current drops below the ICUT current threshold before

its timer expires, the tCUT timer counts back down, but

at 1/16 the rate that it counts up. This allows the current

limit circuitry to tolerate intermittent overload signals with

duty cycles below about 6%; longer duty cycle overloads

will turn the port off.

ICUT is typically set to a lower value than ILIM to allow the

port to tolerate minor faults without current limiting.

Per the IEEE specification, the LTC4274 will automatically

set ILIM to 425mA (shown in bold in Table 5) during in-

rush at port turn-on, and then switch to the programmed

ILIM setting once inrush has completed. To maintain IEEE

compliance, ILIM should kept at 425mA for all Type 1 PDs,

and 850mA if a Type 2 PD is detected. ILIM is automatically

reset to 425mA when a port turns off.

Table 5. Example Current Limit Settings

ILIM (mA)

INTERNAL REGISTER SETTING (hex)

RSENSE = 0.5Ω

RSENSE = 0.25Ω

106

159

213

266

319

372

425

478

531

584

638

744

850

956

1063

1169

1275

1488

1700

1913

2125

2338

2550

2975

ILIM Foldback

The LTC4274 features a two-stage foldback circuit that

reduces the port current if the port voltage falls below

the normal operating voltage. This keeps MOSFET power

dissipation at safe levels for typical 802.3af MOSFETs,

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