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LTC4266A/LTC4266C

4266acfc

APPLICATIONS INFORMATION

Current Limit

Each LTC4266A/LTC4266C port includes two current limit-

ing 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 volt-

age of the main supply (VEE), the type of PSE (Type 1 or

Type 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 specification, the LTC4266A/LTC4266C will

allow the port current 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. 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. If the tCUT timer reaches 60ms (typical) the

port is turned off and the port tCUT fault is set. 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.

The ILIM current limiting circuit is always enabled and

actively limiting port current. The tLIM timer is enabled

only when the programmable tLIMn field is non-zero. 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 threshold is exceeded. When the tLIM timer

reaches 1.7ms (typ) times the programmable tLIMnfieldthe

port is turned off and the port tLIM fault is set. When the

tLIMn field is zero, tLIM behaviors are tracked by the tCUT

timer, which counts up during both ILIM and ICUT events.

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 LTC4266A/LTC4266C will

automatically set ILIM to 425mA (shown in bold in Table 6)

during inrush at port turn-on, and then switch to the

programmed ILIM setting once inrush has completed.

To maintain IEEE compliance, ILIM should be 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 6. 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 LTC4266A/LTC4266C 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 MOS-

FETs, even at extended 802.3at power levels. Current

limit and foldback behavior are programmable on a per-

port basis. Table 6 gives examples of recommended ILIM

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