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LTC4274A/LTC4274C

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APPLICATIONS INFORMATION

Operating Modes

The LTC4274A/LTC4274C can operate in one of four modes:

manual, semi-auto, AUTO pin, or shutdown.

Table 2. Operating Modes

MODE

AUTO

OPMD

DETECT/

CLASS

POWER-UP

AUTOMATIC

ICUT/ILIM

ASSIGNMENT

AUTO Pin

11b

Enabled at

Reset

Automatically

Yes

Reserved

11b

N/A

Semi-auto

10b

Host

Enabled

Upon

Request

Manual

01b

Once Upon

Request

Upon

Request

Shutdown

00b

Disabled

• In manual mode, the port waits for instructions from the

host system before taking any action. It runs a single

detection or classification cycle when commanded to

by the host, and reports the result in its Port Status

on or off the power at any time. This mode should only

be used for diagnostic and test purposes.

• In semi-auto mode, the port repeatedly attempts to

detect and classify any PD attached to it. It reports the

status of these attempts back to the host, and waits for

a command from the host before turning on power to

the port. The host must enable detection (and optionally

classification) for the port before detection will start.

• AUTO pin mode operates the same as semi-auto mode

except that it will automatically turn on the power to the

port if detection is successful. In AUTO pin mode, ICUT

and ILIM values are set automatically by the LTC4274A/

LTC4274C. This operational mode is only valid if the

AUTO pin is high at reset or power-up and remains high

during operation.

• In shutdown mode, the port is disabled and will not

detect or power a PD.

Regardless of which mode it is in, the LTC4274A/LTC4274C

will remove power automatically from a port which gener-

ates a current limit fault. It will also automatically remove

power from any port that generates a disconnect event if

disconnect detection is enabled. The host controller may

also command the port to remove power at any time.

Reset and the AUTO/MID Pins

The initial LTC4274A/LTC4274C configuration depends on

the state of the AUTO and MID pins during reset. Reset

occurs at power-up, or whenever the RESET pin is pulled

low or the global Reset All bit is set. Changing the state

of AUTO or MID after power-up will not properly change

the port behavior of the LTC4274A/LTC4274C until a reset

occurs.

Although typically used with a host controller, the

LTC4274A/LTC4274C can also be used in a standalone

mode with no connection to the serial interface. If there is

no host present, the AUTO pin must be tied high so that, at

reset, the port will be configured to operate automatically.

The port will detect and classify repeatedly until a PD is

discovered, set ICUT and ILIM according to the classifica-

tion results, apply power after successful detection, and

remove power when a PD is disconnected.

Table 3 shows the ICUT and ILIM values that will be auto-

matically set in standalone (AUTO pin) mode, based on

the discovered class.

Table 3. ICUT and ILIM Values in AUTO Pin Mode

CLASS

ICUT

ILIM

Class 1

112mA

425mA

Class 2

206mA

425mA

Class 3 or Class 0

375mA

425mA

Class 4

638mA

850mA

The automatic setting of the ICUT and ILIM values only

occurs if the LTC4274A/LTC4274C is reset with the AUTO

pin high.

If the standalone application is a midspan, the MID pin must

be tied high to enable correct midspan detection timing.

DETECTION

Detection Overview

To avoid damaging network devices that were not designed

to tolerate DC voltage, a PSE must determine whether

the connected device is a real PD before applying power.

The IEEE specification requires that a valid PD have a

common-mode resistance of 25k ±5% at any port volt-

age below 10V. The PSE must accept resistances that fall

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