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LTC4274

4274fa

APPLICATIONS INFORMATION

Operating Modes

The LTC4274 can operate in one of four modes: manual,

semi-auto, AUTO pin, or shutdown.

Table 1. 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 LTC4274.

The AUTO pin must be high at reset to ensure proper

AUTO pin mode operation.

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

detect or power a PD.

Regardless of which mode it is in, the LTC4274 will remove

power automatically from a port that generates a current

limit fault. It will also automatically remove power from a

port that generates a disconnect event if disconnect detec-

tion is enabled. The host controller may also command

the port to remove power at any time.

Reset and the AUTO/MID pins

The initial LTC4274 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. Note that the AUTO pin is only

sampled when a reset occurs. Changing the state of AUTO

or MID after power-up will not change the port behavior

of the LTC4274 until a reset occurs.

Although typically used with a host controller, the LTC4274

can also be used in a standalone mode with no connec-

tion to the serial interface. If there is no host present, the

AUTO pin should 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 classification results,

apply power after successful detection, and remove power

when a PD is disconnected. Similarly, if the standalone

application is a midspan, the MID pin should be tied high

to enable correct midspan detection timing.

Table 2 shows the ICUT and ILIM values that will be

automatically set in AUTO pin mode, based on the dis-

covered class.

Table 2. 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 LTC4274 is reset with the AUTO pin high.

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