LTC4274
16
4274fe
For more information www.linear.com/LTC4274
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
PIN OPMD
DETECT/
CLASS POWER-UP
AUTOMATIC
I
CUT
/I
LIM
ASSIGNMENT
AUTO Pin 1 11b Enabled at
Reset
Automatically Yes
Reserved 0 11b N/A N/A N/A
Semi-auto 0 10b Host
Enabled
Upon
Request
No
Manual 0 01b Once Upon
Request
Upon
Request
No
Shutdown 0 00b Disabled Disabled No
• 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
register. The host system can command the port to
turn 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, I
CUT
and I
LIM
values are set automatically by the LTC4274.
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 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. Changing the state of AUTO
or MID after power-up will not properly 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 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 I
CUT
and I
LIM
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 must be tied high
to enable correct midspan detection timing.
Table 2 shows the I
CUT
and I
LIM
values that will be
automatically set in AUTO pin mode, based on the dis-
covered class.
Table 2. I
CUT
and I
LIM
Values in AUTO Pin Mode
CLASS I
CUT
I
LIM
Class 1 112mA 425mA
Class 2 206mA 425mA
Class 3 or Class 0 375mA 425mA
Class 4 638mA 850mA
The automatic setting of the I
CUT
and I
LIM
values only
occurs if the LTC4274 is reset with the AUTO pin high.
