Datasheet

LTC4274A/LTC4274C
18
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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
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 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 I
CUT
and I
LIM
according to the classifica-
tion results, apply power after successful detection, and
remove power when a PD is disconnected.
Table 3 shows the I
CUT
and I
LIM
values that will be auto-
matically set in standalone (AUTO pin) mode, based on
the discovered class.
Table 3. 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 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