Datasheet

LTC4266
16
4266fe
For more information www.linear.com/LTC4266
ApplicAtions inForMAtion
Operating Modes
The LTC4266 includes four independent ports, each of
which 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 LTC4266. 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 LTC4266 will remove
power automatically from any port that generates 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 com-
mand the port to remove power at any time.
Reset and the AUTO/MID Pins
The initial LTC4266 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 LTC4266 until a reset occurs.
Although typically used with a host controller, the LTC4266
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, all ports will
be configured to operate automatically. Each 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 automati-
cally set in AUTO pin mode, based on the discovered 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 oc-
curs if the LTC4266 is reset with the AUTO pin high.
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 voltage below 10V.
The PSE must accept resistances that fall between 19k and
26.5k, and it must reject resistances above 33kΩ or below
15k (shaded regions in Figure 11). The PSE may choose to
accept or reject resistances in the undefined areas between