LTC4266IGW-PBF View Datasheet(PDF) - Linear Technology
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LTC4266IGW-PBF Datasheet PDF : 30 Pages
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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
DETECT/
PIN OPMD CLASS
AUTOMATIC
ICUT/ILIM
POWER-UP ASSIGNMENT
AUTO Pin 1 11b Enabled at Automatically
Yes
Reset
Reserved 0 11b
N/A
N/A
N/A
Semi-auto 0 10b
Host
Upon
No
Enabled Request
Manual
0 01b Once Upon Upon
No
Request Request
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, ICUT
and ILIM values are set automatically by the LTC4266. 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 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.
16
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. 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 LTC4266 until a reset occurs.
Although typically used with a host controller, the LTC4266
can also be used in a standalone mode with no connection
to the serial interface. If there is no host present, the AUTO
pin should 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 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 automati-
cally set in AUTO pin mode, based on the discovered class.
Table 2. ICUT and ILIM Values in AUTO pin mode
CLASS
ICUT
Class 1
112mA
Class 2
206mA
Class 3 or Class 0
375mA
Class 4
638mA
ILIM
425mA
425mA
425mA
850mA
The automatic setting of the ICUT and ILIM 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
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