LTC4266CUHF-PBF View Datasheet(PDF) - Linear Technology
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LTC4266CUHF-PBF Datasheet PDF : 30 Pages
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LTC4266
APPLICATIONS INFORMATION
The signature detection circuitry is disabled when the port
is initially powered up with the AUTO pin low, in shutdown
mode, or when the corresponding detect enable bit is
cleared.
Detection of Legacy PDs
Proprietary PDs that predate the original IEEE 802.3af
standard are commonly referred to today as legacy de-
vices. One type of legacy PD uses a large common mode
capacitance (>10μF) as the detection signature. Note that
PDs in this range of capacitance are defined as invalid, so
a PSE that detects legacy PDs is technically noncompliant
with the IEEE spec.
The LTC4266 can be configured to detect this type of legacy
PD. Legacy detection is disabled by default, but can be
manually enabled on a per-port basis. When enabled, the
port will report detect good when it sees either a valid IEEE
PD or a high-capacitance legacy PD. With legacy mode
disabled, only valid IEEE PDs will be recognized.
CLASSIFICATION
802.3af Classification
A PD can optionally present a classification signature to
the PSE to indicate the maximum power it will draw while
operating. The IEEE specification defines this signature
as a constant current draw when the PSE port voltage
is in the VCLASS range (between 15.5V and 20.5V), with
the current level indicating one of 5 possible PD classes.
Figure 14 shows a typical PD load line, starting with the
slope of the 25kΩ signature resistor below 10V, then
transitioning to the classification signature current (in
this case, Class 3) in the VCLASS range. Table 4 shows the
possible classification values.
Table 4. Classification Values
CLASS
RESULT
Class 0
No Class Signature Present; Treat Like Class 3
Class 1
3W
Class 2
7W
Class 3
13W
Class 4
25.5W (Type 2)
60
OVER PSE LOAD LINE
50
CURRENT
48mA
40
CLASS 4
30
CLASS 3 33mA
23mA
20
TYPICAL CLASS 2
CLASS 3
14.5mA
10
PD LOAD CLASS 1
LINE
6.5mA
CLASS 0
0
0
5
10
15
20
25
VOLTAGE (VCLASS)
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Figure 13. PD Classification
If classification is enabled, the port will classify the PD
immediately after a successful detection cycle in semi-auto
or AUTO pin modes, or when commanded to in manual
mode. It measures the PD classification signature by ap-
plying 18V for 12ms (both values typical) to the port via
the OUTn pin and measuring the resulting current; it then
reports the discovered class in the port status register. If
the LTC4266 is in AUTO pin mode, it will additionally use
the classification result to set the ICUT and ILIM thresholds.
See the Reset and the AUTO/MID Pins section for more
information.
The classification circuitry is disabled when the port is
initially powered up with the AUTO pin low, in shutdown
mode, or when the corresponding class enable bit is
cleared.
802.3at 2-Event Classification
The 802.3at spec defines two methods of classifying a
Type 2 PD.
One method adds extra fields to the Ethernet LLDP data
protocol; although the LTC4266 is compatible with this
classification method, it cannot perform classification
directly since it doesn’t have access to the data path.
LLDP classification requires the PSE to power the PD as
a standard 802.3af (Type 1) device. It then waits for the
host to perform LLDP communication with the PD and
update the PSE port data. The LTC4266 supports chang-
ing the ILIM and ICUT levels on the fly, allowing the host
to complete LLDP classification.
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