LTC4266 LINER [Linear Technology], LTC4266 Datasheet - Page 18

• Manufacturer Part Number: LTC4266
• Description: Quad IEEE 802.3at Power over Ethernet Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC4266
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 V
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 V
possible classification values.
Table 4. Classification Values
CLASS
Class 0
Class 1
Class 2
Class 3
Class 4
18
CLASS
RESULT
No Class Signature Present; Treat Like Class 3
3W
7W
13W
25.5W (Type 2)
range (between 15.5V and 20.5V), with
CLASS
range. Table 4 shows the
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 I
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 I
to complete LLDP classification.
LIM
60
50
40
30
20
10
and I
0
0
Figure 13. PD Classification
CUT
5
levels on the fly, allowing the host
PD LOAD
TYPICAL
CLASS 3
VOLTAGE (V
LINE
10
CURRENT
CLASS 4
CLASS 3
OVER
CLASS 2
CLASS 1
CLASS 0
CLASS
15
CUT
)
PSE LOAD LINE
48mA
33mA
20
and I
23mA
14.5mA
6.5mA
4266 F13
LIM
25
thresholds.
4266fb