ltc4266a Linear Technology Corporation, ltc4266a Datasheet - Page 20

• Manufacturer Part Number: ltc4266a
• Description: Quad Poe/poe+/ltpoe++ Pse Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTC4266A/LTC4266C
Behavior in AUTO pin mode is similar to semi-auto; however,
after Detect Good is reported and the port is classified (if
classification is enabled), it is automatically powered on
without further intervention. In standalone (AUTO pin)
mode, the I
see the Reset and the AUTO/MID Pin section for more
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 LTC4266A/LTC4266C 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
level indicating one of 5 possible PD classes. Figure 13
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
sification values.
20
CLASS
range (between 15.5V and 20.5V), with the current
CLASS
CUT
range. Table 5 shows the possible clas-
and I
LIM
thresholds are automatically set;
Table 5. Classification Values
CLASS
Class 0
Class 1
Class 2
Class 3
Class 4
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 LTC4266A/LTC4266C is in AUTO pin mode, it will
additionally use the classification result to set the I
and I
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.
LIM
thresholds. See the Reset and the AUTO/MID Pin
60
50
40
30
20
10
0
RESULT
No Class Signature Present; Treat Like Class 3
3W
7W
13W
25.5W (Type 2)
0
Figure 13. PD Classification
5
PD LOAD
CLASS 3
TYPICAL
VOLTAGE (V
LINE
10
CURRENT
CLASS 4
CLASS 3
OVER
CLASS 2
CLASS 1
CLASS 0
CLASS
15
)
PSE LOAD LINE
48mA
33mA
20
23mA
14.5mA
6.5mA
4266AC F13
25
4266acfa
CUT