LTC4274
18
4274fd
APPLICATIONS INFORMATION
The port will then wait for at least 100ms (or 2 seconds if
midspan mode is enabled), and will repeat the detection
cycle to ensure that the data in the Port Status register
is up-to-date.
If the port is in semi-auto mode and high power opera-
tion is enabled, the port will not turn on in response to
a power-on command unless the current detect result is
detect good. Any other detect result will generate a t
START
fault if a power-on command is received. If the port is not
in high power mode, it will ignore the detection result and
apply power when commanded, maintaining backwards
compatibility with the LTC4259A.
Behavior in AUTO pin mode is similar to semi-auto; how-
ever, 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
CUT
and I
LIM
thresholds are automatically set;
see the Reset and the AUTO/MID Pins 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 (>10糉) 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 LTC4274 can be configured to detect this type of
legacy PD. Legacy detection is disabled by default, but
can be manually enabled. 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
CLASS
range (between 15.5V and 20.5V), with the current
level indicating one of 5 possible PD classes. Figure 13
shows a typical PD load line, starting with the slope of the
25k?signature resistor below 10V, then transitioning to
the classification signature current (in this case, Class 3)
in the V
CLASS
range. Table 4 shows the possible clas-
sification 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)
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
Figure 13. PD Classification
VOLTAGE (V
CLASS
)
0
60
50
40
30
20
10
0
5
10
15
20
4274 F13
25
TYPICAL
CLASS 3
PD LOAD
LINE
48mA
33mA
PSE LOAD LINE
23mA
14.5mA
6.5mA
CLASS 4
CLASS 2
CLASS 1
CLASS 0
CLASS 3
OVER
CURRENT
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