ltc4270 Linear Technology Corporation, ltc4270 Datasheet - Page 21

• Manufacturer Part Number: ltc4270
• Description: 12-port Poe/poe+/ltpoe++ Pse Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
the classification signature current (in this case, Class 3)
in the V
sification values.
Table 4. 802.3af and 802.3at 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 LTC4270/LTC4271 is in AUTO pin mode, it will ad-
ditionally use the classification result to set the I
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
CLASS
RESULT
No Class Signature Present; Treat Like Class 3
3W
7W
13W
25.5W (Type 2)
60
50
40
30
20
10
0
0
range. Table 4 shows the possible clas-
Figure 13. PD Classification
5
PD LOAD
TYPICAL
CLASS 3
VOLTAGE (V
LINE
10
CURRENT
CLASS 4
CLASS 3
OVER
CLASS 2
CLASS 1
CLASS 0
CLASS
15
)
PSE LOAD LINE
48mA
33mA
23mA
20
14.5mA
6.5mA
42701 F13
25
CUT
and
802.3at 2-Event Classification
The 802.3at specification defines two methods of classify-
ing a Type 2 PD. A-grade and B-grade LTC4270/LTC4271
parts support 802.3at 2-event classification.
One method adds extra fields to the Ethernet LLDP data
protocol; although the LTC4270/LTC4271 is compatible
with this classification method, it cannot perform clas-
sification 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 LTC4270/LTC4271 supports
changing the I
host to complete LLDP classification.
The second 802.3at classification method, known as
2-event classification or ping-pong, is supported by
the LTC4270/LTC4271. A Type 2 PD that is requesting
more than 13W will indicate Class 4 during normal
802.3af classification. If the LTC4270/LTC4271 sees
Class 4, it forces the port to a specified lower voltage
(called the mark voltage, typically 9V), pauses briefly, and
then re-runs classification to verify the Class 4 reading
(Figure 1). It also sets a bit in the High Power Status register
to indicate that it ran the second classification cycle. The
second cycle alerts the PD that it is connected to a Type
2 PSE which can supply Type 2 power levels.
2-event ping-pong classification is enabled by setting a bit
in the port’s High Power Mode register. Note that a ping-
pong enabled port only runs the second classification cycle
when it detects a Class 4 device; if the first cycle returns
Class 0 to 3, the port determines it is connected to a Type 1
PD and does not run the second classification cycle.
Invalid Type 2 Class Combinations
The 802.3at specification defines a Type 2 PD class signa-
ture as two consecutive Class 4 results; a Class 4 followed
by a Class 0-3 is not a valid signature. In AUTO pin mode,
the LTC4270/LTC4271 will power a detected PD regardless
of the classification results, with one exception: if the PD
presents an invalid Type 2 signature (Class 4 followed by
Class 0 to 3), the LTC4270/LTC4271 will not provide power
and will restart the detection process. To aid in diagnosis,
LIM
and I
LTC4270/LTC4271
CUT
levels on the fly, allowing the
21
42701f