Datasheet

LTC4270/LTC4271
20
42701fc
APPLICATIONS INFORMATION
the classification signature current (in this case, Class 3)
in the V
CLASS
range. Table 5 shows the possible clas-
sification values.
Table 5. 802.3af and 802.3at 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
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
CUT
and
I
LIM
thresholds. See the Reset and the AUTO/MID Pin
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.
VOLTAGE (V
CLASS
)
0
CURRENT (mA)
60
50
40
30
20
10
0
5 10 15 20
42701 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
Figure 13. PD Classification
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
LIM
and I
CUT
levels on the fly, allowing the
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 request-
ing 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
signature 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 excep-
tion: 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.