Datasheet

LTC4290/LTC4271
20
429071fa
For more information www.linear.com/LTC4290
ApplicAtions inForMAtion
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 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 L
TC4290/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.
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 LTC4290/LTC4271
parts support 802.3at 2-event classification.
One method adds extra fields to the Ethernet LLDP data
protocol;
although
the LTC4290/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 (T
ype 1) device. It then waits for
the host to per
form LLDP communication with the PD and
update the PSE port data. The LTC4290/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 LTC4290/LTC4271. A Type 2 PD that is requesting
more than 13W will indicate Class 4 during normal
802.3af classification. If the LTC4290/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 LTC4290/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 LTC4290/LTC4271 will not provide power
and will restart the detection process. To aid in diagnosis,
VOLTAGE (V
CLASS
)
0
CURRENT (mA)
60
50
40
30
20
10
0
5 10 15 20
429071 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