Datasheet

LTC4263
12
4263fe
APPLICATIONS INFORMATION
Figure 4. Classifi cation Load Lines
Classifi cation
An IEEE 802.3af PD has the option of presenting a
classifi cation signature to the PSE to indicate how much
power it will draw when operating. This signature consists
of a specifi c constant-current draw when the PSE port
voltage is between 15.5V and 20.5V, with the current level
indicating the power class to which the PD belongs. Per
the IEEE 802.3af specifi cation, there are fi ve classes and
three power levels for a PD as shown in Table 1. Note that
class 4 is presently reserved by the IEEE for future use.
Figure 4 shows an example PD load line, starting with the
shallow slope of the 25k signature resistor below 10V, then
drawing the classifi cation current (in this case, class 3)
between 15.5V and 20.5V. Also shown is the load line for
the LTC4263. It maintains a low impedance until reaching
current limit at 60mA (typ).
The LTC4263 will classify a port immediately after a
successful detection. It measures the PD classifi cation
signature current by applying 18V (typ) to the port and
measuring the resulting current. The LTC4263 identifi es
the three IEEE power levels and stores the detected class
internally for use by the power management circuitry. In
addition, the LTC4263 allows selectable enforcement of
IEEE classifi cation power levels. With the ENFCLS pin
high, the LTC4263 reduces the I
CUT
current threshold if it
detects class 1 or class 2, thereby insuring that PDs which
violate their advertised class are shut down.
Table 1. IEEE 802.3af Classifi cation, PD Power Consumption, and LTC4263 Enforced Power Output
IEEE 802.3af
CLASS
CLASSIFICATION
CURRENT
MAXIMUM
IEEE ALLOWABLE
PD POWER
LTC4263
ENFORCED I
CUT
THRESHOLD* CLASS DESCRIPTION
0 0mA to 5mA 12.95W 375mA (typ) PD Does Not Implement Classifi cation, Unknown Power
1 8mA to 13mA 3.84W 100mA (typ) Low Power PD
2 16mA to 21mA 6.49W 175mA (typ) Medium Power PD
3 25mA to 31mA 12.95W 375mA (typ) Full Power PD
4 35mA to 45mA 12.95W 375mA (typ) Reserved, Power as Class 0
*Enforced I
CUT
active if ENFCLS pin is high. Otherwise, I
CUT
is 375mA (typ).
V
DD48
– V
OUT
0
CURRENT (mA)
60
50
40
30
20
10
0
5101520
4263 F04
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