LTC4263 LINER [Linear Technology], LTC4263 Datasheet - Page 12

• Manufacturer Part Number: LTC4263
• Description: Single IEEE 802.3af Compliant PSE Controller with Internal Switch
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC4263
Classifi cation
An IEEE 802.3af PD has the option of presenting a clas-
sifi 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 volt-
age is between 15.5V and 20.5V, with the current level
Table 1. IEEE 802.3af Classifi cation, PD Power Consumption, and LTC4263 Enforced Power Output
*Enforced I
12
IEEE 802.3af
CLASS
0
1
2
3
4
CUT
60
50
40
30
20
10
active if ENFCLS pin is high. Otherwise, I
0
Figure 4. Classifi cation Load Lines
0
CLASSIFICATION
16mA to 21mA
25mA to 31mA
35mA to 45mA
8mA to 13mA
0mA to 5mA
CURRENT
5
PD LOAD
CLASS 3
TYPICAL
V
LINE
10
DD48
CURRENT
CLASS 4
CLASS 3
– V
OVER
CLASS 2
CLASS 1
CLASS 0
15
OUT
IEEE ALLOWABLE
PSE LOAD
48mA
33mA
PD POWER
23mA
20
14.5mA
MAXIMUM
6.5mA
LINE
12.95W
12.95W
12.95W
3.84W
6.49W
4263 F04
25
CUT
is 375mA (typ).
ENFORCED I
THRESHOLD*
375mA (typ)
100mA (typ)
175mA (typ)
375mA (typ)
375mA (typ)
LTC4263
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
detects class 1 or class 2, thereby insuring that PDs which
violate their advertised class are shut down.
CUT
PD Does Not Implement Classifi cation, Unknown Power
Low Power PD
Medium Power PD
Full Power PD
Reserved, Power as Class 0
CLASS DESCRIPTION
CUT
current threshold if it
4263fd