LTC4263

12

4263fe

APPLICATIONS INFORMATION

Figure 4. Classification Load Lines

Classification

An IEEE 802.3af PD has the option of presenting a

classification signature to the PSE to indicate how much

power it will draw when operating. This signature consists

of a specific 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 specification, there are five 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 classification 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 classification

signature current by applying 18V (typ) to the port and

measuring the resulting current. The LTC4263 identifies

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 classification power levels. With the ENFCLS pin

detects class 1 or class 2, thereby insuring that PDs which

violate their advertised class are shut down.

Table 1. IEEE 802.3af Classification, PD Power Consumption, and LTC4263 Enforced Power Output

IEEE 802.3af

CLASS

CLASSIFICATION

CURRENT

MAXIMUM

IEEE ALLOWABLE

PD POWER

LTC4263

THRESHOLD*

CLASS DESCRIPTION

0

0mA to 5mA

12.95W

375mA (typ)

PD Does Not Implement Classification, 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

0

60

50

40

30

20

10

0

510

15

20

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