LTC4257

8

4257fb

Measured Current and Measured Voltage. The IEEE has

since removed the Measured Voltage method from the

specification. The LTC4257 is compatible with the IEEE

802.3af standard and also works with the obsolete Mea-

sured Voltage method.

In the Measured Current method (Figure 3), the PSE

presents a fixed voltage between –15.5V and –20V to the

PD. With the input voltage in this range, the LTC4257

asserts a load current from the GND pin through the

associated with each class is shown in Table 2.

In the Measured Voltage method (Figure 4), the PSE drives

a current into the PD and monitors the voltage across the

PD terminals. The PSE current steps between classifica-

tion load current values in order to classify the PD under

test. For PSE probe currents below the PD load current, the

LTC4257 will keep the PD terminal voltage below the

classification voltage range. For PSE probe currents above

the PD load current, the LTC4257 will force the PD terminal

voltage above the classification voltage range.

During classification, a moderate amount of power is

dissipated in the LTC4257. IEEE 802.3af limits the classi-

fication time to 75ms. The LTC4257 is designed to handle

the power dissipation for this time period. If the PSE

Classification

Once the PSE has detected a PD, the PSE may optionally

classify the PD. Classification provides a method for more

efficient allocation of power by allowing the PSE to identify

lower-power PDs and allocate less power for these de-

vices. IEEE 802.3af defines five classes (Table 2) with

varying power levels. The designer selects the appropriate

classification based on the power consumption of the PD.

For each class, there is an associated load current that the

PD asserts onto the line during classification probing. The

PSE measures the PD load current to determine the proper

classification and PD power requirements.

Table 2. Summary of IEEE 802.3af Power Classifications and

MAXIMUM

NOMINAL

LTC4257

POWER LEVELS

CLASSIFICATION

AT INPUT OF PD

LOAD CURRENT

RESISTOR

CLASS

USAGE

(W)

(mA)

(

Ω, 1%)

0

Default

0.44 to 12.95

< 5

Open

1

Optional

0.44 to 3.84

10.5

124

2

Optional

3.84 to 6.49

18.5

68.1

3

Optional

6.49 to 12.95

28

45.3

4

Reserved

Reserved*

40

30.9

*Class 4 is currently reserved and should not be used.

Early revisions of the IEEE 802.3af draft specification

defined two methods that a PSE could use in order to

perform PD classification. These methods are known as

Figure 3. IEEE 802.3af Measured-Current Method

of Classification Probing

Figure 4. IEEE 802.af Measured-Voltage Method

of Classification Probing

APPLICATIO S I FOR ATIO

GND

8

2

4

LTC4257

CONSTANT

LOAD

CURRENT

INTERNAL

TO LTC4257

4257 F03

CURRENT PATH

V

PD

PSE

PSE CURRENT MONITOR

PSE

PROBING

VOLTAGE

SOURCE

–15.5V TO –20.5V

GND

8

2

4

LTC4257

CONSTANT

LOAD

CURRENT

INTERNAL

TO LTC4257

4257 F04

CURRENT PATH

V

PD

PSE

PSE

VOLTAGE

MONITOR

PSE

PROBING

CURRENT

SOURCE

IF PSE PROBING CURRENT < LTC4257 LOAD CURRENT, PD TERMINAL VOLTAGE IS < 15V

IF PSE PROBING CURRENT > LTC4257 LOAD CURRENT, PD TERMINAL VOLTAGE IS > 20V