TPS2370

SLUS537C − AUGUST 2002 − REVISED MARCH 2004

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7

APPLICATION INFORMATION

OVERVIEW

With the addition of power via media-dependent interface (MDI) to the IEEE 802.3af Standard, all data terminal

equipment (DTE) now has the option to receive power over existing cabling that is used for data transmission.

The IEEE 802.3af Standard defines the requirements associated with providing and receiving power over the

existing cabling. The power sourcing equipment (PSE) provides the power on the cable and the powered device

(PD) receives the power. As part of the IEEE 802.3af Standard, the interface between the PSE and PD is

defined as it relates to the detection and classification protocol.

POWER SOURCING EQUIPMENT DETECTION OF A POWERED DEVICE

A powered device (PD) draws power or requests power by participating in a PD detection algorithm. This

algorithm requires the power sourcing equipment (PSE) to probe the link looking for a valid PD. The PSE probes

the link by sending out a voltage between 2.8 V and 10 V across the power lines. A valid PD detects this voltage

and places a resistance of between 23.75 k

Ω and 26.25 kΩ across the power lines. Naturally, the current varies

depending on the input voltage. On detecting this current, the PSE concludes that a valid PD is connected at the

end of the ethernet cable and is requesting power.

If the powered device (PD) is in a state in which it does not accept power, the PD is required to place a

resistance above or below the values listed for a valid PD. On the lower end, a range between 12 k

Ω and

23.75 k

Ω signifies that the PD does not require power. On the higher end, the range is defined to be between

26.25 k

Ω and 45 kΩ . Any resistance value less than 12 kΩ and greater than 45 kΩ, is interpreted by the PSE

as a nonvalid PD detection signature.

The TPS2370 participates in the detection algorithm by activating an internal FET, which connects the DET pin of

the device to VEE. As a result, any resistance connected between VDD and the DET pin of the TPS2370 is, in

effect, across the power lines. This internal FET is active only when input power to the PD is between 2.8 V and

10 V.

POWER SOURCING EQUIPMENT CLASSIFICATION OF A POWERED DEVICE

After the detection phase, the PSE can optionally initiate a classification of the PD. The classification of a PD is

used by the PSE to determine the maximum power required by the PD during normal operation. Five different

levels of classification are defined by the IEEE 802.3af Standard. These levels are shown in Table 2.

Table 2. Powered Device Classification Levels

CLASS

USAGE

POWER DEVICE

POWER

(W)

CLASSIFICATION

CURRENT

(mA)

MIN

MAX

MIN

MAX

0

Default

0.44

12.95

0

4

1

Optional

0.44

3.84

9

12

2

Optional

3.84

6.49

17

20

3

Optional

6.49

12.95

26

30

4

Not allowed

reserved for future use

36

44

Classification of the PD is optionally performed by the PSE only after a valid PD has been detected. To

determine PD classification, the PSE increases the voltage across the power lines to between 15.5 V and 20.5 V.

The amount of current drawn by the PD determines the classification (see Table 2).

When the input voltage to the TPS2370 is between 14.0 V and 20.5 V, the TPS2370 uses an internal regulator to

generate a fixed voltage on the CLASS pin. A resistor connected between the CLASS pin and VEE draws a fixed

amount of current and thereby defines the classification level of the PD.