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LTC4291-1/LTC4292

Rev 0

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Autoclass

IEEE 802.3bt introduces a new optional feature, Autoclass.

Autoclass enables the PSE to reclaim power budget from

single-signature PDs requesting more power than needed

under worst-case operating conditions. 802.3bt does not

specify Autoclass for dual-signature PDs. The LTC4291-1/

LTC4292 fully supports Autoclass.

Prior versions of the 802.3 PoE standard specify mini-

mum PSE output power for worst-case IR drop across

the Ethernet cable and minimum PSE output voltage.

However, a method for the PSE to reclaim over-allocated

power is not specified. When a shorter Ethernet cable is

used, or when the guaranteed PSE output voltage is above

the specified minimum, the specified minimum PSE out-

put power substantially over-allocates power to the PD.

An example PoE system is shown in two versions.

Figure 22 shows a 100W four port PSE servicing three

25.5W PDs over 100m cables. Such a system requires

the PSE to allocate 25.5W per PD and a further 4.5W for

each 100m cable’s IR drop.

The total power allocation is:

3 Ports • (4.5W + 25.5W) = 90W

If an additional 13W PD is plugged into the fourth PSE

port, only 10W is available and the PD cannot be powered.

APPLICATIONS INFORMATION

Figure 22. 100W PoE System with 100m Cables

13W PD

25.5W PD

100m CABLE

4.5W IR DROP

100m CABLE

4.5W IR DROP

100m CABLE

4.5W IR DROP

100W PSE

42911 F22

Figure 23 shows a 100W four port PSE servicing three

25.5W PDs over 10m cables. Such a system requires the

PSE to allocate 25.5W per PD and a further ~0.5W for

each 10m cable’s IR drop.

Without Autoclass, the total power allocation is:

3 ports • (4.5W + 25.5W) = 90W

If an additional 13W PD is plugged into the fourth PSE

port, only 10W is available and the PD cannot be powered

even though the IR drop is much less than in the prior

example.

Assuming the system in Figure 23 is Autoclass-enabled,

the recovered power budget can be used to power addi-

tional ports. During classification, the PSE observes the

PD’s Autoclass request. After power on is completed, the

PD draws its maximum power while the PSE performs

an Autoclass measurement, as specified by 802.3bt. The

PSE in Figure 23 will measure and report 26W of power

consumption for each of the three 25.5W PDs. This result

allows the host to revise the PSE available power budget.

With Autoclass, the total power allocation for Figure 23 is:

3 Ports • 26W (Measured) = 78W

If an additional 13W PD is plugged into the fourth PSE

port, a full 22W is now available and the PD can be suc-

cessfully powered.

Figure 23. 100W PoE System with 10m Cables

13W PD

25.5W PD

10m CABLE

~0.5W IR DROP

10m CABLE

~0.5W IR DROP

10m CABLE

~0.5W IR DROP

100W PSE

42911 F23

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