LT4293 datasheet(29/44 Pages) AD

29 / 44 page

LTC9101-2/LTC9102

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

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.

Autoclass Negotiation Procedure

A PSE may receive an Autoclass request from the PD by

Physical Layer classification or LLDP (by way of the PSE

host). For Physical Layer requests, the Autoclass negotia-

tion procedure listed below is shown in Figure 24.

tACS_MAX (87.5ms)

tACS_MIN (75.5ms)

tCLASS_ACS_MIN

tCLASS_PD_MAX (5ms)

tCLASS_MIN

tCLASS_LCE_MAX

tLCE_MIN

tLCE_MAX

VOUTn

VCLASS

VMARK

IOUTn

CLASS_SIG_4

CLASS_SIG_0

9101-2 F24

Figure 24. Autoclass Negotiation, Voltage and Current

1. PSE begins issuing the long first class event. The PD

class signature is allowed to settle during this time.

2. The PD responds with a class signature corresponding

to its Class. The class signature during this time period

is unrelated to the Autoclass negotiation.

3. The PSE measures the PD class signature during this

time and uses the result for the normal Multi-event

Classification.

4. The PD continues presenting its class signature.

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

9101-2 F22

Figure 22. 100W PoE System with 100m Cables

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

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

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

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

9101-2 F23

Figure 23. 100W PoE System with 10m Cables

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

LT4293 Datasheet(PDF) 29 Page - Analog Devices

LT4293 Datasheet(HTML) 29 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

Datasheet Download

Link URL