LTC4292 datasheet(28/39 Pages) AD

28 / 39 page

Data Sheet

LTC9101-2B/LTC9102

APPLICATIONS INFORMATION

analog.com

Rev. 0 | 28 of 39

Figure 44. 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 45 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

Figure 45. 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 45 is Autoclass-enabled, the recov-

ered power budget can be used to power additional 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 45 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 45 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 successfully 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 negotiation procedure listed below is

shown in Figure 46.

Figure 46. 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.

5. The PSE continues the long class event and does not measure

the class signature current at this time.

6. The PD, if requesting Autoclass, transitions to class signature 0.

If the PD is not requesting Autoclass it continues presenting its

class signature.

7. The PSE measures the Autoclass response of the PD. If class

signature 0 is measured, the PD is requesting Autoclass. When

the measurement is complete the first class event is ended.

8. The PD continues holding the class signature selected in Step 6

until the end of the first class event.

Following the Autoclass negotiation procedure, PSE and PD con-

tinue Physical Layer classification and power up as normal. Re-

gardless of Autoclass, the PD is required to operate below the

negotiated power allocation corresponding to PD assigned Class.

Autoclass Measurement Procedure

Autoclass measurements may be requested by the PD through

Physical Layer classification or, following power on, through LLDP.

Although the LTC9101-2B/LTC9102 is compatible with LLDP-based

Autoclass requests, it cannot receive LLDP Autoclass requests

directly since it does not have access to the data path.

LTC4292 Datasheet(PDF) 28 Page - Analog Devices

LTC4292 Datasheet(HTML) 28 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

Datasheet Download

Link URL