LTC4292 datasheet(30/40 Pages) AD

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LTC4291-1/LTC4292

Rev 0

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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.

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 correspond-

ing 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.

APPLICATIONS INFORMATION

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 continue Physical Layer classification and power up

as normal. Regardless of Autoclass, the PD is required

to operate below the negotiated power allocation corre-

sponding 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 LTC4291-1/LTC4292 is

compatible with LLDP-based Autoclass requests, it can-

not receive LLDP Autoclass requests directly since it does

not have access to the data path.

If the PSE is commanded to perform an Autoclass mea-

surement following a Physical Layer request, the mea-

surement typically begins tAUTO_PSE1 (1.5s typical) after

port inrush is successfully completed. For LLDP-based

Autoclass requests, the measurement begins immediately.

The Autoclass measurement period is tAUTO_PSE2 – tAUTO_

PSE1 (1.8s typical) using a sliding window of tAUTO_WINDOW

(0.2s typical). During the Autoclass measurement period,

the PSE continuously monitors IPORT and VEE, calculat-

ing maximum average power. Following the Autoclass

measurement period, the Autoclass measurements are

reported in the Port Parametric registers.

See the LTC4291 Software Programming documen-

tation for details on enabling Autoclass, the status of

the Autoclass negotiation, reading Autoclass measure-

ment results and dynamically requesting an Autoclass

measurement.

Port Current Readback

The LTC4291-1/LTC4292 measures the current at each

power channel with per-channel A/D converters. The total

port current (sum of both channels) is reported. Port

Figure 24. Autoclass Negotiation, Voltage and Current

tACS_MAX (87.5ms)

tACS_MIN (75.5ms)

tCLASS_ACS_MIN

tCLASS_PD_MAX (5ms)

tCLASS_MIN

tCLASS_LCE_MAX

tLCE_MIN

tLCE_MAX

VOUTnM

VCLASS

VMARK

IOUTnM

CLASS_SIG_4

CLASS_SIG_0

42911 F24

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