LTC4292 datasheet(21/34 Pages) AD

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Data Sheet

LTC9101-2A/LTC9102

APPLICATIONS INFORMATION

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Rev. 0 | 21 of 34

Table 9. Detection Status

MEASURED PD SIGNATURE

(TYPICAL)

DETECTION RESULT

Incomplete or Not Yet Tested

Detect Status Unknown

VPD < 1V

Short Circuit

CPD > 2.7μF

CPD Too High

RPD < 17k

RSIG Too Low

17k < RPD < 29k

Detect Valid

RPD > 29k

RSIG Too High

RPD > 50k

Open Circuit

IPSE = 0 and VPD > 10V

PSE Detected or Port is Precharged

MOSFET Fault

MOSFET Fault Detected

More on Operating Modes

The port’s operating mode determines when the LTC9101-2A/

LTC9102 runs a detection cycle. In manual mode, the port remains

idle until the host orders a detect cycle. It then runs detection,

reports the result, and returns to idle to wait for another command.

In semi-auto mode, the LTC9101-2A/LTC9102 autonomously polls

a port for PDs, but does not apply power until commanded to do

so by the host. The "Detection/Classification Status" registers are

updated at the end of each detection/classification cycle.

In semi-auto mode, if a valid signature resistance is detected and

classification is enabled, the port classifies the PD and reports the

Class result. If a power-on request is not received from the host, the

port waits for 150ms (typical) and repeats the detection cycle. This

periodically refreshes the "Detection/Classification Status" registers.

The LTC9101-2A offers a rich selection of power-on commands for

IEEE compliance, power negotiation, and powering non-standard

devices. Refer to the LTC9101-2A software interface data sheet for

details on the power on pushbuttons.

Detection is disabled for a port when the LTC9101-2A/LTC9102 is

initially powered up, when the port is in shutdown mode, or when

the corresponding "Detect Enable" bit is cleared.

Detection of Legacy PDs

Proprietary PDs that predate the original IEEE 802.3af standard

are commonly referred to as legacy PDs. One type of legacy PD

uses a large common-mode capacitance (>10μF) as the detection

signature.

Legacy PDs may be inferred by detection and classification results.

The LTC9101-2A/LTC9102 provides all the tools necessary to iden-

tify and power on legacy PDs.

CLASSIFICATION

802.3af Classification

A PD may optionally present a classification signature to the PSE

to indicate the maximum power it draws while operating. The IEEE

specification defines this signature as a constant current draw when

the PSE port voltage is in the VCLASS range (between 15.5V and

20.5V), as shown in Figure 39, with the current level indicating

one of five possible PD signatures. Figure 38 shows a typical PD

load line, starting with the slope of the 25k signature resistor below

10V, then transitioning to the classification signature current (in this

case, "Class 3") in the VCLASS range. Table 10 shows the possible

classification values.

Figure 38. PD Classification

Figure 39. Type 1 or 2 PSE, 1-Event Class Sequence

Table 10. Type 1 and Type 2 PD Classification Values

CLASS

RESULT

Class 0

No Class Signature Present; Treat Like Class 3

Class 1

3.84W

Class 2

6.49W

Class 3

13W

Class 4

25.5W (Type 2)

If classification is enabled, the PSE classifies the PD immediately

after a successful detection cycle. The PSE measures the PD clas-

sification signature by applying VCLASS to the port through OUTn

and measuring the resulting current; it then reports the discovered

class in the appropriate "Port Status" register.

Classification is disabled for a port when the port is in shutdown

mode or when the corresponding "Class Enable" bit is cleared.

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