LTC4292 datasheet(20/40 Pages) AD

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LTC4291-1/LTC4292

Rev 0

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APPLICATIONS INFORMATION

Multipoint Detection

The LTC4291-1/LTC4292 uses a multipoint method to

detect PDs. False-positive detections are minimized by

checking for signature resistance with both forced current

and forced voltage measurements.

Initially, two test currents are forced onto the channel (via

the OUTnM pin) and the resulting voltages are measured.

The detection circuitry subtracts the two V-I points to

determine the resistive slope while removing offset caused

by series diodes or leakage at the port (see Figure 13). If

the forced current detection yields a valid signature resis-

tance, two test voltages are then forced onto the channel

and the resulting currents are measured and subtracted.

Both methods must report valid resistances to report a

valid detection. PD signature resistances between 17k

and 29k (typically) are detected as valid and reported as

Detect Good in the corresponding Port Status register or

Channel Status register, as appropriate. Values outside

this range, including open and short circuits, are also

reported. If the channel measures less than 1V during

any forced current test, the detection cycle will abort and

Short Circuit will be reported. Tables 6 and 7 show the

possible detection results.

The PD configuration (single or dual) determines how the

PD is managed during subsequent detection, classifica-

tion and power on procedures. Throughout the remainder

of this data sheet attention will be called to the different

treatment of single-signature and dual-signature PDs.

Connection check is performed with two current measure-

ments, at the same forced voltage, on the first channel.

The second channel is tested for aggressor behavior by

introducing a forced current on the second channel during

the second measurement. Comparison of the two result-

ing current measurements on the first channel allows

for the connected device to be categorized as a single-

signature PD, a dual-signature PD, or an invalid result.

An invalid connection check result is reported when a

device is added or removed during connection check.

DETECTION

Detection Overview

To avoid damaging network devices that were not designed

to tolerate DC voltage, a PSE must determine whether the

connected device is a valid PD before applying power.

The IEEE specification requires that a valid PD have a

common-mode resistance of 25k ±5% at any channel

voltage below 10V. The PSE must accept resistances that

fall between 19k and 26.5k, and it must reject resistances

above 33k or below 15k (shaded regions in Figure 12).

The PSE may choose to accept or reject resistances in

the undefined areas between the must-accept and must-

reject ranges. In particular, the PSE must reject standard

computer Network Interface Cards (NICs), many of which

have 150Ω common-mode termination resistors that will

be damaged if power is applied to them (the black region

at the left of Figure 12).

Figure 12. IEEE 802.3 Signature Resistance Ranges

RESISTANCE

PSE

10k

15k

42911 F12

19k

26.5k

26.25k

23.75k

150 (NIC)

20k

30k

33k

Figure 13. PD Detection

FIRST

DETECTION

POINT

SECOND

DETECTION

POINT

VALID PD

25k SLOPE

240

160

0V-2V

OFFSET

VOLTAGE

42911 F13

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