LT4293 datasheet(23/39 Pages) AD

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Data Sheet

LTC9101-2B/LTC9102

APPLICATIONS INFORMATION

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Rev. 0 | 23 of 39

cation requires that a valid PD has 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

37). 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 37).

Figure 37. IEEE 802.3 Signature Resistance Ranges

Multipoint Detection

The LTC9101-2B/LTC9102 uses a multipoint method to detect PDs.

False-positive detections are minimized by checking for signature

resistance with both forced current and forced voltage measure-

ments.

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

OUTn 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 38). If the forced current detection yields a valid

signature resistance, 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 detec-

tion. PD signature resistances between 17k and 29k (typically) are

detected as valid and reported as Detect Valid in the corresponding

Detection Status register. 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. Table 11 shows the possible

detection results.

Figure 38. PD Detection

Table 11. Detection Status

MEASURED PD SIGNATURE

(TYPICAL)

DETECTION RESULT

Incomplete or Not Yet Tested

Detect Status Unknown

VPD < 1V

Short Circuit

CPD > 2.7μF or VPD > 10V

CPD Too High

RPD < 17k

RSIG Too Low

17k < RPD < 29k

Detect Valid

RPD > 29k

RSIG Too High

RPD > 50k

Open Circuit

PSE Detected

PSE Detected or Port is Precharged

MOSFET Fault

MOSFET Fault Detected

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