LTC4271 datasheet(19/30 Pages) LINER

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LTC4274A/LTC4274C

4274acfc

APPLICATIONS INFORMATION

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

above 33k or below 15k (shaded regions in Figure 11).

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 ports, many of which have 150Ω common-mode

termination resistors that will be damaged if power is ap-

plied to them (the black region at the left of Figure 11).

corresponding Port Status register. Values outside this

range, including open and short-circuits, are also reported.

If the port measures less than 1V at the first forced-current

test, the detection cycle will abort and Short Circuit will

be reported. Table 4 shows the possible detection results.

Table 4. Detection Status

MEASURED PD SIGNATURE

DETECTION RESULT

Incomplete or Not Yet Tested

Detect Status Unknown

<2.4k

Short Circuit

Capacitance > 2.7μF

CPD Too High

2.4k < RPD < 17k

RSIG Too Low

17k < RPD < 29k

Detect Good

>29k

RSIG Too High

>50k

Open Circuit

Voltage > 10V

Port Voltage Outside Detect Range

More On Operating Modes

The port’s operating mode determines when the LTC4274A/

LTC4274C runs a detection cycle. In manual mode, the

port will idle until the host orders a detect cycle. It will

then run detection, report the results, and return to idle

to wait for another command.

In semi-auto mode, the LTC4274A/LTC4274C autono-

mously polls a port for PDs, but it will not apply power

until commanded to do so by the host. The Port Status

valid signature resistance is detected and classification is

enabled, the port will classify the PD and report that result

as well. The port will then wait for at least 100ms (or 2

seconds if midspan mode is enabled), and will repeat the

detection cycle to ensure that the data in the Port Status

If the port is in semi-auto mode and high power opera-

tion is enabled, the port will not turn on in response to

a power-on command unless the current detect result is

Detect Good. Any other detect result will generate a tSTART

fault if a power-on command is received. If the port is not

in high power mode, it will ignore the detection result and

apply power when commanded, maintaining backwards

compatibility with the LTC4259A.

Figure 12. PD Detection

Figure 11. IEEE 802.3af Signature Resistance Ranges

4-Point Detection

The LTC4274A/LTC4274C uses a 4-point detection method

to discover PDs. False-positive detections are minimized by

checking for signature resistance with both forced-current

and forced-voltage measurements. Initially, two test cur-

rents are forced onto the port (via the OUT 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 12). If the forced-current detection

yields a valid signature resistance, two test voltages are

then forced onto the port and the resulting currents are

measured and subtracted. Both methods must report

valid resistances for the port to report a valid detection.

PD signature resistances between 17k and 29k (typically)

are detected as valid and reported as Detect Good in the

RESISTANCE

PSE

0Ω

10k

15k

4274AC F11

19k

26.5k

26.25k

23.75k

150Ω (NIC)

20k

30k

33k

FIRST

DETECTION

POINT

SECOND

DETECTION

POINT

VALID PD

25kΩ SLOPE

275

165

0V-2V

OFFSET

VOLTAGE

4274AC F12

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