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LTC4274

4274fa

APPLICATIONS INFORMATION

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 real PD before applying power. The

IEEE specification requires that a valid PD have a common-

mode resistance of 25k ±5% at any port 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 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 applied to them (the black

region at the left of Figure 11).

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 3

shows the possible detection results.

Table 3. 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

Operating Modes

The port’s operating mode determines when the LTC4274

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 LTC4274 autonomously polls the

port for PDs, but it will not apply power until commanded

to do so by the host. The Port Status register is updated

at the end of each detection cycle. If a valid signature

resistance is detected and classification is enabled, the

port will classify the PD and report that result as well.

Figure 12. PD Detection

Figure 11. IEEE 802.3af Signature Resistance Ranges

4-Point Detection

The LTC4274 uses a 4-point detection method to discover

PDs. False-positive detections are minimized by check-

ing for signature resistance with both forced-current and

forced-voltage measurements. Initially, two test currents

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 Port

Status register. Values outside this range, including open

RESISTANCE

PSE

0Ω

10k

15k

4274 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

4274 F12

LTC4263 Datasheet(PDF) 17 Page - Linear Technology

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