LTC4263CS datasheet(11/24 Pages) LINER

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LTC4263

4263fe

APPLICATIONS INFORMATION

LTC4263 OPERATION

Signature Detection

The IEEE 802.3af speciﬁcation deﬁnes a speciﬁc pair-to-

pair signature resistance used to identify a device that

can accept power via its Ethernet connection. When the

port voltage is below 10V, an IEEE 802.3af compliant

PD will have an input resistance of approximately 25kΩ.

Figure 2 illustrates the relationship between the PD sig-

nature resistance and the required resistance ranges the

PSE must accept and reject. According to the IEEE 802.3af

speciﬁcation, the PSE must accept PDs with signatures

between 19kΩ and 26.5kΩ and may or may not accept

resistances in the two ranges of 15kΩ to 19kΩ and 26.5kΩ

to 33kΩ. The black box in Figure 2 represents the typical

150Ω pair-to-pair termination used in Ethernet devices

like a computer’s network interface card (NIC) that cannot

accept power.

The LTC4263 checks for the signature resistance by forcing

two test currents on the port in sequence and measuring

the resulting voltages. It then subtracts the two V-I points

to determine the resistive slope while removing voltage

offset caused by any series diodes or current offset caused

by leakage at the port (see Figure 3). The LTC4263 will

typically accept any PD resistance between 17kΩ and

29.7kΩ as a valid PD. Values outside this range (exclud-

ing open and short-circuits) are reported to the user by a

code ﬂashed via the LED pin.

The LTC4263 uses a force-current detection method in

order to reduce noise sensitivity and provide a more robust

detection algorithm. The ﬁrst test point is taken by forcing

a test current into the port, waiting a short time to allow

the line to settle and measuring the resulting voltage. This

result is stored and the second current is applied to the

port, allowed to settle and the voltage measured.

The LTC4263 will not power the port if the PD has more

than 5μF in parallel with its signature resistor unless legacy

mode is enabled.

The LTC4263 autonomously tests for a valid PD connected

to the port. It repeatedly queries the port every 580ms, or

every 3.2s if midspan backoff mode is active (see below).

If detection is successful, it performs classiﬁcation and

power management and then powers up the port.

Midspan Backoff

IEEE 802.3af requires the midspan PSE to wait two seconds

after a failed detection before attempting to detect again

unless the port resistance is greater than 500kΩ. This

requirement is to prevent the condition of an endpoint PSE

and a midspan PSE, connected to the same PD at the same

time, from each corrupting the PD signature and preventing

power-on. After the ﬁrst corrupted detection cycle, the

midspan PSE waits while the endpoint PSE completes

detection and turns the port on. If the midspan mode of

the LTC4263 is enabled by connecting the MIDSPAN pin to

VDD5, a 3.2 second delay occurs after every failed detect

cycle unless the result is an open circuit.

Figure 2. IEEE 802.3af Signature Resistance Ranges

Figure 3. PD 2-Point Detection

RESISTANCE

PSE

0Ω

10k

15k

4263 F02

19k

26.5k

26.25k

23.75k

150Ω (NIC)

20k

30k

33k

REJECT

FIRST

DETECTION

POINT

SECOND

DETECTION

POINT

VALID PD

25kΩ SLOPE

255

180

0V-2V

OFFSET

VOLTAGE

4263 F03

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