Si3462-EVB

Rev. 1.2

5

5.1. PSE Detection, Classification, Power-Up, and Power Removal

The basic sequence for applying power is shown in Figure 5. The following is a description of the functions that

must be performed in each phase.

Figure 5. Detection, Classification, Power-Up, and Disconnect Sequence

5.1.1. Detection

During the detection phase, the PSE probes with limited current and voltage to determine if a 25 k signature is

present. A valid PD must present between 23.75 and 26.25 k in the range of 2.8 to 10 V with an offset (due to the

bridge diodes) of up to 1.9 V and a parallel capacitance of between 0.05 and 0.12 µF.

The Si3462-EVB uses a robust 3-point detection sequence to avoid inadvertently powering and damaging a

non-PoE-enabled device.

For endpoint applications, detection must be completed within 500 ms of applying a valid signature. When

configured as a midspan, there is a possibility that the PSE circuit will compete with an endpoint PSE, and, as

required by the IEEE specifications, the Si3462 waits at least two seconds after an unsuccessful detection cycle to

repeat the detection process.

Time

(msec)

2.8 V

10 V

15.5 V

44 V

57 V

20.5 V