AN315

4

Rev. 0.2

The test board survived up to 3500 V. This board was designed for 2500 V isolation. Testing at higher voltage

would require a board design that is capable of withstanding higher voltage or that the Hypot source be removed,

assuming that the cable maintains its charge prior to being plugged in.

This puts the performance of the Si3400 well into the "excellent" CDE category.

4.5. Hot Insertion

Due to the detection process in IEEE STD™ 802.3-2005, which prescribes the use of low voltages and a 5 mA

current limit, hot insertion is not a common problem. However, it is common to have an auxiliary supply as shown

below:

Figure 3. Auxiliary Supply

If the auxiliary supply has a large output capacitor, hot insertion damage may result. This arises from the typical PD

0.1 µF input capacitor being quickly charged, with voltage overshoot resulting from the lead inductance to the

auxiliary power source. While this overshoot will be clamped by the Si3400's protector, for low values of lead

inductance, the peak current can exceed 10 A. A surge limiting resistor will reduce the overshoot and peak current.

A 3

Ω resistor has been found to be an effective value for the surge-limiting resistor in typical applications. Since

this resistor does carry up to the maximum 350 mA of dc PD input current, a 0.5 W resistor is recommended.

Si3400

Si3401

CT1

CT2

SP1

SP2

Vposf

Vneg

Aux Power

48VDC

+

_

Ethernet

Transformer

and RJ45

Diode

>100 V, 1 A

Surge

limiting

resistor