S i 3 4 6 0 - E V B
4
Rev. 1.2
Not Recom
m
en
ded
for N
ew D
esigns.
Please Co
ns
id
er Si3462 for
N
ew
D
esigns.
4.2. Classification
Classification is optional and is performed by applying between 15.5 and 20.5 V to the PD and measuring the
current. The maximum power level that can be drawn by the PD is determined according the following table.
Currents not falling in these ranges maybe be treated as the higher or lower classification level. Currents above
45 mA may be treated as class level 0 or 4, which is 15.4 W in either case.
4.3. Power-Up
After the optional classification step, power is applied. Per the IEEE specification, the PSE must supply between 44
and 57 V to the PD. For longer durations above 50 ms the power may be removed if the maximum power level of
either 4, 7, or 15.4 W is exceeded. If the current exceeds 400 mA, the power must be removed in 75 ms. For
short-circuit protection, output power is removed immediately if the output current exceeds 800 mA or the output
voltage drops below 30 V. If power is removed due to an overload condition, detection must not be attempted again
for at least 2 s.
4.4. Disconnect (Power Removal)
The Si3460 supports a robust disconnect algorithm to avoid false disconnection. Removal of a PD can be sensed
by determining that the dc current is less than 5 mA for between 300 and 400 ms. If the current then exceeds
10 mA for at least 60 ms, the power must not be removed, and the 300–400 ms timer restarts.
Table 1. Classification Levels
Classification Level
Minimum PSE Power Level
Current Measured
Class 0
15.4 W
<5 mA
Class 1
4 W
8–13 mA
Class 2
7 W
16–21 mA
Class 3
15.4 W
25–31 mA
Class 4
15.4 W
35–45 mA
