2.2.3 Detection and Classification
When
the Si3404 is connected via Ethernet cable to a PSE-enabled Ethernet switch, it has to provide a characteristic resistance (~25
kΩ) to the PSE in a given voltage range (2.7–10.1 V). This is called detection. After the PSE detects the PD, the PSE increases the
voltage above the classification threshold 14.5 V. Then, the PD provides the classification current to inform the PSE about its required
power class (Class 1, 2, 3, or 4). Type 1 PSEs cannot provide enough power for a Class 4 PD. Type 2 PSEs have additional voltage
steps before switching on the PD. After an initial classification voltage pulse, the Type 2 PSE reduces the voltage below the mark
threshold level (10 V) then raises it up again to the Class event range. Last, before switching ON the dc-dc, it reduces the voltage
again.
The Si3404 is a Type 1 PD. The following figure represents the typical turning ON procedure of the PD, which includes detection, clas-
sification and PD turn ON.
2.7 V
10.1 V
14.5 V
20.5 V
37 V
57 V
Detection Classification PD turned ON
Voltage
Time
Reset
IEEE 802.3af
startup
37 V - 57 V (af)
Figure 2.2. Powered Device Voltages
2.3 Hotswap Switch
The
hotswap switch is a high voltage switch which separates the PoE inerface from the dc-dc converter domain. The internal hotswap
switch (HSSW) is turned on (conducting) when the PoE interface voltage goes above V
UVLO_R
. It provides limited inrush current until
the dc-dc side capacitor is charged. The hotswap switch turns off (open) if voltage on the HSSW switch is greater than V
HSSW_OFF
.
In overload, the hotswap switch goes into current-limiting mode with a current limit of I
OVL
. It will turn back ON after T
WAITHSSW
elapses
and the dc-dc input capacitor is recharged, meaning the HSO-VNEG voltage is less than V
HSSW_ON
.
Si3404 Data Sheet
System Overview
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