The following figure shows the use case where the Si3474 is used as an 802.3at PSE, without an I
2
C Host. The JP2 AUTO MODE SEL
in this case can be set to either Position 7 or Position 6. The corresponding selection jumpers on the POE_BT_CB will need to be set in
an alternating fashion so that the eight 802.3at ports can drive the RJ45 jacks as shown. Note that the VPWR polarity of the top-row
RJ45 jacks have an opposite polarity relative to the ports on the bottom-row RJ45. Two power supplies are needed for this configura-
tion. The +54 V provides power to PDs as well as to the Si3474 VPWR. The +5 V supply is used to supply power to the Si3474 VDD
(albeit indirectly through isolation)
+54 V
JP2 AUTO MODE SEL
JP1
EXT
+5VDC SEL
USB
+5 V
7 6 5 4 3 2 1
7 6 5 4 3 2 1
2pp Class 4 30 W
2pp Class 3 15 W
B
O
T
T
O
P
VOUT8
VOUT7
VOUT6
VOUT5
VOUT4
VOUT3
VOUT2
VOUT1
VOUT1
VOUT3
VOUT5
VOUT7
VOUT2
VOUT4
VOUT6
VOUT8
Figure 1.4. No I
2
C Host, Eight 802.3at 2-Pair Ports
The figure below shows the use case where the Si3474 is used as an 802.3bt PSE without an I
2
C Host. The JP2 AUTO MODE SEL in
this case can be set to either Position 1, 2, 3, 4, or Position 5. The corresponding selection jumpers on the POE_BT_CB will need to be
set to all "Top" positions. Four of the RJ45 jacks will be active while the other four RJ45 jacks are not connected.
Note that it is also possible to configure the jumpers to all "BOT" instead of all "TOP" (then use the bottom RJ45 jacks). When using the
bottom RJ45s, the VPWR polarity will be inverted relative to when using the top RJ45 jacks. Two power supplies are needed for this
configuration. The +54 V provides power to PDs as well as to the Si3474 VPWR. The +5 V supply is used to supply power to the
Si3474 VDD (albeit indirectly through isolation).
UG425: Si3474 EVB User's Guide
Kit Description and Powering Up the Si3474 EVB
silabs.com
| Building a more connected world.
Rev. 0.1 | 5
