AN296 Silicon_Laboratories, AN296 Datasheet - Page 17

• Manufacturer Part Number: AN296
• Description: Using THE Si3400 AND Si3401 POE PD Controllers IN Isolated AND Non-isolated Designs
• Manufacturer: Silicon_Laboratories

6. Use with an Auxiliary Power Supply
In some applications, it is desirable to be able to use either the power from the RJ45 Power over Ethernet or from
a low-cost auxiliary power supply. This is very easy to do with the Si3400/01, and a 48 V auxiliary supply is shown
in Figure 14.
The auxiliary power source must supply between 41 and 56 V and at least 15 W for class 0 or 3 equipment (less if
the equipment is class 1 or 2). It must also have output that is isolated from earth ground. To prevent damage from
hot insertion suddenly charging the 0.1 µF input capacitor, a 2 Ω surge limiting resistor in series with the auxiliary
power supply is recommended.
This provides a very simple and inexpensive means of providing auxiliary power. The diode bridges in the Si3400/
01 ensure that no power is fed back to the PSE.
The auxiliary power source always provides the power if it is plugged in first because the PSE will not successfully
complete the detection and classification cycle. If the PSE is plugged in first, the auxiliary power or the auxiliary
power source could provide the power, whichever has the greater output voltage. If the auxiliary power source
provides the power, the PSE will generally sense a disconnect.
The Si3400/01 PLOSS signal indicates whether power is being provided from the auxiliary power source.
It is also possible to use a lower-voltage auxiliary power source, such as 12 V by diode OR at the output of the
switching converter, as shown in Figure 15.
Aux Power
and RJ45
Ethernet
transformer
48VDC
Si3400/01
Vposs
+
-
Vss
Optional 4.7 k
Figure 14. 48 V Auxiliary Supply
Figure 15. 12 V Auxiliary Supply
Diode
>100V 1A
Isolated or
power train
Non-Isolated
Rev. 0.8
Surge limiting resistor
SP1
SP2
CT1
CT2
Vposf
12V +
+
Vneg
Aux
Power 12
VDC
Si3400/01
-
_
Regulator
Post
AN296
17