AN296 Silicon_Laboratories, AN296 Datasheet - Page 2

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

AN296
The Power Sourcing Equipment (PSE) supplies 44 to 57 VDC and must be isolated from earth ground. The
powered device (PD) must not consume more than 12.95 W, which translates to no more than 350 mA of steady
state input current, allowing for 20 Ω of cabling resistance between the PSE and PD. This means that with practical
conversion efficiencies, approximately 10 W of regulated power is available to PD devices, making PoE a preferred
alternative for powering devices, such as VoIP phones, wireless routers, and security devices, because it
eliminates the need for a power source, greatly simplifies installation, and allows easy power backup through an
uninterruptible power source (UPS) on the PSE end. The advantages of IEEE 802.3af-compliant equipment
include:
The Si3400/01 is a highly-integrated and efficient PD signature and dc-dc converter integrated circuit. It is fully
compatible with 802.3af and has a two-step inrush current-limiting feature to allow PD designs that are compatible
with pre-standard PSE equipment. It supports PD designs that require isolation between the Ethernet cables and
powered equipment as well as the lower-cost option without isolation for fully-enclosed devices.
This application note covers the basic operation of the Si3400/01 as well as design equations and selection criteria
for signature resistors and capacitors, dc-dc converter power train, input filter, output filter, feedback and
compensation, soft-start, duty cycle limits, and switcher current limit. The Si3400/01 also feature integrated surge
protection, which will be discussed in the final section of this application note.
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This standard provides a standard way for the PSE to recognize that the PD side is PoE-enabled and not supply
power unless a valid signature is detected, thus eliminating the possibility of damaging equipment that is not
PoE-enabled.
This standard provides a method of allowing the 802.3af PD device to supply classification information to the
PSE so that the PSE can determine the load requirements of the multiple pieces of PD equipment it is
powering.
This standard ensures interoperability of PSE and PD devices from different manufacturers.
Rev. 0.8