AN313 Datasheet PDF - Silicon Laboratories

Silicon Laboratories

Part Number AN313
Page 6 Pages

AN313 datasheet pdf
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1. Introduction
Emerging Power over Ethernet (PoE) applications, such
as multi-band and 802.11n access points, often require
more power than the 12.95 W specified by the IEEE
802.3af. This application note outlines how to use the
Si3400 and Si3401 to support powered device (PD)
applications over 15 W output, which corresponds to
www.DataSheeat4bUo.ucotm20 W of input power from the PSE.
The current standard for Power over Ethernet or PoE is
IEEE Std™ 802.3-2005, clause 33. Although there is
currently no ratified standard beyond this, a standard,
commonly referred to as PoE+, is being actively
developed in the p802.3at subcommittee. While this
note attempts to be consistent with the general direction
in discussion by the p802.3at committee, it should be
emphasized that no standard currently exists to support
power above the current specification's 12.95 W
maximum of power available at the PD. The proposed
solutions in this document are intended to serve as
practical solutions for higher power PDs before p802.3at
is ratified.
Based on current discussions in the p802.3at
committee, it appears likely that the minimum PSE
output voltage will be increased, and the maximum
cable resistance will be decreased to support PoE Plus.
It is also likely that there will be a provision for
classifying at the current maximum supported power of
12.95 W input and then negotiating with the PSE for
higher power via a simple data exchange with the
Ethernet switch using, for example, the 802.3 (LLDP)
2. Power and Thermal
The Si3400 and Si3401 were intentionally designed with
"hot swap" switch current limits well above the 350 mA
maximum current specification of 802.3af so as not to
limit the PD from drawing more power from a PSE as
long as the PSE is capable of providing the power. The
typical value is 525 mA for the Si3400 and 550 mA for
the Si3401. Also, the switcher FET is designed for
current levels of up to 4 A with a very low on-resistance
of 0.5 Ω (typical). What this means is that the Si3400
and Si3401 are capable of operating at higher power
levels, subject to thermal constraints and proper thermal
management on the PCB. While the exact voltage and
cable resistance have not yet been determined for the
PoE+ or 802.3at applications, it is estimated that
support of 15 W output power will require a minimum
hot swap switch current limit of about 470 mA. The
Si3401 supports a minimum hot swap current limit of
470 mA.
For a typical 5 V isolated application, the dissipation of
the Si3400 and Si3401 is 1.2 W for the maximum power
of 10 W output with 12.95 W input power. This results in
a thermal rise of 54 °C (plus heating due to other
components), which is close to the typical 160 °C
thermal shutdown temperature specification of the
Si3400 and Si3401 when the ambient temperature is
85 °C. While the Si3400 and Si3401 are designed to
handle the 12.95 W power level, there is simply too
much power dissipation in the Si3400 and Si3401
packages when used beyond this currently-specified
limit. To work around this, it is possible to bypass the
Si3400’s and Si3401's two internal diode bridges using
the circuit shown in Figure 1.
Figure 1. Internal Diode Bridge Bypass
It is recommended that the diode bridges internal to the
Si3400 and Si3401 remain connected so as to allow the
early power loss feature (PLOSS signal) to remain
active. Low-cost diodes, such as 1N4002, can be used
to bypass the diodes on the Si3400 and Si3401
because they have a slightly lower voltage drop. This
will help spread the heat in higher power applications.
For higher efficiency, Schottky diodes can be used. It is
also possible to use just the upper four or lower four
diodes for half the benefit at half the cost.
Using the full bypass, the power in the Si3400 and
Si3401 is reduced to 0.5 W at 5 V/2 A output and
0.83 W at 5 V/3 A (15 W) output. Both of these power
levels are well within the thermal capability of the
Si3400 and Si3401 packages.
With the one-half bypass, the power in the Si3400 and
Si3401 packages is at the 1.2 W maximum for a 5 V/
2.6 A (13 W) load.
Rev. 0.2 12/06
Copyright © 2006 by Silicon Laboratories

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3. Other BOM Considerations
For 802.3af-compliant applications, the basic BOM
recommendations for the Si3400 and Si3401 are
documented in “AN296: Using the Si3400/01 POE PD
Controller in Isolated and Non-Isolated Designs” and the
Si3400 and Si3401 ISO EVB user's guides.
When using the Si3400 or Si3401 at higher power
levels, the following other BOM changes are required:
1. Pulse transformer:
A Coilcraft FA2671 (10:3–3.3 V), FA2672 (10:4–5 V),
or FA2732 (1:1–12 V) (transformer with an EP10
core and 40 µH magnetizing inductance is currently
www.DataSheet4U.rceocmommended. For the higher-power application, an
EP13 core with the same turns ratio and magnetizing
inductance is used.
2. Input Ethernet transformer and jack:
Several vendors are now offering Ethernet
transformers for PoE+, but these are more
commonly not integrated in the Ethernet RJ-45 jack.
4. Recommended Options for Higher
Power Applications
Table 1 lists the recommended options for higher-power,
isolated supply applications.
Table 1. Recommended Options for Higher Power Applications
PD Controller
Circuit Configuration
Output Power
On-chip diode bridges
EP10 or EP13 transformer1
Diode bridge bypass
EP13 transformer
10 W
14–15 W
Yes Si3400ISO-EVB
No2 Si3401ISO-EVB
1. EP13 is recommended for short circuit protection.
2. Available output power will depend on minimum PSE voltage and maximum UTP cable resistance.
2 Rev. 0.2

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5. Conclusion
The Si3400 is designed for a maximum input power of
12.95 W, which is consistent with the IEEE 802.3af
standard. Taking into account cost-effective dc-dc
converter implementations and high-efficiency
performance, this results in a maximum output power of
10 W.
For applications requiring more than 10 W output power,
the Si3401 features an increased hot swap switch
current limit of 470 mA (minimum). This allows the
Si3401 to support 14 to 16 W applications with a small
number of BOM modifications and by selecting an
www.DataSheeat4pUp.croompriate power supply topology.
Si3401-EVB and Si3401ISO-EVB application boards
are available with these recommendations. Complete
circuit and performance data are documented in the
corresponding user’s guides.
Rev. 0.2

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Revision 0.1 to Revision 0.2
„ Si3400 and Si3401 minimum hot swap switch
current limits included.
4 Rev. 0.2

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