AN313 SILABS [Silicon Laboratories], AN313 Datasheet

AN313

Manufacturer Part Number

AN313

Description

USING THE Si3400 AND Si3401 IN HIGH POWER APPLICATIONS

Manufacturer

SILABS [Silicon Laboratories]

Datasheet

1.AN313.pdf (6 pages)

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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

about 20 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

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)

layer.

2. Power and Thermal

Considerations

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

Rev. 0.2 12/06

S I N G T H E

current

S i 3 4 0 0

discussions

A N D

the

Si3401

p802.3at

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.

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.

I N

Figure 1. Internal Diode Bridge Bypass

CT2

CT1

SP1

SP2

I G H

O W E R

Vneg

Vposf

AN313

Si3400

Si3401

P P L I C A T I O N S

AN313

Related parts for AN313

AN313 Summary of contents

Page 1

... 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 2.6 A (13 W) load. Copyright © 2006 by Silicon Laboratories AN313 Vposf Si3400 Si3401 Vneg AN313 ...

Page 2

... AN313 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 ...

Page 3

... Si3401 to support applications with a small number of BOM modifications and by selecting an appropriate 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. high-efficiency Rev. 0.2 AN313 3 ...

Page 4

... AN313 OCUMENT HANGE IST Revision 0.1 to Revision 0.2 Si3400 and Si3401 minimum hot swap switch current limits included. 4 Rev. 0.2 ...

Page 5

... N : OTES Rev. 0.2 AN313 5 ...

Page 6

... AN313 C I ONTACT NFORMATION Silicon Laboratories Inc. 400 West Cesar Chavez Austin, TX 78701 Tel: 1+(512) 416-8500 Fax: 1+(512) 416-9669 Toll Free: 1+(877) 444-3032 Email: PoEinfo@silabs.com www.silabs.com Internet: The information in this document is believed to be accurate in all respects at the time of publication but is subject to change without notice. ...

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