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AN315 ROBUST ELECTRICAL SURGE IMMUNITY F O R P O E PD S T H R O U G H I N T E G R A T E D P R O T E C T I O N 1. Introduction The Si3400 and Si3401 Power over Ethernet (PoE) powered device (PD) controllers are designed in an efficient Silicon on Insulator (SOI) process technology, which enables the integration of a robust surge protection function. Competitive PoE PD devices require external transient voltage suppressors (TVS), such as an SMAJ58A, to provide surge protection. However, due to their inherently wider range of clamping voltage specifications, external TVS-based protectors typically necessitate the use of 100 V processes for the PD controller device. This note discusses some of the detailed considerations regarding the Si3400's proprietary surge protection circuitry that enable the Si3400 and Si3401 devices to yield robust and standards-compliant electrical surge immunity when used in Power over Ethernet powered device applications. 2. Traditional PD Surge Protection Considerations As will be discussed later, surge currents of up to 5 A must be tolerated in PoE applications. The surge protector most often used with PD controllers is an external SMAJ58A TVS diode (D1 in Figure 1). To RJ 45 & Magnetics VSS VSS B1 D1 VPOS RDET VCC VSS M1 Vout Opto 802.3af PD I/F + Switching Regulator RCLASS VNEG VSS SWO DET FB EROUT B2 Figure 1. Traditional PoE PD Interface and Switching Regulator (without High Voltage Device Integration) For example, a standard SMAJ58A TVS from Diodes Incorporated has a maximum operating voltage of 58 V, 25 °C ratings of between 64.4 and 71.2 V at 1 mA, and a clamping voltage under 93.6 V at 4.3 A. While it is possible to sort individual devices for tighter clamp voltage ratings, the typical maximum clamping voltage (including temperature variations) when using this type of protector is close to 100 V. Consequently, most PoE PD controllers are implemented in processes providing a 100 V breakdown voltage. Rev. 0.2 10/06 Copyright © 2006 by Silicon Laboratories Reg AN315 A N315 3. Detailed Description of Si3400 Protection Si3400 and Si3401 protection consists of a stack of ten, high-current 6.2 V Zener diodes. These lower-voltage Zener diodes have a lower temperature and process variation than a higher voltage Zener clamp, such as an SMAJ58A. Consequently, the range of initial breakdown for the complete stack is much tighter: typically 1 kV @ 5 A (10 repetitions) 16.5 kV (both polarities) 15 kV (10 repetitions) > 3.5 kV multiple repetitions (“excellent” category) >175 V for 1 second 3 Ω recommended for good margin The integrated surge protection allows detection of the surge and subsequent shutdown of the switcher as well as steering of surge currents through the on-chip protector for even more robust performance. The tighter distribution of the clamping voltage of the integrated surge protector allows the use of an efficient SOI process, which yields optimal cost and performance compared to solutions requiring an external surge protector. 6. References [1] J. Deatherage and D. Jones, "Multiple Factors Trigger Cable Discharge Events in Ethernet LANs", Electronic Design Dec. 4, 2000. Rev. 0.2 5 A N315 DOCUMENT CHANGE LIST Revision 0.1 to Revision 0.2 Updated Table 1 on page 5. Added reference to IEC 61000-4-2 specification for ESD test. 6 Rev. 0.2 A N315 NOTES: Rev. 0.2 7 A N315 CONTACT INFORMATION 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 Internet: www.silabs.com 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. Silicon Laboratories assumes no responsibility for errors and omissions, and disclaims responsibility for any consequences resulting from the use of information included herein. Additionally, Silicon Laboratories assumes no responsibility for the functioning of undescribed features or parameters. Silicon Laboratories reserves the right to make changes without further notice. Silicon Laboratories makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does Silicon Laboratories assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation consequential or incidental damages. Silicon Laboratories products are not designed, intended, or authorized for use in applications intended to support or sustain life, or for any other application in which the failure of the Silicon Laboratories product could create a situation where personal injury or death may occur. Should Buyer purchase or use Silicon Laboratories products for any such unintended or unauthorized application, Buyer shall indemnify and hold Silicon Laboratories harmless against all claims and damages. Silicon Laboratories and Silicon Labs are trademarks of Silicon Laboratories Inc. Other products or brandnames mentioned herein are trademarks or registered trademarks of their respective holders. 8 Rev. 0.2