01/25/02 16215 Alton Parkway P.O. 57013 Irvine, Califor

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01/25/02 16215 Alton Parkway P.O. 57013 Irvine, Califor

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APPLICATION NOTE Altima AC208 Dual Speed Family Layout Design Guidelines
01/25/02
16215 Alton Parkway
P.O. 57013
Irvine, California 92619-7013
Phone: 949-450-8700
Fax: 949-450-8710
REVISION HISTORY
Revision AC208-AN100-R Date 01/22/02 Change Description Initial release.
Altima Communications, Inc. Wholly Owned Subsidiary Broadcom Corporation P.O. 57013 16215 Alton Parkway Irvine, California 92619-7013 Copyright 2002 Altima Communications, Inc. rights reserved
Broadcom® pulse logo® trademarks Broadcom Corporation and/or subsidiaries United States certain other countries. other trademarks property their respective owners.
Application Note
01/25/02
AC208
TABLE CONTENTS
Overview Ground Plane Integrity Decoupling Between Power Ground. Crosstalk Twisted Pair (TP) Differential Signals. Compliance
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Document AC208-AN100-R Page
AC208
Application Note
01/25/02
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Page Document AC208-AN100-R
Application Note
01/25/02
AC208
OVERVIEW
This application note contains detailed layout design guidelines AC208, AC207, AC206, AC205, which part AC208 product family. AC208 devices running 2.5V power supply include repeater engines 10BASE-T 100BASE-TX low-power 10/100 transceivers, two-port bridge with built-in memory packet buffering. This single-chip solution targeted number applications requiring power robust network tolerance, including, limited dual speed hubs (DSH), gateways routers, repeaters with separate phone connections. high speed designs, designs based AC208 devices must follow strict design rules ensure superior performance. Additionally, design rules pertaining Ethernet transmission lines should followed achieve optimal transceiver performance. Broadcom recommends that designers follow guidelines this application note closely, this particularly critical designers two-layer design achieve most cost-effective system. guidelines follows: Ground Plane Integrity Decoupling between power ground Cross-talk termination twisted-pair differential signals Compliance
These design guidelines apply AC208, AC207, AC206 AC205 equally; however, AC208 used this document represent members AC208 family.
GROUND PLANE INTEGRITY
Maintaining ground plan integrity highest importance two-layer design. This usually issue four-layer PCB. two-layer board design, first need identify reference point system, then provide solid connection that reference point. AC208-based design, place rectangular ground plane underneath chip, connect ground pins (analog/digital) that piece copper. This piece copper ground reference point. Other ground planes provide impedance connections that reference, such wide traces multiple vias.
Ground Plane bottom side, Connect with Reference Point with multiple vias. signals will reference this plane.
Ground Reference Point (comp side)
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Document AC208-AN100-R Overview Page
AC208
Application Note
01/25/02
Chassis ground analog ground should have minimum separation millimeters. separation located between primary side secondary side transformer.
Chassis Ground Plane 45mil
Ground Plane
RJ45
Transformer
Transformer Secondary Side
Transformer Primary Side
DECOUPLING BETWEEN POWER GROUND
Individual decoupling capacitors ground needed analog power pins. ground side decoupling capacitor should have very impedance system reference point (small copper plane under chip shown above). decoupling capacitors should connected between analog power supply analog ground pins. Broadcom recommends using ceramic capacitors better capacitive characteristics over temperature. decoupling digital power pins less critical. However, good design practice provide individual decoupling wherever there physical space PCB. bulk decoupling, Broadcom recommends using capacitors each chip.
De-coupling close possible Provide short impedance ground reference point
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Page Decoupling Between Power Ground Document AC208-AN100-R
Application Note
01/25/02
AC208
CROSSTALK TWISTED PAIR (TP) DIFFERENTIAL SIGNALS
four-layer board, distance between differential pairs provide enough crosstalk isolation, long they referenced same ground plane underneath them. This because distance between layer copper ground plane very small. However, this true two-layer board, where signal pairs farther from ground plane, which makes easier signal coupled from adjacent wires.
Layer trace, 1-1.5 thick, wide, 6-10 more apart Laminate Material, mils thick Ground Plane, thick Core Material, thick Power Plane, thick Laminate Material, mils thick Bottom Layer trace, 1-1.5 thick, wide, 6-10 more apart 4-layers board stack demon graph. (total thickness typical)
diagram above shows typical layer stack-up four-layer PCB. 4-layer board, signals closely referenced adjacent power ground) ground plane. differential pairs 10-20 mils apart, there very little crosstalk between pairs (the differential pairs much closer ground plane than other pairs around).
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Document AC208-AN100-R Crosstalk Twisted Pair (TP) Differential Signals Page
AC208
Application Note
01/25/02
situation different with two-layer board. With same trace width spacing rules, signal pairs relatively closer pairs around them than ground plane ground traces. This creates excessive crosstalk differential pairs routed close proximity extended time (i.e., several inches). Furthermore, amount crosstalk from pair signal line different from amount from pair signal line. This difference becomes differential noise, which degrades system performance significantly.
RXTop Layer trace, 1-1.5 thick, wide, 6-10 more apart
Core Material, mils thick
Bottom Layer Ground Plane signal reference. 2-layers board stack demon graph. (total thickness typical)
minimize crosstalk, keep traces differential pairs short possible. pair routed extended length, observe following guidelines: Keep differential pairs away possible eliminate cross-talk between pair. ground strip layer separate differential pairs. vias connect layer ground strip bottom plane extensively (every several hundred mils) Keep distance differential pairs constant when routing signal. Keep distance differential pairs ground strip constant. mach length differential pairs. Terminate pair closer pins chips rather than transformer. This because crosstalk current depends physical layout signals characteristics board. voltage strength crosstalk signal equal impedance load times crosstalk current. higher input load, higher crosstalk created same board. Because impossible board match input impedance AC208 Kohm), better keep trace from termination input AC208 short possible.
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Page Crosstalk Twisted Pair (TP) Differential Signals Document AC208-AN100-R
Application Note
01/25/02
AC208
RJ45 Line Side
Transformer
Rin~
illustrated above, same crosstalk current, Layer1 generates much less crosstalk voltage input than input Layer2. easier match Ohms impedance transmission line Layer1. Layer2 transmission line, must kept short possible. reduce ground bounce from differential swing driver transformer, apply filter configuration common mode transformer. configuration should applied every port. This very important. Adopting this filter configuration every port improves cable length performance meters.
2.5V 0.1uF 2.5V 49.9ohm 49.9ohm
TXOP Ferite Bead TXON 0.1uF 100ohm
RJ45
0.1uF
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Document AC208-AN100-R Crosstalk Twisted Pair (TP) Differential Signals Page
AC208
Application Note
01/25/02
COMPLIANCE
comply with requirements, keep layout RJ-45 side clean possible. high frequency digital signals signals with sharp rise/fall edges should routed close those differential signals. noise coupled both transmit receive pairs, place Smith terminations close RJ-45 connector.
RJ45
VOID, Absent Copper Plane
Xformer
Ground Plane
System Side Differential
Side Differential
these layout guidelines followed, design achieve optimal Ethernet transceiver performance.
Altima Communications, Inc.
Wholly Owned Subsidiary Broadcom Corporation
16215 Alton Parkway P.O. 57013 Irvine, California 92619-7013 Phone: 949-450-8700 Fax: 949-450-8710
Broadcom® Corporation reserves right make changes without further notice products data herein improve reliability, function, design. Information furnished Broadcom Corporation believed accurate reliable. However, Broadcom Corporation does assume liability arising application this information, application product circuit described herein, neither does convey license under patent rights rights others.
Document AC208-AN100-R

01/25/02 16215 Alton Parkway P.O. 57013 Irvine, Califor

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