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CS61881 Product Technical Brief
Designing a Circuit Board for Both the CS61881 and the LXT381
1. INTRODUCTION
This technical brief describes how to design a circuit board for the Cirrus Logic CS61881 and also accommodate the LXT381. The Cirrus Logic device makes this possible with a minimum of changes; some changes are necessary due to the increased performance and functionality of the Cirrus Logic device. * The CS61881 uses a turns ratio of 1:2 on the receive transformer. The LXT381 uses a 1:1 receive transformer. (See the figures and Table 1.) The increased transformer ratio is needed to prevent the receiver from getting over driven on short cables. The values of R1 and R2 for the CS61881 are different from the equivalent resistors on the LXT381. In the internal impedance matching circuit (Figure 1), the CS61881 uses the same resistor values for all cable impedances. The filter capacitor is also a different value (see Table 1). The Level One part needs 1 k resistors on the receive pins to limit over current conditions. The Cirrus Logic LIUs have internal latch up protection which makes the resistors unnecessary (See Figure 1 and Table 1). For additional latchup protection, the 1 k resistors can be used with the CS61881, but the values of R1 and R2 would then need to be changed according to the impedance of the cable. This is shown in Figure 2 with the values for R1 and R2 in Table 1. The analog JTAG pins on the LXT devices (AT1, AT2) have been replaced by the REF and CBLSEL pins on the Cirrus Logic LIUs.
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1.1 Circuit Differences
The differences in the support circuitry are summarized in the following list. Detailed schematics are shown in the figures below. * The CS61881 uses a turns ratio of 1:1.15 on the transmit transformer. The transmit transformer for the LXT381 has a ratio of 1:2. (See the figures below and Table 1.) The reduced transformer ratio is the result of a more efficient transmitter design which produces significant power savings. The resistors on TTIP and TRING of the Level One part are not used on the CS61881 circuit. In the following figures, this is shown with jumpers on the transmit lines, which would be removed for the Level One device and inserted for the Cirrus Logic LIU. The Cirrus Logic LIU doesn't need the capacitor on the transmit circuit of the Level One device. *
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P.O. Box 17847, Austin, Texas 78760 (512) 445 7222 FAX: (512) 445 7581 http://www.cirrus.com
Copyright Cirrus Logic, Inc. 1999 (All Rights Reserved)
NOV `99 TB451APP1 1
CS61881
68 F 0.1 F + RGND
+3.3V + 0.1 F TGND RV+ TV+ RTIP
1k
C1
RECEIV E LINE
R1
+3.3V VCCIO 0.1 F GNDIO +
R2
RRING
1k
T1 TRANSMIT LINE
TRING +3.3V
75
CS61881 / LXT381 One Channel
R3
TTIP
R4
T2 J2 REF AT1 GND 13.3k Connect jumper J2 to resistor for CS61881, or to AT1 for LXT381
NC
120
CBLSEL J1 AT2
For CS61881, disconnect jumper J1 and set switch for cable type. For LXT381, set switch to NC and connect J1 to AT2.
Figure 1. Circuit for CS61881 (Internal Impedance Matching) or LXT381
Component R1, R2 - internal impedance matching R1, R2 - external impedance matching R3, R4 C1 T1 T2
CS61881 75 15 9.31 0.1F 1:2 1:1.15 120 15 15 0.1F 1:2 1:1.15 75
/ 37.5 11 560 pF 1:1 1:2
LXT381 120
/ 60 11 560 pF 1:1 1:2
Table 1. Component Values
2
TB451APP1
CS61881
68 F 0.1 F + RGND
+3.3V + 0.1 F TGND RV+ TV+ RTIP
1k
C1
RECEIV E LINE
R1
+3.3V VCCIO 0.1 F GNDIO +
R2
RRING
1k
T1 TRANSMIT LINE
TRING CS61881 / LXT381 One Channel NC
120 75
R3
TTIP
R4
T2 J2 REF AT1 GND 13.3k Connect jumper J2 to resistor for CS61881, or to AT1 for LXT381
CBLSEL J1 AT2
For CS61881, disconnect jumper J1 and set switch for cable type. For LXT381, set switch to NC and connect J1 to AT2.
Figure 2. Circuit for CS61881 (External Impedance Matching) or LXT381
Contacting Cirrus Logic Support
For a complete listing of Direct Sales, Distributor, and Sales Representative contacts, visit the Cirrus Logic web site at: http://www.cirrus.com/corporate/contacts/
Preliminary product information describes products which are in production, but for which full characterization data is not yet available. Advance product information describes products which are in development and subject to development changes. Cirrus Logic, Inc. has made best efforts to ensure that the information contained in this document is accurate and reliable. However, the information is subject to change without notice and is provided "AS IS" without warranty of any kind (express or implied). No responsibility is assumed by Cirrus Logic, Inc. for the use of this information, nor for infringements of patents or other rights of third parties. This document is the property of Cirrus Logic, Inc. and implies no license under patents, copyrights, trademarks, or trade secrets. No part of this publication may be copied, reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photographic, or otherwise) without the prior written consent of Cirrus Logic, Inc. Items from any Cirrus Logic website or disk may be printed for use by the user. However, no part of the printout or electronic files may be copied, reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photographic, or otherwise) without the prior written consent of Cirrus Logic, Inc. Furthermore, no part of this publication may be used as a basis for manufacture or sale of any items without the prior written consent of Cirrus Logic, Inc. The names of products of Cirrus Logic, Inc. or other vendors and suppliers appearing in this document may be trademarks or service marks of their respective owners which may be registered in some jurisdictions. A list of Cirrus Logic, Inc. trademarks and service marks can be found at http://www.cirrus.com.
TB451APP1
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