i ntegrated c ircuits d ivision AN-102-r03 www.ixysic.com 1 application note: AN-102 loop current detection for litelink
i ntegrated c ircuits d ivision 2 www.ixysic.com r03 application note: AN-102 1 loop current detection for litelink many daa applications require loop current detection capability especially in voice communications equipment. traditionally, an optocoupler device in series with the telephone line performs this function. while this method can be used with the litelink family of daas such as the cpc5610, it is sometimes desirable to eliminate the voltage drop caused by a series element in the telephone line. for these applications, the circuit shown below should be considered. when the telephone line loop current is greater than 10ma, the rx+ output of the cpc5610 will be biased up at a dc level of 1v. when the loop current drops due to the cpc5610 going on-hook, or disconnection of the remote call, the rx+ line drops to a dc level of approximately 0v. the accompanying schematic shows the rx+ output going to the inverting input of a comparator. the non-inverting input of the comparator goes to a voltage divider in which the dc voltage is fixed at 200mv. the rx+ output will have a 1v dc bias and potentially a maximum 1.1vpp signal (for 0dbm differential) centered about the 1v bias. this situation requires setting a reference that is below the negative half of a signal of 0dbm magnitude yet sufficiently above 0v to facilitate reliable comparis on. considering the above yielded a reference of 200mv. therefore, with an r2 of 25k, r1 yields 619k for a v cc of 5v. for a v cc of 3.3v, changing r1 to 392k maintains an approximate 200mv reference. the output of u1 will go low when loop current is detected. if the loop current is between 5ma and 10ma, then the output of u1 will pulse as the cpc5610 attempts to start-up. below the 5ma loop current value, the output will generally be hi gh all the time. capacitors c1 and c2 are recommended to provide decoupling to the comparator circuit. these components should be placed as close as possible to u1. u1 is an st microelectronics ts861 low voltage rail-to-rail input comparator, however, similar comparators are available from other manufacturers and may be used. ts861 c2 0.1f c1 0.1f 0.1f 0.1f r1 r2 rx- rx+ 1 11 12 rx- rx+ v cc loop v- v+ - + for additional information please vi sit our website at: www.ixysic.com ixys integrated circuits division makes no representations or wa rranties with respect to the a ccuracy or completeness of the co ntents of this publication and reserves the right to make changes to specifications and product descriptions at any time without notice. neither circuit patent licenses nor indemnity ar e expressed or implied. except as set forth in ixys integrated circuits division?s standard terms and condit ions of sale, ixys integrated circuits division assumes no liability whatsoever, a nd disclaims any express or implied warranty, relating to its products including, but not limited to, the implied warranty of merchantability, fitness for a particular purpose, or infringem ent of any intellectual property right. the products described in this document are not designed, intended, authorized or warranted for use as components in systems in tended for surgical implant into the body, or in other applications intended to support or sustain life, or where malfunction of ixys integrated circuits division?s product may resul t in direct physical harm, injury, or death to a person or severe property or environmental damage. ixys integrated circuits divisi on reserves the right to discontinue or make changes to its pr oducts at any time without notice. specification: AN-102-r03 ?copyright 2014, ixys integrated circuits division all rights reserved. printed in usa. 4/8/2014
|
