application note AN584/0393 protection schematics for switching systems ? a. bremond 1. introduction the aim of this note is to summarize the major characteristics of a slic and to propose protection solutions for each configuration. 2. slic function 2.1. slic generalities : the slic function is defined by the acronym borcht : - battery feeding - overvoltage protection - ringing - signalling - codec - hybrid - test the important parameters to define the overvoltage protection are the battery feeding and the ringing signal. 2.1.a/ battery feeding : this sub-function of the slic is characterized by: - the battery voltage typical value (generally between 45 and 65 v) - the tolerance of the voltage value - the possibility to switch from one value to another one (line cards designed to operate equally on normal and long lines) 2.1.b/ ringing signal : for the ringing, two parameters need to be taken into account : - the voltage value (generally between 70 and 100 v rms) - the ringing configuration (fig.1) 2.2. different kinds of slic : there are two slic families : - the slic without integrated ring generator - the slic with integrated ring generator figure 1 : different ringing configurations figure 2a : slic without integrated ring generator 1/7
2.2.a/ slic without integrated ring generator : for this case the slic ic is supplied between ground and the battery voltage (- v bat ). the relay operates the selection of functions, ringing mode in position 2 and the other modes in position 1. (see fig 2a). 2.2.b/ slic with integrated ring generator : this kind of slic, e.g. the l3000 family of sgs-thomson, is supplied between ground, the battery (-v bat ) and a positive voltage (+v b ) up to +72 v. (see fig 2b). 2.3 . goal of the slic protection : the purpose of the protection is to suppress all overvoltages out of the normal operating voltage range of the slic. we have to take into account the two kinds of slic described in section 2.2. 2.2.a/ slic without integrated ring generator : as shown in fig.3, the protection areas are located differently before and after the ring relay. before the relay the protection must operate over the peak value of the ring signal (generally +90v and -190v). as the relay protection does not require a very precise clamping threshold, we usually use a symmetrical overvoltage suppressor (generally + or -200 v). after the relay the protection acts to suppress all spikes above ground and below the battery voltage (-v bat ). it is important to note that the integrated circuit needs a protection threshold as close as possible to the supply voltage. figure 2b : slic with integrated ring generator figure 3 : goal of the protection for the slic without integrated ring generator ? application note 2/7
in certain cases an internal network of diodes allows the output stages of the slic to be oversupplied (see fig.4). 2.3.b/ slic with integrated ring generator : the integrated circuit l3000 of sgs-thomson is presently the only slic of this kind. it operates between ground and the battery voltage for all the modes except for the ringing, where the operating area is located between +v b (up to 72 v) and the battery voltage (-v bat ) (see fig.5). the protection takes into account this fact and operates above +v b and below -v bat . the diodes d1 and d2 (fig.5) act when the l3000 operates out of the ringing mode and when a positive overvoltage is clamped at +v b . the output stages are then temporarily oversupplied at +v b . figure 4 : internal diode network of the tdb7722 figure 6 : typical line card protection at pc board level 3. application diagrams 3.1 . slic without integrated ring generator fig.6 below shows the protection topology for line card protection. the protection is divided into two stages : - the high voltage protection for ring relays (thbtxx series) - the low voltage protection for the slic. here there are two possibilities : - fixed breakdown voltage (thdtxx series). - programmable breakdown voltage (lcpxx series) figure 5 : external diode network used with the l3000 ? application note 3/7
a specific product range for line card protection is available with kit solutions (for ring relays and slic protection) in s08, dil8 and sip3 packages. as shown in figure 7, they are bidirectional functions for relay protection and asymmetrical functions for slic protection. these solutions provide the following advantages : - different surge current capabilities are available. - area used by the protection on the pc board is reduced. - optimum cost/performance compromise is possible. fig. 8 and fig.9 show the product range offered by sgs-thomson, with versions available also in axial from or in the surface mount package sod15. 3.1.1. slic protection with fixed breakdown voltage. figure 7 : line card protection kit trisil dual trisil 3 terminal trisil sod15 sip3 so8 dil8 75a @ 10/1000 m sec 30a @ 10/1000 m sec smthbt 200 thbt200s thbt15011 thbt20011 thbt27011 thbt15012 thbt20012 thbt27012 figure 8 : high voltage protection range = thbtxx sod15 cb429 sil3 so8 dil8 high surge capability 75a @ 10/1000 m sec medium surge capability 30a @ 10/1000 m sec single function dual function dual function smthbt58 tpu58 thdt58s thbt5111 thbt6511 thbt5112 thbt6512 figure 9 : slic protection range fixed breakdown voltage = thdtxx ? application note 4/7
this solution is mainly used to benefit from the high surge current capability of the sod 15 package for ac tests. such high surge performance can only be achieved with this smd package. 3.1.2. slic protection with programmable breakdown voltage figure 10 : surface mount solution in sod 15 figure 11 : monochip programmable breakover voltage solution with this solution, the protection performance is improved by the possibility of adjusting the breakdown voltage to the battery voltage. this function is well suited to variable battery voltage applications such as short / long line switching. ? application note 5/7
figure 12 gives the product range with this function, available in sil4, dil8 and so8 packages. 3.2. slic with integrated ring generator : (slic l3000) many types of slic protection solutions exist, depending on : - the battery voltage - the pc board area available for the protection - the cost / performance compromise 3.2.1. l3000 protection with 2 x l3121b this topology (fig.13) is the most efficient one for this kind of slic. 3.2.2. l3000 protection with 2 x l3100b sil4 so8 dil8 high surge capability 75a @ 10/1000 m sec medium surge capability 30a @ 10/1000 m sec lcp150s lcp1511 lcp1512 figure 12 : slic protection range lcpxx - pro- grammable breakdown voltage figure 13 : l3000 protection with 2 l3121b figure 14 : l3000 protection with 2 l3100b figure 14 has the same electrical behaviour as the previous one but with lower cost. ? application note 6/7
figure 15 : common protection for slic wi- thout integrated ring generator figure 16 : common protection for slic with integrated ring generator 3.2.3. multiline common protection : these types of application (fig.15 and 16) decrease the cost of the protection per line. the major drawbacks of these solutions are : - the short circuit across all the lines when the protection device fires. - the current remaining through the protection device after the surge is too high to allow its automatic switch-off. this must then be effected by a software instruction, e.g. power-down. 4. conclusion the wide range of protection devices on offer from sgs-thomson makes an optimized protection solution possible for every application. information furnished is believed to be accurate and reliable. however, sgs-thom son microelectronics assumes no responsability for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of sgs-thomson microelectronics. specifications mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. sgs-thomson microelectronics products are not authorized for use as critical components in lif e support devices or systems without express written approval of sgs-thomson microelectronics. ? 1995 sgs-thomson microelectronics - printed in italy - all rights reserved. sgs-thomson microelectronics group of companies australia - brazil - france - germany - hong kong - italy - japan - korea - malaysia - malta - morocco - the netherlands - singapore - spain - sweden - switzerland - taiwan - thailand - united kingdom - u.s.a. ? application note 7/7
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