application note l3235 with external ringing by l. vergani, w rossi introduction the sgs-thomson slic kit l3234/l3235 de- signed for pabx application and providing also the ringing generation function (l3234), can be easily used also in system where a standard ring- ing generator is already present and there are dif- ficulties in providing the 100v positive supply for the l3234 ringing injector. circuit implementation here below you can find a detailed description of the ring trip circuitry behavior and its dimen- sioning. for all other characteristics like dc, ac perform- ance, voice path, control interface and other de- tails, the application is the same as the l3234/l3235 kit therefore please refer to the l3234/l3235 datasheet for more informations. AN681/0694 zac za zb cc ch zac zas rpc rpc 3 11 10 8 9 tx ctx ras zb rx 12 cac il cac 100 m f 6.3v 716 22 25 pu gdk power up command for combo 24 oh 20 rng 21 sby to/from card controller rx tx 6 bgnd 13 vcc cvss cvb cvcc vss agnd vbat vss vcc vbat 15 5 1 23 q1 r1 3.3k d1 cs vreg base v cc tip 2 30 w lcp 1512 v bat ring 28 30 w v bat tip r p r p ring r p1 20 w r p1 20 w v bat v ring rr v cc crt rt r3 1m q2 26 4 18 17 19 14 27 rtf vpol lim ref gkf 39k rf 100nf cf cr rlim rr rgf cgf d94tl092a l3235 : analog ground : battery ground must be connected together at a low impedance point qext r2 680k thbt xxx figure 1. this application note shows an easy way, adding few external components, to adapt the l3235 slic to operate together with electromechanical ringing relay and to perform ring/trip function. 1/3
the different operating modes of the l3235 can be programmed through the digital interface based on two input pins sby and rng. when ringing mode is selected, through rng and sby control pins, the l3235 set cs pin to 1 acti- vating, the relay, with 1ma base current into q1 (max. cs current). in this condition the relay is activated and the ringing signal is connec ted to the line through rr and terminated to gnd by rt resistor. dimensioning opportunely the resistors r2 and r3 and the capacitor crt, connected to the base of the transistor q2, it is possible to fix a dc level that will keep the transistor t2 in off condition when no dc current is flowing into the line (tele- phone in on-hook condition). when the phone goes off-hook the negative voltage across ringing resistor force rapidly in conduction the transistor connected to the rtf pin, producing an off-hook detection. once off-hook is detected, the cs pin is auto- matically driven low by l3235, disconnecting the ring relay and therefore performing an auto ring trip function. the key component for good ring trip perform- ances is the crt capacitor. it must have a proper value to reduce the ringing signal that appears on the resistor rt and there- fore on the base of q2. in particular it is important to avoid that the ripple on the q2 base produces an false ring trip de- tection when an high ringing signal is used (90vrms) and five phone are connected in parallel (5ren) with 0 w line resistor and in particular when the value of rr and rt is 400 w (2x400). these are the worst condition that can produce an high ripple level on q2 base, t his effect can be prevented choosing an high value crt. at the same time this capacitor cant have a too high value otherwise the ring trip detection time will be too long in particular for long loops. in this application the crt capacitor has been optimized in order to keep the ring trip time al- ways lower than 100ms even with line resistor greater than 2k w . the components value shown in fig.1 have been chosen in order to obtain the best compromise with the constraints above explaned. in the table below it is possible to observe the minimum value that the capacitor crt must as- sume versus frequency for different rr/rt resis- tor and vring up to 90vrms. 16hz 25hz 40hz 50hz 68hz rr/rt crt 3 270nf 220nf 170nf 150nf 150nf 2x400 crt 3 150nf 150nf 120nf 100nf 82nf 2x200 application note 2/3
information furnished is believed to be accurate and reliable. however, sgs-thomson microelectronics as sumes no responsibi lity 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. spec ifications men- tioned in this publication are subject to ch ange without notice. this publication supersedes and replaces all information previously supplied. sgs-thomson microelectronics products are not authorized for use as critical c omponents in life support devi ces or systems wi thout ex- press written approval of sgs-thomson microelectronics. ? 1995 sgs-thomson microelectronics - all rights reserved sgs-thomson microelectronics group of companies aust ralia - brazil - france - germany - hong kong - i taly - japan - korea - malay sia - malta - morocco - the netherlands - s ingapore - spain - sweden - switzerland - taiwan - thaliand - united k ingdom - u.s.a. application note 3/3
|
