LS356 ETC1 | Alldatasheet
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TELEPHONE SPEECH CIRCUIT WITH MULTIFREQUENCY TONE GENERATOR INTERFACE The LS356 is a monolithic circuit in 16-lead dual sending and receiving amplifiers to compen- in-line plastic package to replace the hybrid sate for line attenuation by sensing either the circuit in telephone set. It works with the same line current or the line voltage. In addition, type of transducers for both transmitter and the LS356 can also work in fixed gain mode. receiver (typical dynamic capsules, but the device _ can also work with piezoceramic ones). Many of [t acts as linear interface for MF, supplying a ¢ \\ ° abilized voltage to the digital chip and de- its electrical characteristics can be controlled by livering to the tne che ME ae means of external components to meet different the Mp61 ¢ the MF tones generated by specifications. : In addition to the speech operation, the LS356 acts as an interface for the MF tone signal (par- : ticularly for M761 C/MOS frequency synthesizer). - The LS356 basic functions are the following: DIP-16 Plastic — It presents the proper DC path for the line (0.4) current. — It handles the voice signal, performing the 2/4 ORDERING NUMBER: teoeeee wires interface and changing the gain on both BLOCK DIAGRAM o — owt © a g hy [n= A A ite i 8 8 pon FSO f H ' OH oe ran roa : i ese rai fT EA rr io t ‘ain niu" @ Q en Loe <b <3. | | aa pas of : 2 Peltor <I t <] R [= | b-pt00 oO oy oO Yoo eee 315 6/86
vA Line voltage (3 ms pulse duration) 220 ~~ i Forward line current 150 mA Ie Reverse line current -150 mA Prot Total power dissipation at Tamp= 70°C 1 WwW Top Operating temperature -45to 70 °C Titg, T) Storage and junction temperature 6510150 °C CONNECTION DIAGRAM {top view) = micineur [pp ref) oc eur ouNe f): ssf} Yoo urns if af) seaneut eras ao. ffs sf) Recewer oureur Sees ffs r2[] receiver outeur pcrecuaton [6 vif] aneursinec ame LINE CURRENT fy tof] weur-crec ame) can conto ffs off cine STF THERMAL DATA Rey j-ame Thermal resistance junction-ambient max 80 °C/W 316
wo ! 320 — St Pr | id tind? x | eco Tle L a |vze18v 7+ TIE] | 6k nn MW ba 3 , LS356 | 13) iy | (221 f 1 wee on fd cnn Ne la Bod ft ° | 0 | — 2000 CT “fof Je cl of £0 FO $- 492012 Fig. 1 Fig. 2 112 to80mA he 12to80mA “h, — [hove [Sionry* rest > rest L oT olf ost, Op ome, - ene, ¥so[ Joon] 50 Jeoon| a a oe de cope co e€ F Cy $921 a YRo, Y cust v cMRR Sidetone= “RO, g,. Vso. Vat Ver Fig. 3 Fig.4 — ° yur rerresoma [SF yet? to80ma a4 S a —+—), F oesy HOw _ 00a test Ac J cen PY ese GQ Brome R=68K9 € | circuit T r Over ‘wo[]eoon i ty Lu 8 AL 4, coe F&F coe Vro Vmo R” Vay Me Vine a 317
ELECTRICAL CHARACTERISTICS (Refer to the test circuits, Vg= 1 to 2V, I_= 12 to 80 mA, $1 and $2 in (al, Tamp= -25 to +50°C, f = 200 to 3400 Hz, unless otherwise specified). ee ee SPEECH OPERATION 1L= 20 mA v IL= 80 mA G, Sending gain for 8 type Tamp™ 28°C f=1KHz Vq=2V type Vr 2mV VgzilV 51 Sending gain flatness Vuiz2mVv frog 1 KHz ivs. frequency) (*) Sending gain flatness for Vg=2V tet 50 MA 2 B type (vs. current) AB type (vs. current) Sending distortion for B #=1KHz Veo= 775 mV type I= 16 mA Veo" 900 mV ‘Sending distortion for f=1KHz Vso" 775 mV [—senanaroniorbwoe [Vane ov ver | | [aero || impedance (pin 1-16) operation $2 in (b) GR Receiving gain for B type Vaqi=03V Va=2V 3 f=1KHz Tamp™ 25°C Vert 15 GR Receiving gain for AB type | Vi= 0.3V Ve=2V -25 f= 1 KHz 3 Tamb= 25°C Ver 1V 42.0 Receiving gain fistness Vpit 0.3V frer 1 KHz #1 (vs. frequency) (*) Receiving gain flatness V@=2V lyet= 50 MA. £05 for B type (vs. current) (*) Receiving gain flatness Vez 2Vv Ipet= 50 MA for AB type (vs. current) Receiving distortion for B f=1KHz Vox 400 mV type Vom 450 mV * Fixed gain mode. 318
ELECTRICAL CHARACTERISTICS (continued) [eee Tra ton [i Yo [or [0] Receiving distortion for f=1 KHz Vom 400 mv « | AB type Voz 450 mv Receiver output Vro= 50 mv a f- impedance (pin 12-13) | Sidetone f=1 KHz Tamp* 25°C 2 Stin (b) ™ | | ete Zm_ Line matching impedance Vaz 03V f=1 KHz 600 | 70 | 2 | 3 | tL 1g Input current for gain control (pin 8) MULTIFREQUENCY SYNTHESIZER INTERFACE Vpp MF supply voltage 24 27 | v (Standby and operation) Ippo MF supply current Standby pe Operation ma | — MF amplifier gain fe in™ KHz 7 Via in= 80 mV Vy} DC input voltage level Var in= 80 mV 03 v (pin 14) Yoo fs onwiorravm [vaste | pte p= fe) [eunmwewvine fe Muting threshold voltage Speech operation [| | [so fv [-] (pin 3) WF epewon bet er] Muting standby current - (pin 3) Muting operating current +10 (bin 3) $$ SSS 319
- DC characteristic The fig. 5 shows the DC equivalent circuit of the LS356. Fig. 5 - Equivalent DC load to the line PINs R ping tk i Ih our PN ee A fixed amount |g of the total available current I, is drained for the proper operation of the circuit. The value of 1, can be programmed externally by changing the value of the bias resistor connected to pin 4 (see block diagram). The minimum value of Ig is 7.5 mA. The voltage V. = 3.8V of the shunt regulator is independent of the line current. The shunt regulator (2) is controlled by a temperature compensated voltage reference (1) (see the block diagram). Fig, 6 shows a more detailed circuit configuration of the shunt regulator. Fig. 6 - Circuit configuration of the shunt regulator nz g a 2 o—t — eo } — _ " | | [leo [Jee | 6 a <f sl 1 i R fa | eS] i + — hy i i Ye i ‘or 86 i > der The difference I, -Ip flows through the shunt regulator being |,, negligible, 1, is an internal constant current generator; hence Vo = Va +1, * Ry = 3.8V. The V,, ly characteristic of the device is therefore similar to a pure resistance in series to a battery. It is important to note that the DC voltage at pin 5 is proportional to the line current (Vs = Vy + Va = {IL-1o) R3 + Vp). nO 320
CIRCUIT DESCRIPTION (continued) 2. Two to four wires conversion The LS356 performs the two wires (line) to four wires (microphone, earphone) conversion by means of a Wheatstone bridge configuration so obtaining the Proper decoupling between sending and receiving signals (see fig. 7). Fig. 7. - Two to four wires conversion i A o Yer fy BN Ti | ite UNE Poort on stim
7 Z _ RI
For a perfect balancing of the bridge om = The AC signal from the microphone is sent to one diagonal of the bridge (pin 6 and 9). A small per- centage of the signal power is lost on Zp (being Zy > Z,_}; the main part is sent to the line via R1. In receiving mode, the AC signal coming from the line is sensed across the second diagonal of the bridge (pin 11 and 10). After amplification it is applied to the receiving capsule. The impedance Zyq is simulated by the shunt regulator that is also intended to work as a transconduc- tance amplifier for the transmission signal. . Ae. The impedance Zy, is defined as ——9-2_. Ale-9 From fig. 6 considering C1 as a short circuit for AC signal, any variation AVe generates a variation: = = . Rp AV, = Va = As * Pa The corresponding current change is = AY, a= 3 Therefore AVe Rs 2m = —& = R38 (1+ Mm = aI (+ Re) See 321
CIRCUIT DESCRIPTION (continued) The total impedance across the line connections (pin 11 and 9) is given by Zu = R1+2Zy // (R2 +25) By choosing Zyy > R1 and Zy > Zy Zc = Zum = R3(1 +Fey Rp The received signal amplitude across pin 11 and 10 can be changed using different values of R1 (of course the relationship Z, /Zg = R1/R2 must be always valid). The received signal is related to R1 value according to the approximated relationship: Va = 2Var Ry +2 Note that by changing the value of R1, the transmission signal current is not changed, being the micro- phone amplifier a transconductance amplifier. 3.Automatic gain control The LS356 automatically adjusts the gain of the sending and receiving amplifiers to compensate for line attenuation. This function is performed by the circuit of fig. 8. Fig. 8 : Yeo YREF GO. big s-1696 The differential stage is progressively unbalanced by changing Vg in the range 1 to 2V (Vrera is an internal reference voltage, temperature compensated). It changes the current Ig, and this current is used as a control quantity for the variable gain stages (am- plifier (4) and (5) in the block diagram). The voitage Vg can be taken: a) from the LS366 itself (both in variable and in fixed mode) and b) from a resistive divider, directly at the end of the line. 1a) In the first case, connecting Vg (pin 8) to the regulator bypass (pin 5) it is possible to obtain a gain characteristic depending on the current. In fact (see fig. 6): Vs = Ve tV7= Ve tly ~lo) RB The starting point of the automatic level control is obtained at | = 25 MA when the drain current 1, =7.5 mA. ToS Pom 322
$$ SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS CIRCUIT DESCRIPTION (continued) Minimum gain is reached for a line current of about 52 mA for the same drain current I, =7.5 mA, When I, is increased by means of the external resistor connected to pin 4, the two above mentioned values of the line current for the starting point and for the minimum gain increase accordingly It is also possible to change the starting point without changing |, by connecting pin 8 to the centre of a resistive divider placed between pin 5 and ground (the total resistance seen by pin 5 must be at least 100 Ks2). In this case, the AGC range increases too; for example using a division 1:1 (50K/50K) the AGC starting point shifts to about |_ = 40 mA, and the minimum gain is obtained at || = 95 mA. In addition to this operation made, the Vg voltage can be maintained constant thus fixing the gain values (Rx, Tx) independently of the line conditions. For this purpose the Vop voltage, available for supplying the MF generator, can be used. b) When gains have to be related to the voltage at the line terminals of the telephone set, it is necessary to obtain Vg from a resistive divider directly connected to the end of the line. This type of operation meets for istance the requirements of the French standard, (See the appli- cation circuit of fig. 12). 4. Transducers interfacing The microphone amplifier (3) has a differential input stage with high impedance (= 40 Ka) so allowing a good matching to the microphone by means of external resistor without affecting the sending gain. The receiving output stage (6) is particularly intended to drive dynamic capsules. (Low output impe- dance, 100 2 max; high current capability, 3 mAp). When a piezoceramic capsule is used, it is useful to increase the receiving gain by increasing R1 value (see the relationship for Va). With very low impedance transducer, DC decoupling by an external capacitor must be provided to pre- venta large DC current flow across the transducer itself due to the receiving output stage offset. 5. Multifrequency interfacing The LS356 acts as a linear interface for the Multifrequency synthesizer M761 according to a logical signal (mute function) present on pin 3. When no key of the keyboard is pressed the mute state is low and the LS356 feeds the M761 through pin 15 with low voltage and low current (standby operation of the M761). The oscillator of the M761 is not operating, When one key is pressed, the M761 sends a “high state” mute condition to the LS 356. A voltage com- parator (8) of LS356 drives internal electronic switches: the voltage and the current delivered by the voltage supply (9) are increased to allow the operation of the oscillator. This extra current is diverted by the receiving and sending section of the LS356 and during this oper- ation the receiving output stage is partially inhibited and the input stages of sending and receiving am- plifiers are switched OFF. A controlled amount of the signalling is allowed to reach the earphone to give a feedback to the subs- criber; the MF amplifier (10) delivers the dial tones to the sending paths. The application circuit shown in fig. 9 fulfils the EUROPE II standard (~6, -8 dBm). If the EUROPE | levels are required (-9, -11 dBm) an external divider must be used (fig. 10). The mute function can be used also when a temporary inhibition of the output signal is requested. 323
a
APPLICATION INFORMATION
Fig. 9 - Application circuit with multifrequency (EUROPE || STD) a Bs Bolsa ds ty L956 (Fen TTT | MF cone es ‘| ane of i Ti | |s{}eeal he. j« s2na : 2 43s ‘t) cet _ Py 23" ; ; 4H | Ral} at] We Fig. 10 - Application circuit with multifrequency (EUROPE |) —, an =o ts xt a6 STAR Me mput 19356 cS we | 2066 sl eat i GoKA ea i ” | 2 a 324
APPLICATION INFORMATION (continued) Fig. 11 - Sending and Fig, 12 - Application circuit without multifrequency receiving gain vs. line cur- © rent (application circuit of 7 nel CEG Lf et Lp co aN | leona sass [ifn Pitty | 7 “nee am) hed a, mal } 4B af) [ae Lone Fig. 13 - Application circuit with gain controlled by line voltage (French standard) cuseen incu” Absa 1A Gl —+# 29 LX hur wT Sa 7 00 bs ef SS ype seoxa, 109 hs a aye ! 3 — LINE > 18, ts 356 } hooKn 22Ka| 1000pF 4 1 tt | De|q ! P, og ca ye \\ i ron | > - LT ~ — rer 326
a APPLICATION INFORMATION (continued) Fig 14 - Application circuit with fixed gain operation Fig. 15 - External mute function 1S v LS 356 18 OPhs M761 |ute L356 2 4 9 5 . Speech Sa Lh a) with multifrequency ee [* 33.0] 8 $-4697 Mute Lsa56 ‘Speect 2 - paver) Ry = 0 Max gain condition Ry = 0 Min gain condition b) without multifrequency In addition to the above mentioned applications, different values for the external components can be used in order to satisfy different requirements. The following table (refer to the application circuit of fig. 9) can help the designers. [commen [vom | rome | Note 39.22 Rt controls the receiving gain. When high current values are allowed, R1 must be able to dissipate up to 1W.
3920 Bridge Resistors The ratio R2/R1 fixes the amount of
signal delivered to the line. R1 helps in fixing the DC characteristic (see R3 note). Line current sensing. | The relationships involving R3 are: Fixing DC charac- _ . fora = Zui" (20R3//Zp) + RI — ge ce Zelz = Viz IL-to) (R38 + RI) + Vor Vo=3.8V. Without any problem it is possible to have @ Zyqi_ ranging from 600 up to 900 2. As far as the power diss pation is concerned, see R1 note. 326
APPLICATION INFORMATION (continued) | cone [vam | ramme | Note
13 KQ Bias Resistor ‘The suggested value assures the mini-
mum operating current. It is possible to increose the supply current by decreasing R4 (they are inversely proportional), in order to achieve the shifting of the AGC starting point. 22K Balance Network It is possible to change RS and R6 values | in order to improve the matching to dif- ferent lines; in any case: 10 Ka 2a. Fo Za At Zp = RS +RE/Xcq R7-RT 100 2 Receiver impedance R7 and R7’ must be equal; the suggested matching value is good for matching to dynamic capsule; there is no problem in increasing and decreasing (down to 0 92) this value. A DC decoupling must be inserted when low resistance levels are used to stop the current due to the receiver output offset voltage (max 200 mV). Microphone The suggested value is typical for a impedance dynamic microphone, but it is possible matching to choose RB in a wide range. c 10 uF Regulator AC bypass | A value greater than 10 uF gives a system start time too high for low current Tine during MF operation; a lower value gives an alteration of the AC line im- pedance at low frequency. c2 47 nF Matching to a C2 changes with the characteristics of capacitive line the transmission line, c3 82 nF Receiving gain C3 depends on balancing and line im- flatness pedance versus frequency cS 0.33 uF DC filtering The CS range is from 0.1 uF to 0.47 uF. The lowest value is ripple limited, the higher value is starting up time limited. a ce 10 uF Receiving output DC See note for R7, R7’. decoupling Receiving input DC decoupling 327