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| LS656 TELEPHONE SPEECH CIRCUIT WITH MULTIFREQUENCY TONE GENERATOR INTERFACE .PRESENTSTHEPROPERDCPATHFORTHE .HANDLES .ACTSASLI LINE CURRENT, PARTICULAR CARE BEING PAID TO HAVE LOW VOLTAGE DROP THE VOICE SIGNAL, PERFORMING THE 2/4 WIRES INTERFACE AND CHANGING THE GAIN ON BOTH SENDING AND RECEIVING AMPLIFIERS TO COMPENSATE FOR LINE ATTENUATION BY SENSING EITHER THE LINE CURRENT OR THE LINE VOLTAGE. IN ADDITION, THE LS656 CAN ALSO WORK IN FIXED GAIN MODE NEAR INTERFACE FOR MF, SUPPLYING A STABILIZED VOLTAGE TO THE DIGITALCHIP AND DELIVERING TO THE LINE THE MF TONES GENERATED BY THE M761 DIP16 SO20L ORDERING NUMBERS : LS656AB (DIP16) LS656AD1 (SO20) DESCRIPTION The LS656 is a monolithic integrated circuit in 16lead plastic package to replace the hybrid circuit in telephone set. It works with the same type of transducers for both transmitter and receiver (typically dynamic capsules). Many of its electrical charBLOCK DIAGRAM (DIP16) acteristics can be controlled by means of external components to meet different specifications. In addition to the speech operation, the LS656 acts as an interface for the MF tonesignal (particularly for M761 C/MOS frequency synthesizer). June 1993 1/15 LS656 ABSOLUTE MAXIMUM RATINGS Symbol VL IL IL Ptot Top Tstg, Tj Forward Line Current Reverse Line Current Total Power Dissipation at Tamb = 70 C Operating Temperature Storage and Junction Temperature Parameter Line Voltage (3 ms pulse duration) Value 22 150 - 150 1 - 45 to 70 - 65 to 150 Unit V mA mA W C C THERMAL DATA Symbol R th j-amb Parameter Thermal Resistance Junction-ambient Max Value 80 Unit C/W PIN CONNECTIONS (top view) DIP-16 SO-20L TEST CIRCUITS 2/15 LS656 Figure 1. Figure 2. V = 0.1 V CMR Side tone = VRO VMI Gs = VSO VMI Figure 3. Figure 4. GR = VRO VRI GMF = VMO VMF 3/15 LS656 ELECTRICAL CHARACTERISTICS (refer to the test circuits, VG = 1 to 2V, IL = 12 to 80mA, S1, S2 and S3 in (a), Tamb = - 25 to + 50oC, f = 200 to 3400Hz, unless otherwise specified) Symbol SPEECH OPERATION VL Line Voltage Tamb = 25C IL = 12mA IL = 30mA IL = 60mA f = 1kHz Tamb = 25C, f = 1kHz, VMI = 2mV IL = 25mA IL = 50mA VMI = 2mV, fref = 1kHz VMI = 3 mV, Iref = 50mA, S3 in (b) f = 1kHz, IL = 16mA Vso = 775mV Vso = 900mV VMI =0V; VG = 1V; S1 in (b) VMI = 2mV VMI = 2mV, S2 in (b) VRI = 0.3V, f = 1kHz, Tamb = 25C IL = 25mA IL = 50mA VRI = 0.3V, fref = 1kHz VRI = 0.3V, Iref = 50mA, S3 in (b) f = 1kHz, IL = 15mA VRO = 400mV VRO = 450mV VRI = 0V; VG = 1V; S1 in (b) VRO = 50mV f = 1kHz, Tamb = 25C, S1 in (b) VRI = 0.3V, f = 1kHz 500 600 150 30 36 700 - 10 40 - 30 - 71 V 3.4 4.0 5.1 7.0 dB dB 48 44 -1 -1 51 47 +1 +1 dB dB 2 2 2 3 10 % % dBmp k dB dB -6 - 11 -1 -1 -3 -8 +1 +1 dB dB % 3 10 V dB A 3 - 2 3 - 3 3 3 2 - 2 3 1 2 - Parameter Test Conditions Min. Typ. Max. Unit Fig. CMR GS Common Mode Rejection Sending Gain 50 Sending Gain Flatness (versus frequency) Sending Gain Flatness (versus current) Sending Distortion Sending Noise Microphone Input Impedance (pin 1-16) Sending Gain in MF Operation GR Receiving gain Receiving Gain Flatness (vs. freq.) Receiving Gain Flatness (vs. current) Receiving Distortion Receiving Noise Receiving Ouptut Impedance (pins 12-13) Sidetone ZML I8 Line Matching Impedance Input Current for Gain Control (pin 8) 4/15 LS656 ELECTRICAL CHARACTERISTICS (continued) Symbol Parameter Test Conditions Min. Typ. Max. Unit Fig. MULTIFREQUENCY SYNTHESIZER INTERFACE VDD IDD MF Supply Voltage Stand by and Operation MF Supply Current Stand by Operation MF Amplifier Gain VI RI d DC Input Voltage Level (pin 14) Input Impedance (pin 14) Distortion Starting Delay Time Muting Threshold Voltage (pin3) Muting Stand by Current (pin 3) ) Muting Operating Current (pin 3) S2 in (b) Speech Operation MF Operation 1.6 - 10 + 10 S2 in (b) S2 in (b) fMF in = 1kHz, VMF in = 80mV VMF in = 80mV VMF in = 80mV VMF in = 150mVp, IL > 17mA 60 4 5 1 2.4 0.5 2 15 VDD x 0.3 17 2.5 2.7 V mA mA dB V k % ms V V A A - - - 4 - - 4 - - - - - CIRCUIT DESCRIPTION 1. DC Characteristic The fig. 5 shows the DC equivalent circuit of the LS656. A fixed amount Io of the total available current IL is drained for the proper operation of the circuit. The value of Io can be programmed externally by changing the value of the bias resistor connected to pin 4 (see block diagram). The minimum value of Io is 7.5 mA. The voltage Vo = 37 V 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 Figure 5 : Equivalent DC Load to the Line. block diagram). Fig. 6 shows a more detailed circuit configurationof the shunt regulator. ThedifferenceIL-Io flowsthrough theshunt regulator being Ib negligible. Ia is an internal constant current generator ; hence Vo = VB + Ia . Ra = 3.7 V. The VL, IL 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 (V5 = V7 + VB = (IL Io) R3 + VB). The DC characteristic of the LS656 is shown in fig. 7. 5/15 LS656 Figure 6 : Circuit Configuration of the Shunt Regulator. Figure 7 : DC Characteristic. 2. Two to Four Wires Conversion TheLS656 performsthe 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. 8). ZL R1 = For a perfect balancing of the bridge ZB R2 The AC signal from the microphone is sent to one diagonalof the bridge (pin 6 and 9). A small percentage of the signal power is lost on ZB (being ZB >> ZL) ; the main part is sent to the line via R1. In receiving mode, the AC signal coming from the line is 6/15 sensed across the second diagonal of the bridge (pin 11 and 10). After amplification it is appliedto the receiving capsule. The impedance ZM is simulated by the shunt regulator that is also intended to work as a transconductance amplifier for the transmission signal. V6 - 9 The impedance ZM is defined as I6 - 9 From fig. 6 considering C1 as a short circuit for AC signal, any variation V6 generates a variation : Rb V7 = VA = V6 Ra + Rb LS656 The corresponding current is V7 I = R3 Therefore Ra V6 = R3 1 + I Rb The total impedance across the line connections (pin 11 and 9) is given by ZM = ZML = R1 + ZM//(R2 + ZB) By choosing ZM R1 and ZB ZM Ra ZML ZM = R3 1 + Rb The received signal amplitude across pin 11 and 10 can be changed using different value of R1 (of course the relationship ZL/ZB = R1/R2 must be always valid). The received signal is related to R1 value according to the approximated relationship : VR = 2 VRI Io isincreased 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 possibleto changethe starting point without changingIo by connecting pin 8 to the centre of a resistive divider placed betweenpin 5 and ground (the total resistance seen by pin 5 must be at least 100 K). In this case, the AGC range increases too; for example using a division 1 : 1 (50 K/50 K) the AGC starting point shifts toaboutIL =40 mA, and the minimum gain is obtained at IL = 95 mA. In addition to this operation mode, the VG voltage can be maintained constant thus fixing the gain value (Rx, Tx) independently of the line conditions. For this purpose the VDD 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 obtainVG froma resistive divider directly connected to the end of the line. This type of operationmeets the requirements of the French standard. (See the application circuit of fig. 13). 4. Transducer Interfacing The microphone amplifier (3) has a differential input stage with high impedance ( 40k) 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 impedance (100 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 VR). Whit very low impedance transducer, DC decoupling by an external capacitor must be provided to prevent a large DC current flow across the transducer itself due to the receiving output stage offset. 5. Multifrequency Interfacing The LS656 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 LS656 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 LS656. A voltage com7/15 R1 R1 + ZM Note that by changing the value of R1, the transmission signal current is not changed, being the microphone amplifier a transconductance amplifier. 3. Automatic Gain Control The LS656 automatically adjusts the gain of the sending and receiving amplifiers to compensate for line attenuation. This function is performed by the circuit of fig. 9. The differential stage is progressively unbalanced by changing VG in the range 1 to 2 V (VREFG 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 (amplifier (4) and (5) in the block diagram). The voltage VG can be taken : a) from the LS656 itself (both in variable and in fixed mode) and. b) from a resistive divider, directly at the end of the line. a) In the first case, connectingVG (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) V5 = VB + V7 VB = (IL - I o) R3 Thestarting point of theautomaticlevel controlis obtained at IL = 25 mA when the drain current Io = 7.5 mA. Minimum gain is reached for a line current of about 50 mA for the same drain current Io = 7.5 mA. When LS656 parator (8) of LS656 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 LS656 and during this operation the receiving output stage is partially inhibited and the input stages of sending and receiving amplifiers are switched OFF. Figure 8 : Two to Four Wires Conversion. A controlled amount of the signalling is allowed to reach the earphone to give a feedback to the subscriber ; the MF amplifier (10) delivers the dial tones to the sending paths. The mute function can be used also when a temporary inhibition of the output signal is requested. The application circuit shown in fig. 10 fulfils the EUROPE II standard (-6, -8 dBm). If the EUROPE I levels are required (-9, -11 dBm) an external divider Figure 9. 8/15 LS656 APPLICATION INFORMATION Figure 10 : Application Circuit with Multifrequency (Europe II STD). Figure 11 : Application Circuit with Multifrequency (Europe I STD). 9/15 LS656 Figure 12 : Sending and Receiving Gain vs. Line Current (application circuit of fig. 10). Figure 13 : Application Circuit without Multifrequency. 10/15 LS656 Figure 14 : Application Circuit with Gain Controlled by Line Voltage (french standard). Figure 15 : Application Circuit with Fixed Gain Operation. Ry = 0 Rx = 0 Main gain condition Main gain condition 11/15 LS656 Figure 16 : External Mute Function. a) with multifrequency 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. 10) can help the designers. Component R1 R2 R3 Value 30 330 30 Purpose Bridge Resistors Note R1 controls the receiving gain. When high current values are allowed, R1 must be able to dissipate up to 1 W. The Ratio R2/R1 fixes the amount of signal delivered to the line. R1 helps in fixing the DC characteristics (see R3 note). The relationships involving R3 are : ZL//ZML and ZML = (20 R3//ZB) + R1, GS = K R3 VL = (IL - IO) (R3 + R1) + V0 ; V0 = 3.7V Without any problem it is possible to have a ZML ranging from 600 up to 900. As far as the power dissipation is concerned, see R1 note. The suggested value assures the minimum operating current. It is possible to increase the supply current by decreasing R4 (they are inversely proportional), in order to achieve the shifting of the AGC starting point. (see fig. 16). After R4 changement, so It's possible to change R5 and R6 values in order to improve the ZL R1 = , ZB = R5 + R6//XC4 matching to different lines ; in any case : ZB R2 Line Current Sensing Fixing DC Characteristic R4 13k Bias Resistor R5 R6 R7-R7' 2.2k 6.8k 100 Balance Network R8 C1 C2 C3 C4 C5 C6-C7 C8 C9 Receiver R7 and R7', must be equal ; the suggested value is good for Impedance Matching matching to dynamic capsule ; there is no problem in increasing and decreasing (down to 0) this value. A DC decoupling must be inserted when low resistance levels are used to stop 200 Microphone Impedance Matchin 10F Regulator AC A value greater than 10 F gives a system start time too high for byPass low current line during MF operation ; a lower value gives an alteration of the AC line impedance at low frequency. 47nF Matching to a C2 changes with the characteristics of the transmission line. Capacitive Line 82nF Receiving Gain C3 depends on balancing and line impedance versus frequency. Flatness 15nF Balance Network See note for R5, R6. 0.33F DC Filtering The C5 range is from 0.1 F to 0.47 F. The lowest value is ripple limited, the higher value is starting up time limited. 1000pF RF byPass 100F Receiving Output See note for R7, R7. DC Decoupling 1 F Receiving Input DC Decoupling 12/15 LS656 DIP16 PACKAGE MECHANICAL DATA DIM. Min. a1 B b b1 D E e e3 F i L Z 3.3 1.27 8.5 2.54 17.78 7.1 5.1 0.130 0.050 0.51 0.77 0.5 0.25 20 0.335 0.100 0.700 0.280 0.201 DIP16.TBL mm Typ. Max. Min. 0.020 1.65 0.030 inch Typ. Max. 0.065 0.020 0.010 0.787 a1 I b1 b Z B e3 e L E D 16 9 1 8 13/15 PM-DIP16.EPS F LS656 SO20 PACKAGE MECHANICAL DATA DIM. Min. A a1 a2 b b1 C c1 D E e e3 F L M S 7.4 0.5 12.6 10 1.27 11.43 7.6 1.27 0.75 8 (max.) o mm Typ. Max. 2.65 0.1 0.2 2.45 0.35 0.23 0.5 45 (typ.) 13.0 10.65 0.496 0.394 o inch Min. Typ. Max. 0.104 0.004 0.008 0.096 0.014 0.009 0.020 0.019 0.013 0.49 0.32 0.510 SO20L.TBL PM-SO20L.EPS 0.419 0.050 0.450 0.291 0.020 0.300 0.050 0.030 L a2 A C c1 e3 E D M 20 11 1 10 14/15 F a1 b e s b1 LS656 Information furnished is believed to be accurate and reliable. However, SGS-THOMSON Microelectronics assumes no responsibility 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 life support devices or systems without express written approval of SGS-THOMSON Microelectronics. (c) 1994 SGS-THOMSON Microelectronics - 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 - Thaliand - United Kingdom - U.S.A. 15/15 |
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