L3281 LOW VOLTAGE TELEPHONE SPEECH CIRCUITS PRELIMINARY DATA OPERATION DOWN TO 1.6V / 6.5mA DTMF & BEEP TONE INPUTS EXTERNAL MUTING FOR EARPHONE AND MICROPHONE SUITABLE FOR DYNAMIC EARPHONE AND DYNAMIC OR ELECTRET MICROPHONE AGC CONTROL ON BOTH SENDING AND RECEIVING DESCRIPTION The L3281 is an electronic speech circuit developed to replace hybrid circuits in telephone sets that can be operated in parallel with other phones. DIP14 SO14 ORDERING NUMBERS: L3281AB L3281AD1 BLOCK DIAGRAM June 1993 1/10 This is advanced information on a new product now in development or undergoing evaluation. Details are subject to change without notice. L3281 PIN CONNECTION (top view) ABSOLUTE MAXIMUM RATINGS Symbol Parameter Value DIP-14 SO-14 Unit VL Line Voltage (3 ms pulse) 15 V IL Line Current 150 mA Ptot Total Power Dissipation, Tamb = 55°C Top Operating Temperature 1.0 – 20 to 55 0.6 °C W Tj Junction Temperature – 65 to 150 °C THERMAL DATA Symbol Rth j-amb 2/10 Parameter Thermal Resistance Junction Ambient Max Value DIP-14 SO-14 90 130 Unit °C/W L3281 TEST CIRCUITS Figure 1. Figure 2. Figure 3. 3/10 L3281 ELECTRICAL CHARACTERISTICS IL = 20 to 100mA; R4 =( 51Ω // diode) + 33Ω; T = 25°C; f = 1kHz; Unless Otherwise Specified Symbol Vl CMRR Gtx Parameter Line Voltage Test Condition IL IL IL IL Min. = 6.5mA = 20mA = 50mA = 80mA Common Mode Rej. Ratio Typ. Max. Unit 1.65 3.4 6.0 8 3.7 6.5 9.5 V V V V 50 dB Sending Gain Vmi = 10mV; IL = 20mA 30 31.5 33 dB DGtx Delta Sending Gain Vmi = 10mV; IL = 70mA – 7.2 –5.7 –4.2 dB THDtx Sending Distortion Vso = 700mV; IL = 20mA 5 % N tx Sending Noise Vmi = 0V; IL = 50mA Zml Mic. Input Impedance Vmi = 10mV Grx – 70 dB 40 kΩ Receiving Gain IL = 20mA; Vri = 0.2V – 10.7 – 9.2 – 7.7 dB DGrx Delta Receiving Gain IL = 70mA; Vri = 0.2V – 7.2 – 5.7 – 4.2 dB THD rx Receiving Distortion Vro = 350mV; Load = 350Ω Vro = 300mV; IL = 10 mA 5 5 % % N rx Receiving Noise Vri = 0V Zro Rec. Output Impedance Load = 200Ω; Vro = 50V 10 Sidetone Vmi = 10mV 10 20 600 700 Zm Line Match. Impedance Vri = 0.2V 500 Vso Sending Output Voltage IL = 6.5mA; THD = 5% 100 Iro 0.5 Receiving Output Current IL = 6.5mA; THD = 5% MU lo Mute Input Low Dialing Mode MU hl Mute Input Open Speaking Mode Gmf DTMF Gain Vmf IN = 10mV Rmf DTMF Input Impedance THDmf DTMF Distortion Vmf LN = 140mV Gbeep Beeptone Gain Vbeep IN = 25mV R beep Beeptone Input Impedance Vbeep IN = 100mV; IL = 20mA Vz Zener Voltage (Pin 5) Iz = 1 mA Ileak Leakage Current,V pin5 = 3V 4/10 Ω dB Ω mV mAp µA 50 100 1 µA 14.5 16 17.5 dB 5 10 kΩ 5 8.5 5.5 Beeptone Distortion THD beep µV 100 4.2 8 kΩ 0.5 5 5.1 6.2 20 % dB % V µA L3281 LOGIC OF MUTE SWITCHING MUTE DTMF BEEP MIC IMP RX IMP LOW ACTIVE TO LINE ACTIVE TO EARPHONE MUTED MUTED ACTIVE ACTIVE (DIAL) OUTPUT OUTPUT OPEN ACTIVE TO LINE ACTIVE TO EARPHONE (SPEECH) OUTPUT OUTPUT CIRCUIT DESCRIPTION TWO TO FOUR WIRE CONVERSION The L3281AB is based on a Wheastone bridge configuration. To balance the bridge the following relation must be satisfied: ZI / / Zm = R2 Zb R3 The AC signal from the microphone is sent to one diagonal of the bridge (pins 1 and 3). A small percentage of the signal power is lost on Zb (being Zb > (Zm//Zi)); the main part is sent to the line via R2. In receiving mode, the AC signal coming from the LINE is sensed across the second diagonal of the bridge (pins 12 and 2). The impedance Zm and Zb can be complex. Figure 4: 2/4 Wire Conversion 5/10 L3281 DC CHARACTERISTIC The fig.5 shows the equivalent simplified circuit of the DC regulator that provides to give the opportune DC impedance Zdc. Idc Z4 ⋅ VL = RB ⋅ ⋅ (RA + RB) + VD + VR1 equivalent. The zener voltage will be: 70K Vz = + 1 Vbe 13.6K It is possible to supply 1mA to the electrete voltage if VL > (1mA + Iz) ⋅ Rz + Vz ⋅ Figure 6: Low Voltage Speech Circuit. ⋅ RA + 1 + VD + VR1 VL = (Idc Z4) RB since RA = RB ⋅ ⋅ VL = (Idc Z4 2) + VD + VR1 When IL 18 mA and considering neglectible the VD + VR1 variation versus line current : ∆ VL ZDC = = 2 Z4 ∆ Idc At IL = 6.5 mA no current flows through Z4 but only in the rest of the circuit for internal biasing (Io;Ia). The bias current Io is fixed by the resistor R2. The line voltage in this case is : ⋅ VL = Ia RA + VR1 = 1.6 V The Fig.6 shows the DC characteristic (voltage between pin 12 and pin 3 versus line current). The device own an equivalent zener voltage at pin 5 that can be used as supply voltage for electret microphone (see Block Diagram). The value of the resistor R2 and the capacitor C2 should be chosen in order to not affect the AC line inpedance. The Fig.7 shows the zener Figure 5: Equivalent Simplified Circuit 6/10 Figure 7: Zener Equivalent. L3281 AC CHARACTERISTIC The AC Impedance measured at line terminals is equal to: 1 Zm = (R1 + ) / / ( R2 + R3 + Zb ) jwC1 The value of the capacitor C1 must be In the range of 22 µF to 100 µF. The external resistor R1 can be replaced by a resistor/capacitor network in order to realize a complex Impedance Zm. TRANSMITTING CIRCUIT The first block of the TX stage is basically a differential amplifier which converts voltage to current. The inputs are internally polarized at 300 mVdc. The differential Input impedance is 60 KΩ to allow a good matching to microphone. The AGC in TX is function of voltage at pin 14 in order to decrease to max gain of 5.5dB to 6.0dB when the line current increases. RECEIVING CIRCUIT Fig.9 shows the equivalent receiving circuit. The differential input of RX signal across R2+R3 is transferred to the AGC block when the mute signal (pin 10) is not active. The AGC in RX is a function of the voltage at pin 14 and decreases the gain when the line current increases (5.5dB to 6.0dB). The final stage is a single ended amplifier with low output impedance optimized to drive magnetic/dynamic transducers. Figure 8: Equivalent Transmitting Circuit. Figure 9: Equivalent Receiving Circuit. 7/10 L3281 DIP14 PACKAGE MECHANICAL DATA mm DIM. MIN. a1 0.51 B 1.39 TYP. MAX. MIN. TYP. MAX. 0.020 1.65 0.055 0.065 b 0.5 0.020 b1 0.25 0.010 D 20 0.787 E 8.5 0.335 e 2.54 0.100 e3 15.24 0.600 F 7.1 0.280 I 5.1 0.201 L Z 8/10 inch 3.3 1.27 0.130 2.54 0.050 0.100 L3281 SO14 PACKAGE MECHANICAL DATA DIM. mm MIN. TYP. A a1 inch MAX. MIN. TYP. 1.75 0.1 0.069 0.2 a2 MAX. 0.004 0.008 1.6 0.063 b 0.35 0.46 0.014 0.018 b1 0.19 0.25 0.007 0.010 C 0.5 0.020 c1 45° (typ.) D 8.55 8.75 0.336 0.344 E 5.8 6.2 0.228 0.244 e 1.27 0.050 e3 7.62 0.300 F 3.8 4.0 0.15 0.157 L 0.5 1.27 0.020 0.050 M S 0.68 0.027 8° (max.) 9/10 L3281 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. 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. 10/10