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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
DIP14
SO14
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. BLOCK DIAGRAM
ORDERING NUMBERS: L3281AB L3281AD1
June 1993
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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 VL IL Ptot Top Tj Parameter Line Voltage (3 ms pulse) Line Current Total Power Dissipation, Tamb = 55C Operating Temperature Junction Temperature 1.0 - 20 to 55 - 65 to 150 Value DIP-14 15 150 0.6 SO-14 Unit V mA W C C
THERMAL DATA
Symbol Rth j-amb Parameter Thermal Resistance Junction Ambient Max Value DIP-14 90 SO-14 130 Unit C/W
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L3281
TEST CIRCUITS Figure 1.
Figure 2.
Figure 3.
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L3281
ELECTRICAL CHARACTERISTICS IL = 20 to 100mA; R4 =( 51 // diode) + 33; T = 25C; f = 1kHz; Unless Otherwise Specified
Symbol Vl Parameter Line Voltage IL IL IL IL Test Condition = 6.5mA = 20mA = 50mA = 80mA 50 Vmi = 10mV; IL = 20mA Vmi = 10mV; IL = 70mA Vso = 700mV; IL = 20mA Vmi = 0V; IL = 50mA Vmi = 10mV IL = 20mA; Vri = 0.2V IL = 70mA; Vri = 0.2V Vro = 350mV; Load = 350 Vro = 300mV; IL = 10 mA Vri = 0V Load = 200; Vro = 50V Vmi = 10mV Vri = 0.2V IL = 6.5mA; THD = 5% IL = 6.5mA; THD = 5% Dialing Mode Speaking Mode Vmf IN = 10mV Vmf LN = 140mV Vbeep IN = 25mV 5.5 Vbeep IN = 100mV; IL = 20mA Iz = 1 mA 4.2 8.5 8 0.5 5.1 20 5 6.2 14.5 5 16 10 5 500 100 0.5 50 100 1 17.5 100 10 10 600 20 700 40 - 10.7 - 7.2 - 9.2 - 5.7 - 7.7 - 4.2 5 5 - 70 30 - 7.2 31.5 -5.7 33 -4.2 5 Min. Typ. 1.65 3.4 6.0 8 Max. 3.7 6.5 9.5 Unit V V V V dB dB dB % dB k dB dB % % V dB mV mAp A A dB k % dB k % V A
CMRR Gtx DGtx THDtx N tx Zml Grx DGrx THD rx N rx Zro Zm Vso Iro MU lo MU hl Gmf Rmf THDmf Gbeep R beep THD beep Vz Ileak
Common Mode Rej. Ratio Sending Gain Delta Sending Gain Sending Distortion Sending Noise Mic. Input Impedance Receiving Gain Delta Receiving Gain Receiving Distortion Receiving Noise Rec. Output Impedance Sidetone Line Match. Impedance Sending Output Voltage Receiving Output Current Mute Input Low Mute Input Open DTMF Gain DTMF Input Impedance DTMF Distortion Beeptone Gain Beeptone Input Impedance Beeptone Distortion Zener Voltage (Pin 5) Leakage Current,V pin5 = 3V
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L3281
LOGIC OF MUTE SWITCHING
MUTE LOW (DIAL) OPEN (SPEECH) DTMF ACTIVE TO LINE OUTPUT ACTIVE TO LINE OUTPUT BEEP ACTIVE TO EARPHONE OUTPUT ACTIVE TO EARPHONE OUTPUT ACTIVE ACTIVE MIC IMP MUTED RX IMP MUTED
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 Figure 4: 2/4 Wire Conversion
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.
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L3281
DC CHARACTERISTIC The fig.5 shows the equivalent simplified circuit of the DC regulator that provides to give the opportune DC impedance Zdc. 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
VL =
Idc Z4
RB
(RA + RB) + VD + VR1
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 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 :
Z4
Figure 7: Zener Equivalent.
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
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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 F to 100 F. 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 K to allow Figure 8: Equivalent Transmitting Circuit. 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 9: Equivalent Receiving Circuit.
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L3281
DIP14 PACKAGE MECHANICAL DATA
DIM. MIN. a1 B b b1 D E e e3 F I L Z 1.27 3.3 2.54 0.050 8.5 2.54 15.24 7.1 5.1 0.130 0.100 0.51 1.39 0.5 0.25 20 0.335 0.100 0.600 0.280 0.201 1.65 mm TYP. MAX. MIN. 0.020 0.055 0.020 0.010 0.787 0.065 inch TYP. MAX.
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L3281
SO14 PACKAGE MECHANICAL DATA
DIM. A a1 a2 b b1 C c1 D E e e3 F L M S 3.8 0.5 8.55 5.8 1.27 7.62 4.0 1.27 0.68 8 (max.) 0.15 0.020 8.75 6.2 0.35 0.19 0.5 45 (typ.) 0.336 0.228 0.050 0.300 0.157 0.050 0.027 0.344 0.244 0.1 mm MIN. TYP. MAX. 1.75 0.2 1.6 0.46 0.25 0.014 0.007 0.020 0.004 MIN. inch TYP. MAX. 0.069 0.008 0.063 0.018 0.010
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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. (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.
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