LM1818 Electronically Switched Audio Tape System General Description Features The LM1818 is a linear integrated circuit containing all of the active electronics necessary for building a tape recorder deck (excluding the bias oscillator). The electronic functions on the chip include: a microphone and playback preamplifier, record and playback amplifiers, a meter driving circuit, and an automatic input level control circuit. The IC features complete internal electronic switching between the record and playback modes of operation. The multipole switch used in previous systems to switch between record and playback modes is replaced by a single pole switch, thereby allowing for more flexibility and reliability in the recorder design.* Y Y Y Y Y Y Electronic record/play switching 85 dB power supply rejection Motional peak level meter circuitry Low noise preamplifier circuitry 3.5V to 18V supply operation Provision for external low noise input transistor *Monaural operation, Figure 9 . Typical Applications TL/H/7894 – 1 FIGURE 1. Stereo Application Circuit (Left Channel Shown), VS e 15V Order Number LM1818N See NS Package Number N20A C1995 National Semiconductor Corporation TL/H/7894 RRD-B30M115/Printed in U. S. A. LM1818 Electronically Switched Audio Tape System April 1987 Absolute Maximum Ratings Operating Temperature If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/Distributors for availability and specifications. Supply Voltage Package Dissipation, (Note 1) Storage Temperature 0§ C to a 70§ C Junction Temperature 150§ C Minimum Voltage on Any Pin Maximum Voltage on Pins 2 and 5 Maximum Current Out of Pin 14 Lead Temperature (Soldering, 10 sec.) 18V 1560 mW b 65§ C to a 150§ C b 0.1 VDC 0.1 VDC 5 mADC 260§ C Electrical Characteristics VCC e 6V, TA e 25§ C, See Test Circuits (Figures 2 and 3) Parameter Conditions Operating Supply Voltage Range Min Typ 3.5 Supply Current Test Circuit (Figure 2) Turn-ON Time Externally Programmable Playback Signal to Noise 5 Units 18 VDC 12 mA 400 ms DIN Eq. (3180 and 120 ms), 20 – 20 kHz, RS e 0, Unweighted, VREF e 1 mV at 400 Hz 74 dB Flat Gain, 20–20 kHz, RS e 0, ALC OFF, VREF e 1 mV at 1 kHz, Unweighted 69 dB Fast Turn-ON Charging Current Pins 16 and 17 200 mA Record and Playback Preamplifier Open Loop Voltage Gain f e 100 Hz 100 dB Preamplifier Input Impedance Pin 16 or Pin 17 50 kX Preamplifier Input Referred PSRR 1 kHz Ð Flat Gain 85 dB 0.5 V 0.5 mA 80 dB Record Signal to Noise 50 Max Bias Voltage on Pin 18 in Play Mode or Pin 15 in Record Mode Monitor Amplifier Input Bias Current Pins 11 and 12 Monitor Amplifier Open Loop Voltage Gain Record or Playback, f e 100 Hz Monitor Output Current Capability Pins 9 and 10, Source Current Available 400 750 mA Monitor Amplifier Output Swing RL e 10k, AC Load 1.2 1.65 Vrms THD, All Amplifiers At 1 kHz, 40 dB Closed Loop Gain 0.05 % Record-Playback Switching Time As in Test Circuit 50 ms Input ALC Range DVIN for DVOUT e 8 dB 40 dB 25 mVrms 2 kX Input Voltage on ALC Pin for Start of ALC Action ALC Input Impedance ALC Attack Time C13 e 10 mF 7 ms ALC Decay Time R17 e % , C13 e 10 mF 30 sec Meter Output Gain 100 mVrms at 1 kHz into Pin 4 Meter Output Current Capability 800 2 mVDC mADC Note 1: For operation in ambient temperatures above 25§ C, the device must be derated based on a 150§ C maximum junction temperature and a thermal resistance of 80§ C/W junction to ambient. 2 FIGURE 2. General Test Circuit TL/H/7894 – 2 Test Circuits 3 Test Circuits (Continued) TL/H/7894 – 3 FIGURE 3. Noise Test Circuit 4 Equivalent Schematic Diagram TL/H/7894 – 4 FIGURE 4 5 Typical Performance Characteristics Automatic Level Control (ALC) Response Characteristic Preamp Input Noise Voltage Preamp Input Noise Current TL/H/7894 – 5 Application Hints transistor. The amplifiers are stable for all gains above 5 and have a typical open loop gain of 100 dB. R8 and R9 enable C6 to be quickly charged and set the DC gain. Internal biasing provides a DC voltage independent of temperature at pin 17 so that the preamplifier DC output will remain relatively constant with temperature. Supply decoupling is provided by an internal regulator. Additional decoupling can be added for the input stages by increasing the size of the capacitor on pin 20 of the IC. A fast charging circuit is connected to the preamplifiers’ input capacitors (pins 16 and 17) to decrease the turn-ON time. Larger input capacitors decrease the noise by reducing the source impedance at lower frequencies where 1/f noise current produces an input noise voltage. The input resistance of the preamplifiers is typically 50 kX. PREAMPLIFIERS (Figure 5) There are 2 identical preamplifiers with 1 common output pin on the IC. One amplifies low level inputs such as a microphone in the record mode and another amplifies the signal from the playback head in the playback mode. The amplifiers use a common capacitor, C6, to set the low frequency pole of the closed loop responses. On the playback amplifier, the collector of the input device is made available so that an external low noise device can be connected in critical applications. When using an external low noise transistor, pins 17 and 18 of the IC are shorted together to ensure that the internal input transistor is turned OFF and the external transistor’s collector is tied to pin 19. The input and feedback connections are now made to the external input Quiescent DC Output Voltage # VDC e 1 a R9 R8 where RE e J (0.5 b 50 c 10b6R2)V if R2 a R3 l 10 RE R8R9 R8 a R9 AC Voltage Gain R4 a AAC e R3 1 a sC5R3 a1 R2 TL/H/7894 – 7 TL/H/7894–6 FIGURE 5. Preamplifier 6 Application Hints (Continued) ed transistor. The impedance of the saturated transistor forms a voltage divider with the source impedance of a series resistor (R1 in Figure 9 ). The input signal is decreased as the ALC transistor is increasingly forward biased. The ALC transistor will be forward biased when the preamplifiers’s AC output (pin 14), coupled to the combination ALCmeter drive input (pin 4) reaches 40 mV peak (25 mVrms). The gain of the ALC loop is such that a preamp input signal increase of 10 dB will result in a 2 dB increase on the AC output of the preamplifier. If greater than 25 mVrms is desired at the output of the preamp, a series resistor can be added between the preamp output coupling capacitor and the ALC input (pin 4). The input impedance of the ALC circuit is 2 kX; therefore, if a 2 kX series resistor is added, ALC action will begin at 50 mVrms. The ALC memory capacitor connected to pin 6 has the additional function of amplifier anti-pop control; for this reason, it is necessary that a capacitor be connected to pin 6 even if ALC is not used. MONITOR AND RECORD AMPLIFIERS (Figure 6) The monitor and record amplifiers share common input and feedback connections but have separate outputs. During playback, the input signal is amplified and appears only at the playback monitor output. Because the outputs are separate, different feedback components can be used and, as a result, totally different responses can be set. The amplifiers are stable for all closed loop gains above 3 and have an open loop gain of typically 80 dB. The outputs are capable of supplying a minimum of 400 mA into a load and swing within 500 mV of either VCC or ground. If more than 400 mA is needed to drive a load, an external pull-up resistor on the output of these amplifiers can increase the load driving capability. AUTOMATIC LEVEL CONTROLÐALC (Figure 7) The automatic level control provides a constant output level for a wide range of record source input levels. The ALC works on the varying impedance characteristic of a saturat- Record gain e 1 a R15 R14 Playback gain e 1 a R16 R14 TL/H/7894 – 8 FIGURE 6. Monitor Amplifier 7 Application Hints (Continued) TL/H/7894 – 9 FIGURE 7. Auto Level-Meter Circuit meter output pin is between 0 VDC and 0.7 VDC there is a 50 mA discharge current; when the pin is between 0.7V and 1.1V there is no internal discharge current; and when the voltage on pin 8 is greater than 1.1V there is a discharge equivalent to a 3.3k resistor across the memory capacitor. These different discharge rates allow the meter circuit to display fast, accurate responses on the lower portion of the meter display, slow responses in the higher portion of the meter display, and rapid discharge when the voltage is above the maximum reading the meter can display. The resistor in series with the meter can be adjusted such that the previously mentioned responses coincide with the proper points (0 VU and a 3 VU) on the meter scale. METER DRIVINGÐMOTIONAL PEAK LEVEL RESPONSE (Figure 7) The meter drive output (pin 8) is capable of supplying 1–2 mA at a filtered DC voltage that is typically equal to 10 times the RMS value of the signal applied to the ALC-meter drive input (pin 4). The RC network connected to pin 7 of the IC determines the memory constant of the meter circuit. It is therefore possible to store the peak input signal by giving this RC network a long time constant, or read the instantaneous signal level by giving this RC network a very short time constant (i.e., no capacitor). This memory capacitor is discharged within the integrated circuit at a discharge rate related to the DC level on the meter output pin. When the 8 Application Hints (Continued) Anti-Pop Circuitry (Figure 8) t2 e R13 C11 In VCC b CC The capacitor on pin 3 is used in a time delay system in conjunction with C13, the ALC capacitor, to suppress pops when switching between record and playback. Figure 8 illustrates how this is done. The output amplifier, either record or playback, is shut off prior to switching and carefully rebiased after switching takes place. It is therefore required that a proper ratio is selected between the ALC capacitor and the logic input RC time constant. The ALC capacitor must be discharged to 0.7V within the time it takes the logic input capacitor to: 1) charge from VCC/2 to 0.7 VCC when switching from record to playback, or 2) discharge from VCC/2 to 0.3 VCC when switching from playback to record. These times would normally be similar; however, the ALC capacitor can be charged to a different initial value depending upon the input to the ALC circuit. The maximum value to which the ALC memory capacitor will normally charge is 3.2V, therefore, the maximum time allowed for discharging C13 is given by: t1 e Ð 0.3 V ( R13 C11 In Ð 0.5 V ( VCC e 0.51 R13 C11 CC To be sure that C13 is completely discharged, let t2 l t1. R13 C11 l t1 (72 ms) e e 141 ms 0.51 0.51 If C11 e 10 mF, R13 e 15 kX R13 should be kept to a value less than 50 kX to insure that bias current existing from pin 3 does not cause an offset voltage above 200 mV. Typically this bias current is less than 3 mA. Record Playback Switch When the voltage on pin 3 of the IC is greater than 0.5 VCC, the internal record-playback switch switches into the playback mode. During playback the record preamplifier remains partially biased but the input signal to this preamp does not appear at the preamplifier output. In addition, during the playback mode, the record monitor output (pin 9) is disabled and the ALC circuit operates to minimize the signal into the record preamp input. The meter circuit is operational in the playback as well as the record mode. Similarly, during the record mode, the playback preamp input is ignored and the playback monitor output is disabled. In addition, a pin is available to hold one side of the record head at ground potential while sinking up to 500 mA of AC bias and record current. (3.2V b 0.7V) (C13 c DV) e C13 I1 350 mA e C13 c 7.2 c 104 If C13 e 10 mF, t1 e 72 ms It is now necessary to determine the minimum value for the R/P logic capacitor. This is done by computing the time between the 2 voltage switching points using the exponential equations for a single RC network. TL/H/7894 – 10 FIGURE 8A. Anti-Pop Circuit 9 Application Hints (Continued) TL/H/7894 – 11 FIGURE 8B. Waveform for Anti-Pop Circuit External Components (Refer to Figure 9 , Monaural Application Circuit) Component External Component Function Normal Range of Value R1 Used in conjunction with varying impedance of pin 5, forming a resistor divider network to reduce input level in automatic level control circuit. C2 Forms a noise reduction system by varying bandwidth as a function of the changing impedance on pin 5. With a small input signal, the bandwidth is reduced by R1 and C2. As the input level increases, so does the bandwidth. C1, C3 Coupling capacitors. Because these are part of the source impedance, it is important to use the larger values to keep low frequency source impedance at a minimum. C4 Radio frequency interference roll-off capacitor 100 pF – 300 pF R2 R3 R4 C5 Playback response equalization. C5 and R3 form a pole in the amplifier response at 50 Hz. C5 and R4 form a zero in the response at 1.3 kHz for 120 ms equalization and 2.3 kHz for 70 ms equalization. 50X – 200X 47 kX – 3.3 MX 2 kX – 200 kX R5 R6 Microphone preamplifier gain equalization 50X – 200X 5 kX – 200 kX R7 R8 R9 C6 C7 DC feedback path. Provides a low impedance path to the negative input in order to sink the 50 mA negative input amplifier current. C6, R9, R7 and C7 provide isolation from the output so that adequate gain can be obtained at 20 Hz. This 2-pole technique also provides fast turn-ON settling time. C8 Preamplifier output to monitor amplifier input coupling C9 ALC coupling capacitor. Note that ALC input impedance is 2 kX R10 R11 R12 C10 These components bias the monitor amplifier output to half supply since the amplifier is unity gain at DC. This allows for maximum output swing on a varying supply. 10 500X –20 kX 0.01 mF – 0.5 mF 0.5 mF –10 mF 0 – 2 kX 200X –5 kX 1 kX –30 kX 200 mF – 1000 mF 0 – 100 mF 0.05 mF –1 mF 0.1 mF –5 mF 10 kX – 100 kX 10 kX – 100 kX 10 kX – 100 kX 1 mF – 100 mF External Components (Refer to Figure 9 , Monaural Application Circuit) (Continued) Component External Component Function Normal Range of Value C11 R13 Exponentially falling or rising signal on pin 3 determines sequencing, time delay, and operational mode of the record/play anti-pop circuitry. See antipop diagram. 0 – 10 mF 0 –50 kX R14 R15 R16 C12 R16, R14 and C12 determine monitor amplifier response in the play mode. R15, R14 and C12 determine monitor amplifier response in the record mode. 1k – 100k 30 kX –3 MX 30 kX –3 MX 0.1 mF –20 mF C13 R17 Determines decay response on ALC characteristic and reduces amplifier pop 5 mF –20mF 100k – % C14 R18 Determines time constant of meter driving circuitry 0.1 mF –10 mF 100k – % R19 Meter sensitivity adjust 10 kX – 100 kX C15 Record output DC blocking capacitor C16 Play output DC blocking capacitor 0.1 mF –10 mF C17 R21 R22 Changes record output response to approximate a constant current output in conjunction with record head impedance resulting in proper recording equalization 500 pF – 0.1mF 5 kX – 100 kX 5 kX – 100 kX C18 Preamplifier supply decoupling capacitor. Note that large value capacitor will increase turn-ON time 0.1 mF – 500 mF C19 Supply decoupling capacitor C20 Decouples bias oscillator supply R23 Allows bias level adjustment R24 Adjusts DC erase current in DC erase machines (for AC erase, see ‘‘Stereo Application Circuit,’’ Figure 1 ) L1 C21 Optional bias trap C22 Bias Roll-Off H1 Record/play head H2 Erase head (DC type, AC optional) 1 mF –10 mF 100 mF – 1000 mF 10 mF – 500mF 0 – 1 kX 1 mH – 30 mH 100 pF – 2000 pF 0.001 mF – 0.01 mF 100X – 500X; 70 mH – 300 mH 10X – 300X 11 Typical Applications (Continued) TOKO America, Inc. 1250 Feehanville Drive Mount Prospect, IL 60056 TEL: (312) 297-0070 TL/H/7894 – 12 FIGURE 9A. Monaural Application Circuit 12 Typical Applications (Continued) TL/H/7894 – 13 TL/H/7894 – 14 FIGURE 9B. Level Diagram for Monaural Application Circuit 13 LM1818 Electronically Switched Audio Tape System Physical Dimensions inches (millimeters) Molded Dual-In-Line Package (N) Order Number LM1818N NS Package Number N20A LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform, when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. National Semiconductor Corporation 1111 West Bardin Road Arlington, TX 76017 Tel: 1(800) 272-9959 Fax: 1(800) 737-7018 2. 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