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LM1893 Folha de dados(PDF) 11 Page - National Semiconductor (TI) |
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LM1893 Folha de dados(HTML) 11 Page - National Semiconductor (TI) |
11 / 24 page Component Selection (Continued) power capability than standard avalanche diodes rated for equal DC dissipation Metal oxide varistors have not proven useful because of their inferior clamping coefficient and are not recommended Specifications for an example minimum diode are given in Figure 13 Breakdown Voltage 44 – 49V 1mA Maximum Leakage 1mA 40V Capacitance 300 pF BV Maximum Clamp Voltage 645V 78A Peak Non-Repetitive Pulse Power 10 kW for 1 ms (REA Standard Exponential Pulse) Surge Current 70A for 1120s FIGURE 13 Key specifications for a recommended transient suppressor ZT available from General Semiconductor 2001 West Tenth Place Tempe AZ 85281 602 – 968-3101 part no SA40A RT RT acts as a voltage divider with ZT absorbing transient energy that attempts to pull the Carrier Input pin above 44V Make the resistor a carbon composition 14W When exper- iments discharging CC charged to the peak-to-peak 620V AC thru a 1X power line were carried out film resistors blew open-circuit DT This Schottky diode is placed in parallel with the CCT chip’s substrate diode to pass the majority of the current drawn from ground when the Carrier Input or Carrier Output is pulled below ground by a larger-than-twice-the supply-swing on the tank Note that ZT is in parallel with the substrate diode but is ineffective due to its high forward voltage drop and high diffusion capacitance caused by its low forward speed Tests proved that a 1N5818 kept a receive-path functional with a 20X boost transmitter with a 71 transform- er attempted to swing the receiver’s Carrier IO to g100V (300 mA peak ground current in the receiver) Without DT the receiver momentarily stops functioning at a 100 times lower ground current This diode is not needed if the Carrier IO never swings below ground If your CCT systems all run on the same regulated voltage with all matched transformers and turns ratios it is not needed Otherwise it is THE RECEIVER The receiver and transmitter share components CC T1 CQ RT ZT CO RO and peripheral supply and bias components that are not in need of change for RX mode operation Val- ues for the balance of the components are now found Line-Frequency Rejection To use the ultimate sensitivity of the device fully 110 dB of 115 V 60 Hz attenuation is required between the line and the limiter amplifier output Using the circuit topology of Fig- ure 4 the combined attenuation of the CC T1 highpass the tuned transformer and the bandpass filter attenuation of the limiter amplifier give far more line rejection than the above-stated minimum However if some other CCT line coupling circuit is used line rejection will become important to the system designer Receiver input power supply rejection (PSRR) and common- mode rejection (CMRR) are one-in-the-same using the sup- ply-referenced signal input of Figure 4 Ripple swings both differential inputs of the Norton amp equally while the sin- gle-ended input signal swings only the positive input Overall PSRR consists of the input CMRR (set by the input stage component matching) and the ripple-frequency attenuation of the input amplifier bandpass response that passes carrier frequency but stops low frequencies A typical 1% resistor and 1 mV n-p-n mirror offsets give 26 dB of attenuation the bandpass gives 54 dB 120 Hz attenuation for an overall 80 dB PSRR to allow tens of volts of ripple before impacting ultimate sensitivity CC A value was chosen earlier Knowing T1’s secondary induc- tance allows a check of LC line attenuation using Figure 14 CL The Norton input limiter amplifier has a bandpass filter for enhanced receiver selectivity noise immunity and line fre- quency rejection The nominal response curve for FO e 50 kHz is shown in Figure 15 The 300 kHz pole is fixed The 50 kHz pole is set by CL’s value After CL is found the resulting line frequency attenuation is found for the bandpass filter Use Figure 15 to find a CL value given for FO The approxi- mate line frequency attenuation of the bandpass filter may then be found in Figure 16 Figure 15 returns a value for CL 33% larger than nominal giving a low frequency pole 33% low to allow for component tolerances TLH6750 – 15 FIGURE 14 The 60 Hz line rejection of the highpass filter made up of CC and T1’s line-side winding (neglecting capacitive coupling) TLH6750 – 16 TLH6750 – 17 FIGURE 15 Given FO CL is found Also shown is the input amplifier’s small signal amplitude response 11 |
Nº de peça semelhante - LM1893 |
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Descrição semelhante - LM1893 |
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