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LM2893N Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM2893N Datasheet(HTML) 10 Page - National Semiconductor (TI) |
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10 / 24 page  Component Selection (Continued) CC Capacitor CC’s primary function is to block the power line voltage from T1’s line-side winding Also CC and T1’s line- side winding comprise a LC highpass filter The self-induc- tance of T1 is far too low to support a direct line connection CC must have a low enough impedance at FO to allow T1 to drive transmitted energy onto the line To drive a 14X power line the impedance of CC should be below 14X Use Figure 9 to find the reactive impedance of CC to check that it is less than the line impedance Then check Figure 10 to see that the power line current is small enough to keep T1 well out of saturation the recommended transformers can withstand a 10 Amp-turn magnetizing force (1 Amp through the worst-case 10 turn line-side winding) Caution is required when choosing CC to avoid series reso- nance of the series combination of CC the transformer in- ductance and the reflected tank impedance The low resist- ance of the network under series resonance will load the line possibly decreasing range For your particular line cou- pling circuit measure for series resonance using some ex- pected line impedance load RB This base-bleed resistor turns QB off quickly - important since the amplifier output swing is about 200V ms An RB below about 24X will conduct excessive current and over- load the chip amplifier and is not recommended TLH6750 – 11 FIGURE 9 CC’s impedance should be as a rule-of-thumb smaller than the lowest expected line impedance RG This resistor in parallel with the internal 10X resistor fixes the current gain of the output amplifier and so the output current amplitude Figure 11 gives output current and mini- mum AC current gain hfe for QB when RG is used to boost output current QB The boost gain transistor QB must be fast Double-diffused devices with 50 MHz FT’s work slower transistors (epi-base types) do not preserve a sinusoidal waveform when FO is high or will cause the output amp to oscillate QB must have a certain minimum hfe for given boost levels as shown in Figure 11 Figure 12 shows the power QB must dissipate continuously operating with a shorted output BVCER (R e RB) must be 60V or greater and QB must have adequate SOA for transient survival ZT Unfortunately potentially damaging transient energy passes through transformer T1 onto the Carrier IO pin (instanta- neous power of greater than 1 kW has been measured us- ing the recommended transformers) For self protection the Carrier IO has an internal 44V voltage clamp with a 20X series resistance A parallel low impedance 44V external transient suppression diode will then conduct the lion’s share of any current when transients force the Carrier IO to a high voltage TLH6750 – 12 FIGURE 10 The AC line-induced current passed by CC TLH6750 – 13 FIGURE 11 Output amplifier current and required min QB hfe versus gain-setting resistor RG TLH6750 – 14 FIGURE 12 Boost transistor power dissipation versus amplifier output current ZT must be used unless some precaution is taken to protect the Carrier IO pin from line transients or transients caused when stored line energy in CC is discharged by the random phase of power line connection and disconnection Worst case CC may discharge a full peak-to-peak line voltage into the tuned circuit Another way to reduce the need for ZT is by placing another magnetic circuit in the signal path that relies on a high but easily saturated permeability to couple a primary and secondary winding - a toroidal transformer for example Toroids cost more than ZT Use an avalanche diode designed specifically for transient suppression they have orders of magnitude higher pulse 10 |
Similar Part No. - LM2893N |
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Similar Description - LM2893N |
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