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LM12CL データシート(PDF) 11 Page - National Semiconductor (TI) |
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LM12CL データシート(HTML) 11 Page - National Semiconductor (TI) |
11 / 14 page Application Information (Continued) where Z L is the magnitude of the load impedance and θ its phase angle. Maximum average dissipation occurs below maximum output swing for θ < 40˚. The instantaneous power dissipation over the conducting half cycle of one output transistor is shown here. Power dis- sipation is near zero on the other half cycle. The output level is that resulting in maximum peak and average dissipation. Plots are given for a resistive and a series RL load. The latter is representative of a 4 Ω loudspeaker operating below reso- nance and would be the worst case condition in most audio applications. The peak dissipation of each transistor is about four times average. In ac applications, power capability is of- ten limited by the peak ratings of the power transistor. The pulse thermal resistance of the LM12 is specified for constant power pulse duration. Establishing an exact equivalency between constant-power pulses and those en- countered in practice is not easy. However, for sine waves, reasonable estimates can be made at any frequency by as- suming a constant power pulse amplitude given by: where φ = 60˚ and θ is the absolute value of the phase angle of Z L. Equivalent pulse width is tON ≅ 0.4τ for θ = 0 and tON ≅ 0.2τ for θ≥ 20˚, where τ is the period of the output wave- form. DISSIPATION DRIVING MOTORS A motor with a locked rotor looks like an inductance in series with a resistance, for purposes of determining driver dissipa- tion. With slow-response servos, the maximum signal ampli- tude at frequencies where motor inductance is significant can be so small that motor inductance does not have to be taken into account. If this is the case, the motor can be treated as a simple, resistive load as long as the rotor speed is low enough that the back emf is small by comparison to the supply voltage of the driver transistor. A permanent-magnet motor can build up a back emf that is equal to the output swing of the op amp driving it. Reversing this motor from full speed requires the output drive transistor to operate, initially, along a loadline based upon the motor resistance and total supply voltage. Worst case, this loadline will have to be within the continuous dissipation rating of the drive transistor; but system dynamics may permit taking ad- vantage of the higher pulse ratings. Motor inductance can cause added stress if system response is fast. Shunt- and series-wound motors can generate back emf’s that are considerably more than the total supply voltage, re- sulting in even higher peak dissipation than a permanent-magnet motor having the same locked-rotor re- sistance. VOLTAGE REGULATOR DISSIPATION The pass transistor dissipation of a voltage regulator is eas- ily determined in the operating mode. Maximum continuous dissipation occurs with high line voltage and maximum load current. As discussed earlier, ripple voltage can be averaged if peak ratings are not exceeded; however, a higher average voltage will be required to insure that the pass transistor does not saturate at the ripple minimum. Conditions during start-up can be more complex. If the input voltage increases slowly such that the regulator does not go into current limit charging output capacitance, there are no problems. If not, load capacitance and load characteristics must be taken into account. This is also the case if automatic restart is required in recovering from overloads. Automatic restart or start-up with fast-rising input voltages cannot be guaranteed unless the continuous dissipation rat- ing of the pass transistor is adequate to supply the load cur- rent continuously at all voltages below the regulated output voltage. In this regard, the LM12 performs much better than IC regulators using foldback current limit, especially with high-line input voltage above 20V. POWER LIMITING Should the power ratings of the LM12 be exceeded, dynamic safe-area protection is activated. Waveforms with this power limiting are shown for the LM12 driving ±26V at 30 Hz into 3 Ω in series with 24 mH (θ = 45˚). With an inductive load, the output clamps to the supplies in power limit, as above. With resistive loads, the output voltage drops in limit. Behavior with more complex RCL loads is between these extremes. Secondary thermal limit is activated should the case tem- perature exceed 150˚C. This thermal limit shuts down the IC completely (open output) until the case temperature drops to about 145˚C. Recovery may take several seconds. POWER SUPPLIES Power op amps do not require regulated supplies. However, the worst-case output power is determined by the low-line supply voltage in the ripple trough. The worst-case power dissipation is established by the average supply voltage with high-line conditions. The loss in power output that can be guaranteed is the square of the ratio of these two voltages. Relatively simple off-line switching power supplies can pro- vide voltage conversion, line isolation and 5-percent regula- tion while reducing size and weight. The regulation against ripple and line variations can provide a substantial increase in the power output that can be guar- DS008704-26 DS008704-27 www.national.com 11 |
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同様の説明 - LM12CL |
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