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LM12CLK Arkusz danych(PDF) 5 Page - National Semiconductor (TI) |
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LM12CLK Arkusz danych(HTML) 5 Page - National Semiconductor (TI) |
5 / 14 page Application Information (Continued) wide variety of designs with all sorts of fault conditions. A few simple precautions will eliminate these problems. One would do well to read the section on supply bypassing, lead inductance, output clamp diodes, ground loops and reactive loading before doing any experimentation. Should there be problems with erratic operation, blow-outs, excessive distortion or oscillation, another look at these sections is in order. The management and protection circuitry can also affect op- eration. Should the total supply voltage exceed ratings or drop below 15–20V, the op amp shuts off completely. Case temperatures above 150˚C also cause shut down until the temperature drops to 145˚C. This may take several seconds, depending on the thermal system. Activation of the dynamic safe-area protection causes both the main feedback loop to lose control and a reduction in output power, with possible oscillations. In ac applications, the dynamic protection will cause waveform distortion. Since the LM12 is well protected against thermal overloads, the suggestions for determining power dissipation and heat sink requirements are presented last. SUPPLY BYPASSING All op amps should have their supply leads bypassed with low-inductance capacitors having short leads and located close to the package terminals to avoid spurious oscillation problems. Power op amps require larger bypass capacitors. The LM12 is stable with good-quality electrolytic bypass ca- pacitors greater than 20 µF. Other considerations may re- quire larger capacitors. The current in the supply leads is a rectified component of the load current. If adequate bypassing is not provided, this distorted signal can be fed back into internal circuitry. Low distortion at high frequencies requires that the supplies be bypassed with 470 µF or more, at the package terminals. LEAD INDUCTANCE With ordinary op amps, lead-inductance problems are usu- ally restricted to supply bypassing. Power op amps are also sensitive to inductance in the output lead, particularly with heavy capacitive loading. Feedback to the input should be taken directly from the output terminal, minimizing common inductance with the load. Sensing to a remote load must be accompanied by a high-frequency feedback path directly from the output terminal. Lead inductance can also cause voltage surges on the supplies. With long leads to the power source, energy stored in the lead inductance when the out- put is shorted can be dumped back into the supply bypass capacitors when the short is removed. The magnitude of this transient is reduced by increasing the size of the bypass ca- pacitor near the IC. With 20 µF local bypass, these voltage surges are important only if the lead length exceeds a couple feet (> 1 µH lead inductance). Twisting together the supply and ground leads minimizes the effect. GROUND LOOPS With fast, high-current circuitry, all sorts of problems can arise from improper grounding. In general, difficulties can be avoided by returning all grounds separately to a common point. Sometimes this is impractical. When compromising, special attention should be paid to the ground returns for the supply bypasses, load and input signal. Ground planes also help to provide proper grounding. Many problems unrelated to system performance can be traced to the grounding of line-operated test equipment used for system checkout. Hidden paths are particularly difficult to sort out when several pieces of test equipment are used but can be minimized by using current probes or the new iso- lated oscilloscope pre-amplifiers. Eliminating any direct ground connection between the signal generator and the os- cilloscope synchronization input solves one common prob- lem. OUTPUT CLAMP DIODES When a push-pull amplifier goes into power limit while driv- ing an inductive load, the stored energy in the load induc- tance can drive the output outside the supplies. Although the LM12 has internal clamp diodes that can handle several am- peres for a few milliseconds, extreme conditions can cause destruction of the IC. The internal clamp diodes are imper- fect in that about half the clamp current flows into the supply to which the output is clamped while the other half flows across the supplies. Therefore, the use of external diodes to clamp the output to the power supplies is strongly recom- mended. This is particularly important with higher supply voltages. Experience has demonstrated that hard-wire shorting the output to the supplies can induce random failures if these ex- ternal clamp diodes are not used and the supply voltages are above ±20V. Therefore it is prudent to use outputclamp di- odes even when the load is not particularly inductive. This also applies to experimental setups in that blowouts have been observed when diodes were not used. In packaged equipment, it may be possible to eliminate these diodes, pro- viding that fault conditions can be controlled. Heat sinking of the clamp diodes is usually unimportant in that they only clamp current transients. Forward drop with 15A fault transients is of greater concern. Usually, these transients die out rapidly. The clamp to the negative supply can have somewhat reduced effectiveness under worst case conditions should the forward drop exceed 1.0V. Mounting this diode to the power op amp heat sink improves the situ- ation. Although the need has only been demonstrated with some motor loads, including a third diode (D3 above) will eliminate any concern about the clamp diodes. This diode, however, must be capable of dissipating continuous power as determined by the negative supply current of the op amp. REACTIVE LOADING The LM12 is normally stable with resistive, inductive or smaller capacitive loads. Larger capacitive loads interact with the open-loop output resistance (about 1 Ω) to reduce the phase margin of the feedback loop, ultimately causing oscillation. The critical capacitance depends upon the feed- back applied around the amplifier; a unity-gain follower can handle about 0.01 µF, while more than 1 µF does not cause problems if the loop gain is ten. With loop gains greater than unity, a speedup capacitor across the feedback resistor will DS008704-6 www.national.com 5 |
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