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LM12CL データシート(PDF) 6 Page - National Semiconductor (TI) |
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LM12CL データシート(HTML) 6 Page - National Semiconductor (TI) |
6 / 14 page Application Information (Continued) aid stability. In all cases, the op amp will behave predictably only if the supplies are properly bypassed, ground loops are controlled and high-frequency feedback is derived directly from the output terminal, as recommended earlier. So-called capacitive loads are not always capacitive. A high-Q capacitor in combination with long leads can present a series-resonant load to the op amp. In practice, this is not usually a problem; but the situation should be kept in mind. Large capacitive loads (including series-resonant) can be accommodated by isolating the feedback amplifier from the load as shown above. The inductor gives low output imped- ance at lower frequencies while providing an isolating im- pedance at high frequencies. The resistor kills the Q of se- ries resonant circuits formed by capacitive loads. A low inductance, carbon-composition resistor is recommended. Optimum values of L and R depend upon the feedback gain and expected nature of the load, but are not critical. A 4 µH inductor is obtained with 14 turns of number 18 wire, close spaced, around a one-inch-diameter form. The LM12 can be made stable for all loads with a large ca- pacitor on the output, as shown above. This compensation gives the lowest possible closed-loop output impedance at high frequencies and the best load-transient response. It is appropriate for such applications as voltage regulators. A feedback capacitor, C 1, is connected directly to the output pin of the IC. The output capacitor, C 2, is connected at the output terminal with short leads. Single-point grounding to avoid dc and ac ground loops is advised. The impedance, Z 1, is the wire connecting the op amp output to the load capacitor. About 3-inches of number-18 wire (70 nH) gives good stability and 18-inches (400 nH) begins to degrade load-transient response. The minimum load ca- pacitance is 47 µF, if a solid-tantalum capacitor with an equivalent series resistance (ESR) of 0.1 Ω is used. Electro- lytic capacitors work as well, although capacitance may have to be increased to 200 µF to bring ESR below 0.1 Ω. Loop stability is not the only concern when op amps are op- erated with reactive loads. With time-varying signals, power dissipation can also increase markedly. This is particularly true with the combination of capacitive loads and high-frequency excitation. INPUT COMPENSATION The LM12 is prone to low-amplitude oscillation bursts com- ing out of saturation if the high-frequency loop gain is near unity. The voltage follower connection is most susceptible. This glitching can be eliminated at the expense of small-signal bandwidth using input compensation. Input compensation can also be used in combination with LR load isolation to improve capacitive load stability. An example of a voltage follower with input compensation is shown here. The R 2C2 combination across the input works with R 1 to reduce feedback at high frequencies without greatly affecting response below 100 kHz. A lead capacitor, C 1, improves phase margin at the unity-gain crossover fre- quency. Proper operation requires that the output impedance of the circuitry driving the follower be well under 1 k Ω at fre- quencies up to a few hundred kilohertz. Extending input compensation to the integrator connection is shown here. Both the follower and this integrator will handle 1 µF capacitive loading without LR output isolation. CURRENT DRIVE This circuit provides an output current proportional to the in- put voltage. Current drive is sometimes preferred for servo motors because it aids in stabilizing the servo loop by reduc- ing phase lag caused by motor inductance. In applications requiring high output resistance, such as operational power supplies running in the current mode, matching of the feed- back resistors to 0.01% is required. Alternately, an adjust- able resistor can be used for trimming. DS008704-7 DS008704-8 DS008704-9 DS008704-10 DS008704-11 www.national.com 6 |
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同様の説明 - LM12CL |
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