K04RLM12 Datasheet, PDF (10/23 Pages) Texas Instruments – Operational Amplifier

K04RLM12, LM12CL
OBSOLETE
SNOSBY8D – MAY 1999 – REVISED APRIL 2013
www.ti.com
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 1Ω) to reduce the phase margin of the feedback loop, ultimately
causing oscillation. The critical capacitance depends upon the feedback 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 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 impedance at lower frequencies while providing an
isolating impedance at high frequencies. The resistor kills the Q of series 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 capacitor 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, C₁, is connected directly to the output pin of the IC. The output capacitor, C₂, is connected
at the output terminal with short leads. Single-point grounding to avoid dc and ac ground loops is advised.
The impedance, Z₁, 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 capacitance is 47 μF, if a solid-tantalum capacitor with an equivalent series resistance (ESR) of
0.1Ω is used. Electrolytic capacitors work as well, although capacitance may have to be increased to 200 μF to
bring ESR below 0.1Ω.
Loop stability is not the only concern when op amps are operated 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.
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