LM12CL National Semiconductor, LM12CL Datasheet - Page 6

• Manufacturer Part Number: LM12CL
• Description: 80W Operational Amplifier
• Manufacturer: National Semiconductor

www.national.com
Application Information
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
pin of the IC. The output capacitor, C
output terminal with short leads. Single-point grounding to
avoid dc and ac ground loops is advised.
The impedance, Z
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
lytic capacitors work as well, although capacitance may have
to be increased to 200 µF to bring ESR below 0.1 .
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.
1
, is the wire connecting the op amp output
1
, is connected directly to the output
2
, is connected at the
(Continued)
DS008704-8
DS008704-7
is used. Electro-
6
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
with R
greatly affecting response below 100 kHz. A lead capacitor,
C
quency. Proper operation requires that the output impedance
of the circuitry driving the follower be well under 1 k
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.
1
, improves phase margin at the unity-gain crossover fre-
1
to reduce feedback at high frequencies without
2
C
2
combination across the input works
DS008704-10
DS008704-9
DS008704-11
at fre-