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LM12CL データシート(PDF) 5 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
部品番号 LM12CL
部品情報  80W Operational Amplifier
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メーカー  NSC [National Semiconductor (TI)]
ホームページ  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM12CL データシート(HTML) 5 Page - National Semiconductor (TI)

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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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