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Número de pieza LM12CLK
Descripción 80W Operational Amplifier
Fabricantes National Semiconductor 
Logotipo National Semiconductor Logotipo



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May 1999
LM12CL
80W Operational Amplifier
General Description
The LM12 is a power op amp capable of driving ±25V at
±10A while operating from ±30V supplies. The monolithic IC
can deliver 80W of sine wave power into a 4load with
0.01% distortion. Power bandwidth is 60 kHz. Further, a
peak dissipation capability of 800W allows it to handle reac-
tive loads such as transducers, actuators or small motors
without derating. Important features include:
input protection
controlled turn on
thermal limiting
overvoltage shutdown
output-current limiting
dynamic safe-area protection
The IC delivers ±10A output current at any output voltage
yet is completely protected against overloads, including
shorts to the supplies. The dynamic safe-area protection is
provided by instantaneous peak-temperature limiting within
the power transistor array.
The turn-on characteristics are controlled by keeping the
output open-circuited until the total supply voltage reaches
14V. The output is also opened as the case temperature ex-
ceeds 150˚C or as the supply voltage approaches the
BVCEO of the output transistors. The IC withstands overvolt-
ages to 80V.
This monolithic op amp is compensated for unity-gain feed-
back, with a small-signal bandwidth of 700 kHz. Slew rate is
9V/µs, even as a follower. Distortion and capacitive-load sta-
bility rival that of the best designs using complementary out-
put transistors. Further, the IC withstands large differential
input voltages and is well behaved should the
common-mode range be exceeded.
The LM12 establishes that monolithic ICs can deliver consid-
erable output power without resorting to complex switching
schemes. Devices can be paralleled or bridged for even
greater output capability. Applications include operational
power supplies, high-voltage regulators, high-quality audio
amplifiers, tape-head positioners, x-y plotters or other
servo-control systems.
The LM12 is supplied in a four-lead, TO-3 package with V−
on the case. A gold-eutectic die-attach to a molybdenum in-
terface is used to avoid thermal fatigue problems. The LM12
is specified for either military or commercial temperature
range.
Connection Diagram
Typical Application*
DS008704-1
4-pin glass epoxy TO-3
socket is available from
AUGAT INC.
Part number 8112-AG7
Bottom View
Order Number LM12CLK
See NS Package Number K04A
*Low distortion (0.01%) audio amplifier
DS008704-2
© 1999 National Semiconductor Corporation DS008704
www.national.com

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LM12CLK pdf
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.
DS008704-6
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) 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
5 www.national.com

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LM12CLK arduino
Application Information (Continued)
where ZL is the magnitude of the load impedance and θ its
phase angle. Maximum average dissipation occurs below
maximum output swing for θ < 40˚.
DS008704-26
The instantaneous power dissipation over the conducting
half cycle of one output transistor is shown here. Power dis-
sipation is near zero on the other half cycle. The output level
is that resulting in maximum peak and average dissipation.
Plots are given for a resistive and a series RL load. The latter
is representative of a 4loudspeaker operating below reso-
nance and would be the worst case condition in most audio
applications. The peak dissipation of each transistor is about
four times average. In ac applications, power capability is of-
ten limited by the peak ratings of the power transistor.
The pulse thermal resistance of the LM12 is specified for
constant power pulse duration. Establishing an exact
equivalency between constant-power pulses and those en-
countered in practice is not easy. However, for sine waves,
reasonable estimates can be made at any frequency by as-
suming a constant power pulse amplitude given by:
sulting in even higher peak dissipation than a
permanent-magnet motor having the same locked-rotor re-
sistance.
VOLTAGE REGULATOR DISSIPATION
The pass transistor dissipation of a voltage regulator is eas-
ily determined in the operating mode. Maximum continuous
dissipation occurs with high line voltage and maximum load
current. As discussed earlier, ripple voltage can be averaged
if peak ratings are not exceeded; however, a higher average
voltage will be required to insure that the pass transistor
does not saturate at the ripple minimum.
Conditions during start-up can be more complex. If the input
voltage increases slowly such that the regulator does not go
into current limit charging output capacitance, there are no
problems. If not, load capacitance and load characteristics
must be taken into account. This is also the case if automatic
restart is required in recovering from overloads.
Automatic restart or start-up with fast-rising input voltages
cannot be guaranteed unless the continuous dissipation rat-
ing of the pass transistor is adequate to supply the load cur-
rent continuously at all voltages below the regulated output
voltage. In this regard, the LM12 performs much better than
IC regulators using foldback current limit, especially with
high-line input voltage above 20V.
POWER LIMITING
where φ = 60˚ and θ is the absolute value of the phase angle
of ZL. Equivalent pulse width is tON 0.4τ for θ = 0 and tON
0.2τ for θ ≥ 20˚, where τ is the period of the output wave-
form.
DISSIPATION DRIVING MOTORS
A motor with a locked rotor looks like an inductance in series
with a resistance, for purposes of determining driver dissipa-
tion. With slow-response servos, the maximum signal ampli-
tude at frequencies where motor inductance is significant
can be so small that motor inductance does not have to be
taken into account. If this is the case, the motor can be
treated as a simple, resistive load as long as the rotor speed
is low enough that the back emf is small by comparison to
the supply voltage of the driver transistor.
A permanent-magnet motor can build up a back emf that is
equal to the output swing of the op amp driving it. Reversing
this motor from full speed requires the output drive transistor
to operate, initially, along a loadline based upon the motor
resistance and total supply voltage. Worst case, this loadline
will have to be within the continuous dissipation rating of the
drive transistor; but system dynamics may permit taking ad-
vantage of the higher pulse ratings. Motor inductance can
cause added stress if system response is fast.
Shunt- and series-wound motors can generate back emf’s
that are considerably more than the total supply voltage, re-
DS008704-27
Should the power ratings of the LM12 be exceeded, dynamic
safe-area protection is activated. Waveforms with this power
limiting are shown for the LM12 driving ±26V at 30 Hz into
3in series with 24 mH (θ = 45˚). With an inductive load, the
output clamps to the supplies in power limit, as above. With
resistive loads, the output voltage drops in limit. Behavior
with more complex RCL loads is between these extremes.
Secondary thermal limit is activated should the case tem-
perature exceed 150˚C. This thermal limit shuts down the IC
completely (open output) until the case temperature drops to
about 145˚C. Recovery may take several seconds.
POWER SUPPLIES
Power op amps do not require regulated supplies. However,
the worst-case output power is determined by the low-line
supply voltage in the ripple trough. The worst-case power
dissipation is established by the average supply voltage with
high-line conditions. The loss in power output that can be
guaranteed is the square of the ratio of these two voltages.
Relatively simple off-line switching power supplies can pro-
vide voltage conversion, line isolation and 5-percent regula-
tion while reducing size and weight.
The regulation against ripple and line variations can provide
a substantial increase in the power output that can be guar-
11 www.national.com

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