LM12CL80W Operational AmplifierGeneral DescriptionThe LM12 is a power op amp capable of driving ±25V at±10A while operating from ±30V supplies. The monolithic ICcan deliver 80W of sine wave power into a 4Ω load with0.01% distortion. Power bandwidth is 60 kHz. Further, apeak dissipation capability of 800W allows it to handle reac-tive loads such as transducers, actuators or small motorswithout derating. Important features include:

• input protection• controlled turn on• thermal limiting• overvoltage shutdown• output-current limiting• dynamic safe-area protectionThe IC delivers ±10A output current at any output voltageyet is completely protected against overloads, includingshorts to the supplies. The dynamic safe-area protection isprovided by instantaneous peak-temperature limiting withinthe power transistor array.

The turn-on characteristics are controlled by keeping theoutput open-circuited until the total supply voltage reaches14V. The output is also opened as the case temperatureexceeds 150˚C or as the supply voltage approaches theBVCEO 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 is9V/µs, even as a follower. Distortion and capacitive-loadstability rival that of the best designs using complementaryoutput transistors. Further, the IC withstands large differen-tial input voltages and is well behaved should the common-mode range be exceeded.

The LM12 establishes that monolithic ICs can deliver con-siderable output power without resorting to complex switch-ing schemes. Devices can be paralleled or bridged for evengreater output capability. Applications include operationalpower supplies, high-voltage regulators, high-quality audioamplifiers, 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 molybdenuminterface is used to avoid thermal fatigue problems. TheLM12 is specified for either military or commercial tempera-ture range.

Connection Diagram

00870401

4-pin glass epoxy TO-3

socket is available from

AUGAT INC.

Part number 8112-AG7

Bottom ViewOrder Number LM12CLK

See NS Package Number K04A

Typical Application*

00870402

*Low distortion (0.01%) audio amplifier

May 1999LM

12CL

80WO

perationalAm

plifier

© 2004 National Semiconductor Corporation DS008704 www.national.com

Absolute Maximum Ratings (Note 1)If Military/Aerospace specified devices are required,please contact the National Semiconductor Sales Office/Distributors for availability and specifications.

Total Supply Voltage (Note 1) 80V

Input Voltage (Note 2)

Output Current Internally Limited

Junction Temperature (Note 3)

Storage Temperature Range −65˚C to 150˚C

Lead Temperature

(Soldering, 10 seconds) 300˚C

Operating RatingsTotal Supply Voltage 15V to 60V

Case Temperature (Note 4) 0˚C to 70˚C

Electrical Characteristics (Note 4)Parameter Conditions Typ

25˚CLM12CL Units

Limits

Input Offset Voltage ±10V ≤ VS ≤ ±0.5 VMAX, VCM = 0 2 15/20 mV (max)Input Bias Current V− + 4V ≤ VCM ≤ V+ −2V 0.15 0.7/1.0 µA (max)Input Offset Current V− +4V ≤ VCM ≤ V+ −2V 0.03 0.2/0.3 µA (max)Common Mode V− +4V ≤ VCM ≤ V+ −2V 86 70/65 dB (min)Rejection

Power Supply V+ = 0.5 VMAX, 90 70/65 dB (min)

Rejection −6V ≥ V− ≥ −0.5 VMAXV− = −0.5 VMAX, 110 75/70 dB (min)

6V ≤ V+ ≤ 0.5 VMAXOutput Saturation tON = 1 ms,

Threshold ∆VIN = 5 (10 ) mV,IOUT = 1A 1.8 2.2/2.5 V (max)

8A 4 5/7 V (max)

10A 5 V (max)

Large Signal Voltage tON = 2 ms,

Gain VSAT = 2V, IOUT = 0 100 30/20 V/mV (min)

VSAT = 8V, RL = 4Ω 50 15/10 V/mV (min)Thermal Gradient PDISS = 50W, tON = 65 ms 30 100 µV/W (max)

Feedback

Output-Current Limit tON = 10 ms, VDISS = 10V 13 16 A (max)

tON = 100 ms, VDISS = 58V 1.5 0.9/0.6 A (min)

1.5 1.7 A (max)

Power Dissipation tON = 100 ms, VDISS = 20V 100 80/55 W (min)

Rating VDISS = 58V 80 52/35 W (min)

DC Thermal Resistance (Note 5) VDISS = 20V 2.3 2.9 ˚C/W (max)

VDISS = 58V 2.7 4.5 ˚C/W (max)

AC Thermal Resistance (Note 5) 1.6 2.1 ˚C/W (max)

Supply Current VOUT = 0, IOUT = 0 60 120/140 mA (max)

Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. The maximum voltage for which the LM12 is guaranteed tooperate is given in the operating ratings and in Note 4. With inductive loads or output shorts, other restrictions described in applications section apply.

Note 2: Neither input should exceed the supply voltage by more than 50 volts nor should the voltage between one input and any other terminal exceed 60 volts.

Note 3: Operating junction temperature is internally limited near 225˚C within the power transistor and 160˚C for the control circuitry.

Note 4: The supply voltage is ±30V (VMAX = 60V), unless otherwise specified. The voltage across the conducting output transistor (supply to output) is VDISS andinternal power dissipation is PDISS. Temperature range is 0˚C ≤ TC ≤ 70˚C where TC is the case temperature. Standard typeface indicates limits at 25˚C whileboldface type refers to limits or special conditions over full temperature range. With no heat sink, the package will heat at a rate of 35˚C/sec per 100W ofinternal dissipation.

Note 5: This thermal resistance is based upon a peak temperature of 200˚C in the center of the power transistor and a case temperature of 25˚C measured at thecenter of the package bottom. The maximum junction temperature of the control circuitry can be estimated based upon a dc thermal resistance of 0.9˚C/W or an acthermal resistance of 0.6˚C/W for any operating voltage.

Although the output and supply leads are resistant to electrostatic discharges from handling, the input leads are not.The part should be treated accordingly.

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Output-Transistor Ratings (guaranteed)Safe Area DC Thermal Resistance

00870431 00870432

Pulse Thermal Resistance

00870433

Typical Performance CharacteristicsPulse Power Limit Pulse Power Limit

00870434 00870435

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Typical Performance Characteristics (Continued)

Peak Output Current Output Saturation Voltage

00870436 00870437

Large Signal Response Follower Pulse Response

00870438 00870439

Large Signal Gain Thermal Response

00870440 00870441

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Typical Performance Characteristics (Continued)

Total Harmonic Distortion Frequency Response

00870442 00870443

Output Impedance Power Supply Rejection

00870444 00870445

Input Bias Current Input Noise Voltage

00870446 00870447

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Typical Performance Characteristics (Continued)

Common Mode Rejection Supply Current

00870448 00870449

Supply Current Cross-Supply Current

00870450 00870451

Application Information

GENERAL

Twenty five years ago the operational amplifier was a spe-cialized design tool used primarily for analog computation.However, the availability of low cost IC op amps in the late1960’s prompted their use in rather mundane applications,replacing a few discrete components. Once a few basicprinciples are mastered, op amps can be used to give ex-ceptionally good results in a wide range of applications whileminimizing both cost and design effort.

The availability of a monolithic power op amp now promisesto extend these advantages to high-power designs. Someconventional applications are given here to illustrate op ampdesign principles as they relate to power circuitry. The inevi-table fall in prices, as the economies of volume productionare realized, will prompt their use in applications that mightnow seem trivial. Replacing single power transistors with anop amp will become economical because of improved per-formance, simplification of attendant circuitry, vastly im-proved fault protection, greater reliability and the reduction ofdesign time.

Power op amps introduce new factors into the design equa-tion. With current transients above 10A, both the inductanceand resistance of wire interconnects become important in anumber of ways. Further, power ratings are a crucial factor indetermining performance. But the power capability of the IC

cannot be realized unless it is properly mounted to an ad-equate heat sink. Thus, thermal design is of major impor-tance with power op amps.

This application summary starts off by identifying the originof strange problems observed while using the LM12 in awide variety of designs with all sorts of fault conditions. A fewsimple precautions will eliminate these problems. Onewould do well to read the section on supply bypassing,lead inductance, output clamp diodes, ground loops andreactive loading before doing any experimentation.Should there be problems with erratic operation, blow-outs, excessive distortion or oscillation, another look atthese sections is in order.

The management and protection circuitry can also affectoperation. Should the total supply voltage exceed ratings ordrop below 15–20V, the op amp shuts off completely. Casetemperatures above 150˚C also cause shut down until thetemperature drops to 145˚C. This may take several seconds,depending on the thermal system. Activation of the dynamicsafe-area protection causes both the main feedback loop tolose control and a reduction in output power, with possibleoscillations. In ac applications, the dynamic protection willcause waveform distortion. Since the LM12 is well protectedagainst thermal overloads, the suggestions for determiningpower dissipation and heat sink requirements are presentedlast.

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Application Information (Continued)SUPPLY BYPASSING

All op amps should have their supply leads bypassed withlow-inductance capacitors having short leads and locatedclose to the package terminals to avoid spurious oscillationproblems. Power op amps require larger bypass capacitors.The LM12 is stable with good-quality electrolytic bypasscapacitors greater than 20 µF. Other considerations mayrequire larger capacitors.

The current in the supply leads is a rectified component ofthe load current. If adequate bypassing is not provided, thisdistorted signal can be fed back into internal circuitry. Lowdistortion at high frequencies requires that the supplies bebypassed 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 alsosensitive to inductance in the output lead, particularly withheavy capacitive loading. Feedback to the input should betaken directly from the output terminal, minimizing commoninductance with the load. Sensing to a remote load must beaccompanied by a high-frequency feedback path directlyfrom the output terminal. Lead inductance can also causevoltage surges on the supplies. With long leads to the powersource, energy stored in the lead inductance when the out-put is shorted can be dumped back into the supply bypasscapacitors when the short is removed. The magnitude of thistransient is reduced by increasing the size of the bypasscapacitor near the IC. With 20 µF local bypass, these voltagesurges are important only if the lead length exceeds a couplefeet (> 1 µH lead inductance). Twisting together the supplyand ground leads minimizes the effect.

GROUND LOOPS

With fast, high-current circuitry, all sorts of problems canarise from improper grounding. In general, difficulties can beavoided by returning all grounds separately to a commonpoint. Sometimes this is impractical. When compromising,special attention should be paid to the ground returns for thesupply bypasses, load and input signal. Ground planes alsohelp to provide proper grounding.

Many problems unrelated to system performance can betraced to the grounding of line-operated test equipment usedfor system checkout. Hidden paths are particularly difficult tosort out when several pieces of test equipment are used butcan be minimized by using current probes or the new iso-lated oscilloscope pre-amplifiers. Eliminating any directground connection between the signal generator and theoscilloscope synchronization input solves one commonproblem.

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 theLM12 has internal clamp diodes that can handle severalamperes for a few milliseconds, extreme conditions cancause destruction of the IC. The internal clamp diodes areimperfect in that about half the clamp current flows into thesupply to which the output is clamped while the other halfflows across the supplies. Therefore, the use of externaldiodes to clamp the output to the power supplies is stronglyrecommended. This is particularly important with higher sup-ply voltages.

Experience has demonstrated that hard-wire shorting theoutput to the supplies can induce random failures if theseexternal clamp diodes are not used and the supply voltagesare above ±20V. Therefore it is prudent to use outputclampdiodes even when the load is not particularly inductive. Thisalso applies to experimental setups in that blowouts havebeen observed when diodes were not used. In packagedequipment, it may be possible to eliminate these diodes,providing that fault conditions can be controlled.

00870406

Heat sinking of the clamp diodes is usually unimportant inthat they only clamp current transients. Forward drop with15A fault transients is of greater concern. Usually, thesetransients die out rapidly. The clamp to the negative supplycan have somewhat reduced effectiveness under worst caseconditions should the forward drop exceed 1.0V. Mountingthis diode to the power op amp heat sink improves thesituation. Although the need has only been demonstratedwith some motor loads, including a third diode (D3 above)will eliminate any concern about the clamp diodes. Thisdiode, however, must be capable of dissipating continuouspower as determined by the negative supply current of theop amp.

REACTIVE LOADING

The LM12 is normally stable with resistive, inductive orsmaller capacitive loads. Larger capacitive loads interactwith the open-loop output resistance (about 1Ω) to reducethe phase margin of the feedback loop, ultimately causingoscillation. The critical capacitance depends upon the feed-back applied around the amplifier; a unity-gain follower canhandle about 0.01 µF, while more than 1 µF does not causeproblems if the loop gain is ten. With loop gains greater thanunity, a speedup capacitor across the feedback resistor willaid stability. In all cases, the op amp will behave predictablyonly if the supplies are properly bypassed, ground loops arecontrolled and high-frequency feedback is derived directlyfrom the output terminal, as recommended earlier.

So-called capacitive loads are not always capacitive. Ahigh-Q capacitor in combination with long leads can presenta series-resonant load to the op amp. In practice, this is notusually a problem; but the situation should be kept in mind.

00870407

Large capacitive loads (including series-resonant) can beaccommodated by isolating the feedback amplifier from theload as shown above. The inductor gives low output imped-ance at lower frequencies while providing an isolating im-

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Application Information (Continued)pedance at high frequencies. The resistor kills the Q ofseries resonant circuits formed by capacitive loads. A lowinductance, carbon-composition resistor is recommended.Optimum values of L and R depend upon the feedback gainand expected nature of the load, but are not critical. A 4 µHinductor is obtained with 14 turns of number 18 wire, closespaced, around a one-inch-diameter form.

00870408

The LM12 can be made stable for all loads with a largecapacitor on the output, as shown above. This compensationgives the lowest possible closed-loop output impedance athigh frequencies and the best load-transient response. It isappropriate for such applications as voltage regulators.

A feedback capacitor, C1, is connected directly to the outputpin of the IC. The output capacitor, C2, is connected at theoutput terminal with short leads. Single-point grounding toavoid dc and ac ground loops is advised.

The impedance, Z1, is the wire connecting the op amp outputto the load capacitor. About 3-inches of number-18 wire(70 nH) gives good stability and 18-inches (400 nH) beginsto degrade load-transient response. The minimum load ca-pacitance is 47 µF, if a solid-tantalum capacitor with anequivalent series resistance (ESR) of 0.1Ω is used. Electro-lytic capacitors work as well, although capacitance may haveto be increased to 200 µF to bring ESR below 0.1Ω.Loop stability is not the only concern when op amps areoperated with reactive loads. With time-varying signals,power dissipation can also increase markedly. This is par-ticularly true with the combination of capacitive loads andhigh-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 nearunity. The voltage follower connection is most susceptible.This glitching can be eliminated at the expense of small-signal bandwidth using input compensation. Input compen-sation can also be used in combination with LR load isolationto improve capacitive load stability.

00870409

An example of a voltage follower with input compensation isshown here. The R2C2 combination across the input workswith R1 to reduce feedback at high frequencies withoutgreatly affecting response below 100 kHz. A lead capacitor,C1, improves phase margin at the unity-gain crossover fre-

quency. Proper operation requires that the output impedanceof the circuitry driving the follower be well under 1 kΩ atfrequencies up to a few hundred kilohertz.

00870410

Extending input compensation to the integrator connection isshown here. Both the follower and this integrator will handle1 µF capacitive loading without LR output isolation.

CURRENT DRIVE

00870411

This circuit provides an output current proportional to theinput voltage. Current drive is sometimes preferred for servomotors because it aids in stabilizing the servo loop by reduc-ing phase lag caused by motor inductance. In applicationsrequiring high output resistance, such as operational powersupplies 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.

PARALLEL OPERATION

00870412

Output drive beyond the capability of one power amplifiercan be provided as shown here. The power op amps arewired as followers and connected in parallel with the outputscoupled through equalization resistors. A standard, high-voltage op amp is used to provide voltage gain. Overallfeedback compensates for the voltage dropped across theequalization resistors.

With parallel operation, there may be an increase in un-loaded supply current related to the offset voltage across the

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Application Information (Continued)equalization resistors. More output buffers, with individualequalization resistors, may be added to meet even higherdrive requirements.

00870413

This connection allows increased output capability withoutrequiring a separate control amplifier. The output buffer, A2,provides load current through R5 equal to that supplied bythe main amplifier, A1, through R4. Again, more output buff-ers can be added.

Current sharing among paralleled amplifiers can be affectedby gain error as the power-bandwidth limit is approached. Inthe first circuit, the operating current increase will dependupon the matching of high-frequency characteristics. In thesecond circuit, however, the entire input error of A2 appearsacross R4 and R5. The supply current increase can causepower limiting to be activated as the slew limit is ap-proached. This will not damage the LM12. It can be avoidedin both cases by connecting A1 as an inverting amplifier andrestricting bandwidth with C1.

SINGLE-SUPPLY OPERATION

00870414

Although op amps are usually operated from dual supplies,single-supply operation is practical. This bridge amplifiersupplies bi-directional current drive to a servo motor whileoperating from a single positive supply. The output is easilyconverted to voltage drive by shorting R6 and connecting R7to the output of A2, rather than A1.

Either input may be grounded, with bi-directional drive pro-vided to the other. It is also possible to connect one input toa positive reference, with the input signal varying about thisvoltage. If the reference voltage is above 5V, R2 and R3 arenot required.

HIGH VOLTAGE AMPLIFIERS

00870415

The voltage swing delivered to the load can be doubled byusing the bridge connection shown here. Output clamping tothe supplies can be provided by using a bridge-rectifierassembly.

00870416

One limitation of the standard bridge connection is that theload cannot be returned to ground. This can be circum-vented by operating the bridge with floating supplies, asshown above. For single-ended drive, either input can begrounded.

00870417

This circuit shows how two amplifiers can be cascaded todouble output swing. The advantage over the bridge is thatthe output can be increased with any number of stages,although separate supplies are required for each.

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Application Information (Continued)

00870418

Discrete transistors can be used to increase output drive to±70V at ±10A as shown above. With proper thermal design,the IC will provide safe-area protection for the external tran-sistors. Voltage gain is about thirty.

OPERATIONAL POWER SUPPLY

00870419

Note: Supply voltages for the LM318s are ±15V

External current limit can be provided for a power op amp asshown above. The positive and negative current limits canbe set precisely and independently. Fast response is as-sured by D1 and D2. Adjustment range can be set down tozero with potentiometers R3 and R7. Alternately, the limit canbe programmed from a voltage supplied to R2 and R6. This isthe set up required for an operational power supply orvoltage-programmable power source.

SERVO AMPLIFIERS

When making servo systems with a power op amp, there isa temptation to use it for frequency shaping to stabilize theservo loop. Sometimes this works; other times there arebetter ways; and occasionally it just doesn’t fly. Usually it’s amatter of how quickly and to what accuracy the servo muststabilize.

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Application Information (Continued)

00870420

This motor/tachometer servo gives an output speed propor-tional to input voltage. A low-level op amp is used for fre-quency shaping while the power op amp provides currentdrive to the motor. Current drive eliminates loop phase shiftdue to motor inductance and makes high-performance ser-vos easier to stabilize.

00870421

This position servo uses an op amp to develop the ratesignal electrically instead of using a tachometer. In high-performance servos, rate signals must be developed withlarge error signals well beyond saturation of the motor drive.Using a separate op amp with a feedback clamp allows therate signal to be developed properly with position errorsmore than an order of magnitude beyond the loop-saturationlevel as long as the photodiode sensors are positioned withthis in mind.

VOLTAGE REGULATORS

00870422

An op amp can be used as a positive or negative regulator.Unlike most regulators, it can sink current to absorb energydumped back into the output. This positive regulator has a0–50V output range.

00870423

Dual supplies are not required to use an op amp as a voltageregulator if zero output is not required. This 4V to 50Vregulator operates from a single supply. Should the op ampnot be able to absorb enough energy to control an overvolt-age condition, a SCR will crowbar the output.

REMOTE SENSING

00870424

Remote sensing as shown above allows the op amp tocorrect for dc drops in cables connecting the load. Even so,cable drop will affect transient response. Degradation can beminimized by using twisted, heavy-gauge wires on the out-put line. Normally, common and one input are connectedtogether at the sending end.

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Application Information (Continued)AUDIO AMPLIFIERS

00870425

A power amplifier suitable for use in high-quality audio equip-ment is shown above. Harmonic distortion is about 0.01-percent. Intermodulation distortion (60 Hz/7 kHz, 4:1) mea-sured 0.015-percent. Transient response and saturationrecovery are clean, and the 9 V/µs slew rate of the LM12virtually eliminates transient intermodulation distortion. Us-ing separate amplifiers to drive low- and high-frequencyspeakers gets rid of high-level crossover networks and at-tenuators. Further, it prevents clipping on the low-frequencychannel from distorting the high frequencies.

DETERMINING MAXIMUM DISSIPATION

It is a simple matter to establish power requirements for anop amp driving a resistive load at frequencies well below10 Hz. Maximum dissipation occurs when the output is atone-half the supply voltage with high-line conditions. Theindividual output transistors must be rated to handle thispower continuously at the maximum expected case tem-perature. The power rating is limited by the maximum junc-tion temperature as determined by

TJ = TC + PDISS θJC,where TC is the case temperature as measured at the centerof the package bottom, PDISS is the maximum power dissi-pation and θJC is the thermal resistance at the operatingvoltage of the output transistor. Recommended maximumjunction temperatures are 200˚C within the power transistorand 150˚C for the control circuitry.

If there is ripple on the supply bus, it is valid to use theaverage value in worst-case calculations as long as the peakrating of the power transistor is not exceeded at the ripplepeak. With 120 Hz ripple, this is 1.5 times the continuouspower rating.

Dissipation requirements are not so easily established withtime varying output signals, especially with reactive loads.Both peak and continuous dissipation ratings must be takeninto account, and these depend on the signal waveform aswell as load characteristics.

With a sine wave output, analysis is fairly straightforward.With supply voltages of ±VS, the maximum average powerdissipation of both output transistors is

where ZL is the magnitude of the load impedance and θ itsphase angle. Maximum average dissipation occurs belowmaximum output swing for θ < 40˚.

00870426

The instantaneous power dissipation over the conductinghalf cycle of one output transistor is shown here. Powerdissipation is near zero on the other half cycle. The outputlevel is that resulting in maximum peak and average dissi-pation. Plots are given for a resistive and a series RL load.The latter is representative of a 4Ω loudspeaker operatingbelow resonance and would be the worst case condition inmost audio applications. The peak dissipation of each tran-sistor is about four times average. In ac applications, powercapability is often limited by the peak ratings of the powertransistor.

The pulse thermal resistance of the LM12 is specified forconstant power pulse duration. Establishing an exactequivalency 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 byassuming a constant power pulse amplitude given by:

where φ = 60˚ and θ is the absolute value of the phase angleof 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 outputwaveform.

DISSIPATION DRIVING MOTORS

A motor with a locked rotor looks like an inductance in serieswith a resistance, for purposes of determining driver dissipa-tion. With slow-response servos, the maximum signal ampli-tude at frequencies where motor inductance is significantcan be so small that motor inductance does not have to betaken into account. If this is the case, the motor can betreated as a simple, resistive load as long as the rotor speedis low enough that the back emf is small by comparison tothe supply voltage of the driver transistor.

A permanent-magnet motor can build up a back emf that isequal to the output swing of the op amp driving it. Reversingthis motor from full speed requires the output drive transistorto operate, initially, along a loadline based upon the motorresistance and total supply voltage. Worst case, this loadline

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Application Information (Continued)will have to be within the continuous dissipation rating of thedrive transistor; but system dynamics may permit takingadvantage of the higher pulse ratings. Motor inductance cancause added stress if system response is fast.

Shunt- and series-wound motors can generate back emf’sthat are considerably more than the total supply voltage,resulting in even higher peak dissipation than a permanent-magnet motor having the same locked-rotor resistance.

VOLTAGE REGULATOR DISSIPATION

The pass transistor dissipation of a voltage regulator iseasily determined in the operating mode. Maximum continu-ous dissipation occurs with high line voltage and maximumload current. As discussed earlier, ripple voltage can beaveraged if peak ratings are not exceeded; however, ahigher average voltage will be required to insure that thepass transistor does not saturate at the ripple minimum.

Conditions during start-up can be more complex. If the inputvoltage increases slowly such that the regulator does not gointo current limit charging output capacitance, there are noproblems. If not, load capacitance and load characteristicsmust be taken into account. This is also the case if automaticrestart is required in recovering from overloads.

Automatic restart or start-up with fast-rising input voltagescannot be guaranteed unless the continuous dissipation rat-ing of the pass transistor is adequate to supply the loadcurrent continuously at all voltages below the regulated out-put voltage. In this regard, the LM12 performs much betterthan IC regulators using foldback current limit, especiallywith high-line input voltage above 20V.

POWER LIMITING

00870427

Should the power ratings of the LM12 be exceeded, dynamicsafe-area protection is activated. Waveforms with this powerlimiting are shown for the LM12 driving ±26V at 30 Hz into3Ω in series with 24 mH (θ = 45˚). With an inductive load, theoutput clamps to the supplies in power limit, as above. Withresistive loads, the output voltage drops in limit. Behaviorwith 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 ICcompletely (open output) until the case temperature drops toabout 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-linesupply voltage in the ripple trough. The worst-case powerdissipation is established by the average supply voltage with

high-line conditions. The loss in power output that can beguaranteed 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 providea substantial increase in the power output that can be guar-anteed under worst-case conditions. In addition, switchingpower supplies can convert low-voltage power sources suchas automotive batteries up to regulated, dual, high-voltagesupplies optimized for powering power op amps.

HEAT SINKING

A semiconductor manufacturer has no control over heat sinkdesign. Temperature rating can only be based upon casetemperature as measured at the center of the package bot-tom. With power pulses of longer duration than 100 ms, casetemperature is almost entirely dependent on heat sink de-sign and the mounting of the IC to the heat sink.

00870428

The design of heat sink is beyond the scope of this work.Convection-cooled heat sinks are available commercially,and their manufacturers should be consulted for ratings. Thepreceding figure is a rough guide for temperature rise as afunction of fin area (both sides) available for convectioncooling.

Proper mounting of the IC is required to minimize the thermaldrop between the package and the heat sink. The heat sinkmust also have enough metal under the package to conductheat from the center of the package bottom to the finswithout excessive temperature drop.

A thermal grease such as Wakefield type 120 or ThermalloyThermacote should be used when mounting the package tothe heat sink. Without this compound, thermal resistance willbe no better than 0.5˚C/W, and probably much worse. Withthe compound, thermal resistance will be 0.2˚C/W or less,assuming under 0.005 inch combined flatness runout for thepackage and heat sink. Proper torquing of the mountingbolts is important. Four to six inch-pounds is recommended.

Should it be necessary to isolate V− from the heat sink, aninsulating washer is required. Hard washers like beryliumoxide, anodized aluminum and mica require the use of ther-mal compound on both faces. Two-mil mica washers aremost common, giving about 0.4˚C/W interface resistancewith the compound. Silicone-rubber washers are also avail-able. A 0.5˚C/W thermal resistance is claimed without ther-mal compound. Experience has shown that these rubberwashers deteriorate and must be replaced should the IC bedismounted.

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Application Information (Continued)“Isostrate” insulating pads for four-lead TO-3 packages areavailable from Power Devices, Inc. Thermal grease is notrequired, and the insulators should not be reused.

Definition of TermsInput offset voltage: The absolute value of the voltagebetween the input terminals with the output voltage andcurrent at zero.

Input bias current: The absolute value of the average of thetwo input currents with the output voltage and current atzero.

Input offset current: The absolute value of the difference inthe two input currents with the output voltage and current atzero.

Common-mode rejection: The ratio of the input voltagerange to the change in offset voltage between the extremes.

Supply-voltage rejection: The ratio of the specified supply-voltage change to the change in offset voltage between theextremes.

Output saturation threshold: The output swing limit for aspecified input drive beyond that required for zero output. Itis measured with respect to the supply to which the output isswinging.

Large signal voltage gain: The ratio of the output voltageswing to the differential input voltage required to drive theoutput from zero to either swing limit. The output swing limit

is the supply voltage less a specified quasi-saturation volt-age. A pulse of short enough duration to minimize thermaleffects is used as a measurement signal.

Thermal gradient feedback: The input offset voltagechange caused by thermal gradients generated by heating ofthe output transistors, but not the package. This effect isdelayed by several milliseconds and results in increasedgain error below 100 Hz.

Output-current limit: The output current with a fixed outputvoltage and a large input overdrive. The limiting currentdrops with time once the protection circuitry is activated.

Power dissipation rating: The power that can be dissi-pated for a specified time interval without activating theprotection circuitry. For time intervals in excess of 100 ms,dissipation capability is determined by heat sinking of the ICpackage rather than by the IC itself.

Thermal resistance: The peak, junction-temperature rise,per unit of internal power dissipation, above the case tem-perature as measured at the center of the package bottom.The dc thermal resistance applies when one output transis-tor is operating continuously. The ac thermal resistance ap-plies with the output transistors conducting alternately at ahigh enough frequency that the peak capability of neithertransistor is exceeded.

Supply current: The current required from the powersource to operate the amplifier with the output voltage andcurrent at zero.

Equivalent Schematic (excluding active protection circuitry)

00870429

LM12

CL

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Physical Dimensions inches (millimeters)unless otherwise noted

4-Lead TO-3 Steel Package (K)Order Number LM12CLK

NS Package Number K04A

National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reservesthe right at any time without notice to change said circuitry and specifications.

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1. Life support devices or systems are devices or systemswhich, (a) are intended for surgical implant into the body, or(b) support or sustain life, and whose failure to perform whenproperly used in accordance with instructions for useprovided in the labeling, can be reasonably expected to resultin a significant injury to the user.

2. A critical component is any component of a life supportdevice or system whose failure to perform can be reasonablyexpected to cause the failure of the life support device orsystem, or to affect its safety or effectiveness.

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LM12C

L80W

OperationalA

mplifier

LM12CLGeneral DescriptionConnection DiagramTypical Application*Absolute Maximum RatingsOperating RatingsElectrical Characteristics Output-Transistor Ratings (guaranteed)Typical Performance CharacteristicsApplication InformationGENERALSUPPLY BYPASSINGLEAD INDUCTANCEGROUND LOOPSOUTPUT CLAMP DIODESREACTIVE LOADINGINPUT COMPENSATIONCURRENT DRIVEPARALLEL OPERATIONSINGLE-SUPPLY OPERATIONHIGH VOLTAGE AMPLIFIERSOPERATIONAL POWER SUPPLYSERVO AMPLIFIERSVOLTAGE REGULATORSREMOTE SENSINGAUDIO AMPLIFIERSDETERMINING MAXIMUM DISSIPATIONDISSIPATION DRIVING MOTORSVOLTAGE REGULATOR DISSIPATIONPOWER LIMITINGPOWER SUPPLIESHEAT SINKING

Definition of TermsEquivalent Schematic Physical Dimensions