lm12cl.pdf

March 1995

LM12 80W Operational Amplifier

General Description

The LM12 is a power op amp capable of driving g 25V at

g 10A while operating from g 30V supplies. The monolithic

IC can deliver 80W of sine wave power into a 4 X load 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:

Y input protection

Y controlled turn on

Y thermal limiting

Y overvoltage shutdown

Y output-current limiting

Y dynamic safe-area protection

The IC delivers g 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

exceeds 150 § C or as the supply voltage approaches the

BV CEO 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/ m s, even as a follower. Distortion and capacitive-load

stability rival that of the best designs using complementary

output 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 even

greater output capability. Applications include operational

power supplies, high-voltage regulators, high-quality audio

amplifiers, tape-head positioners, x-y plotters or other ser-

vo-control systems.

The LM12 is supplied in a four-lead, TO-3 package with V b

on the case. A gold-eutectic die-attach to a molybdenum

interface is used to avoid thermal fatigue problems. The

LM12 is specified for either military or commercial tempera-

ture range.

Connection Diagram

Typical Application*

4-pin glass epoxy TO-3

socket is available from

AUGAT INC.

Part number 8112-AG7

TL/H/8704±2

TL/H/8704±1

*Low distortion (0.01%) audio amplifier

Bottom View

Order Number LM12CLK

See NS Package Number K04A

C 1995 National Semiconductor Corporation

TL/H/8704

RRD-B30M115/Printed in U. S. A.

Absolute Maximum Ratings

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales

Office/Distributors for availability and specifications.

Total Supply Voltage (Note 1)

Junction Temperature

(Note 3)

Storage Temperature Range

b 65 § Cto150 § C

Lead Temperature (Soldering, 10 seconds)

300 § C

80V

Operating Ratings

Total Supply Voltage

Input Voltage

(Note 2)

Output Current

Internally Limited

15V to 60V

Case Temperature (Note 4)

0 § Cto70 § C

Electrical Characteristics (Note 4)

Parameter

Conditions

Typ

LM12CL

Units

25 § C

Limits

Input Offset Voltage

g 10V s V S s g 0.5 V MAX ,

15/20

mV (max)

V CM e 0

Input Bias Current

V b a 4V s V CM s V a b 2V

0.15

0.7/1.0

m A (max)

Input Offset Current

V b a 4V s V CM s V a b 2V

0.03

0.2/0.3

m A (max)

Common Mode

V b a 4V s V CM s V a b 2V

70/65

dB (min)

Rejection

Power Supply

V a e 0.5 V MAX ,

70/65

dB (min)

Rejection

b 6V t V b t b 0.5 V MAX

V b eb 0.5 V MAX ,

6V s V a s 0.5 V MAX

110

75/70

dB (min)

Output Saturation

t ON e 1 ms,

Threshold

D V IN e 5(10) mV,

I OUT e 1A

1.8

2.2/2.5

V (max)

5/7

V (max)

10A

V (max)

Large Signal Voltage

t ON e 2 ms,

Gain

V SAT e 2V, I OUT e 0

100

30/20

V/mV (min)

V SAT e 8V, R L e 4 X

15/10

V/mV (min)

Thermal Gradient

P DISS e 50W, t ON e 65 ms

100

m V/W (max)

Feedback

Output-Current

t ON e 10 ms, V DISS e 10V

A (max)

Limit

t ON e 100 ms, V DISS e 58V 1.5 0.9/0.6 A (min)

1.5 1.7 A (max)

Power Dissipation t ON e 100 ms, V DISS e 20V 100 80/55 W (min)

Rating V DISS e 58V 80 52/35 W (min)

DC Thermal Resistance (Note 5) V DISS e 20V 2.3 2.9 § C/W (max)

V DISS e 58V 2.7 4.5 § C/W (max)

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

Supply Current V OUT e 0, I OUT e 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 to

operate 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 g 30V (V MAX e 60V), unless otherwise specified. The voltage across the conducting output transistor (supply to output) is V DISS and

internal power dissipation is P DISS . Temperature range is 0 § C s T C s 70 § C where T C is the case temperature. Standard typeface indicates limits at 25 § C while

boldface 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 of

internal 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 the

center 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 ac

thermal 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.

Output-Transistor Ratings (guaranteed)

Safe Area

DC Thermal Resistance

Pulse Thermal Resistance

TL/H/8704±3

Typical Performance Characteristics

Pulse Power Limit

Peak Output Current

Output Saturation Voltage

Large Signal Response

Follower Pulse Response

TL/H/8704±4

Typical Performance Characteristics (Continued)

Large Signal Gain

Thermal Response

Total Harmonic Distortion

Frequency Response

Output Impedance

Power Supply Rejection

Input Bias Current

Input Noise Voltage

Common Mode Rejection

Supply Current

Cross-Supply Current

TL/H/8704±5

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 late

1960's prompted their use in rather mundane applications,

replacing a few discrete components. Once a few basic

principles are mastered, op amps can be used to give ex-

ceptionally good results in a wide range of applications

while minimizing both cost and design effort.

The availability of a monolithic power op amp now promises

to extend these advantages to high-power designs. Some

conventional applications are given here to illustrate op amp

design principles as they relate to power circuitry. The inevi-

table fall in prices, as the economies of volume production

are realized, will prompt their use in applications that might

now seem trivial. Replacing single power transistors with an

op amp will become economical because of improved per-

formance, simplification of attendant circuitry, vastly im-

proved fault protection, greater reliability and the reduction

of design time.

Power op amps introduce new factors into the design equa-

tion. With current transients above 10A, both the inductance

and resistance of wire interconnects become important in a

number of ways. Further, power ratings are a crucial factor

in determining performance. But the power capability of the

IC cannot be realized unless it is properly mounted to an

adequate heat sink. Thus, thermal design is of major impor-

tance with power op amps.

This application summary starts off by identifying the origin

of strange problems observed while using the LM12 in a

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

operation. 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 sec-

onds, depending on the thermal system. Activation of the

dynamic safe-area protection causes both the main feed-

back 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 require-

ments 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 m F. 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 m F or more, at the package terminals.

LEAD INDUCTANCE

With ordinary op amps, lead-inductance problems are usual-

ly 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

capacitor near the IC. With 20 m F local bypass, these volt-

age surges are important only if the lead length exceeds a

couple feet ( l 1 m H 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 dif-

ficult to sort out when several pieces of test equipment are

used but can be minimized by using current probes or the

new isolated oscilloscope pre-amplifiers. Eliminating any di-

rect ground connection between the signal generator and

the oscilloscope synchronization input solves one common

problem.

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

amperes for a few milliseconds, extreme conditions can

cause destruction of the IC. The internal clamp diodes are

imperfect 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

recommended. This is particularly important with higher sup-

ply voltages.

Experience has demonstrated that hard-wire shorting the

output to the supplies can induce random failures if these

external clamp diodes are not used and the supply voltages

are above g 20V. Therefore it is prudent to use output-

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