K04RLM12 Datasheet, PDF (8/23 Pages) Texas Instruments – Operational Amplifier

K04RLM12, LM12CL
OBSOLETE
SNOSBY8D – MAY 1999 – REVISED APRIL 2013
APPLICATION INFORMATION
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GENERAL
Twenty five years ago the operational amplifier was a specialized 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 exceptionally 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 inevitable 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 performance, simplification of attendant circuitry, vastly improved fault
protection, greater reliability and the reduction of design time.
Power op amps introduce new factors into the design equation. 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 importance 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 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 capacitors greater than 20 μF. Other
considerations may require 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 usually 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 output 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 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.
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