K04RLM12 Datasheet, PDF (9/23 Pages) Texas Instruments – Operational Amplifier

OBSOLETE
K04RLM12, LM12CL
www.ti.com
SNOSBY8D – MAY 1999 – REVISED APRIL 2013
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 isolated oscilloscope pre-amplifiers.
Eliminating any direct 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 driving an inductive load, the stored energy in the load
inductance 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 supply 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 ±20V. Therefore it is prudent to use
outputclamp diodes 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, providing 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 situation. 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.
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