K04RLM12 Datasheet, PDF (18/23 Pages) Texas Instruments – Operational Amplifier

K04RLM12, LM12CL
OBSOLETE
SNOSBY8D – MAY 1999 – REVISED APRIL 2013
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The pulse thermal resistance of the LM12 is specified for constant power pulse duration. Establishing an exact
equivalency between constant-power pulses and those encountered in practice is not easy. However, for sine
waves, reasonable estimates can be made at any frequency by assuming a constant power pulse amplitude
given by:
where
• φ = 60°
• θ is the absolute value of the phase angle of Z_L
(3)
Equivalent pulse width is t_ON ≃ 0.4τ for θ = 0 and t_ON ≃ 0.2τ for θ ≥ 20°, where τ is the period of the output
waveform.
DISSIPATION DRIVING MOTORS
A motor with a locked rotor looks like an inductance in series with a resistance, for purposes of determining
driver dissipation. With slow-response servos, the maximum signal amplitude 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 advantage 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, 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 is easily 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 ensured unless the continuous dissipation
rating of the pass transistor is adequate to supply the load current 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.
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