LM12CL National Semiconductor, LM12CL Datasheet - Page 11

• Manufacturer Part Number: LM12CL
• Description: 80W Operational Amplifier
• Manufacturer: National Semiconductor

Application Information
where Z
phase angle. Maximum average dissipation occurs below
maximum output swing for
The instantaneous power dissipation over the conducting
half cycle of one output transistor is shown here. Power dis-
sipation is near zero on the other half cycle. The output level
is that resulting in maximum peak and average dissipation.
Plots are given for a resistive and a series RL load. The latter
is representative of a 4
nance and would be the worst case condition in most audio
applications. The peak dissipation of each transistor is about
four times average. In ac applications, power capability is of-
ten limited by the peak ratings of the power transistor.
The pulse thermal resistance of the LM12 is specified for
constant power pulse duration. Establishing an exact
equivalency 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 by as-
suming a constant power pulse amplitude given by:
where
of Z
form.
DISSIPATION DRIVING MOTORS
A motor with a locked rotor looks like an inductance in series
with a resistance, for purposes of determining driver dissipa-
tion. With slow-response servos, the maximum signal ampli-
tude 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 ad-
vantage 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, re-
0.2 for
L
. Equivalent pulse width is t
L
= 60˚ and is the absolute value of the phase angle
is the magnitude of the load impedance and
20˚, where
loudspeaker operating below reso-
is the period of the output wave-
<
40˚.
ON
0.4 for
DS008704-26
(Continued)
= 0 and t
ON
its
11
sulting
permanent-magnet motor having the same locked-rotor re-
sistance.
VOLTAGE REGULATOR DISSIPATION
The pass transistor dissipation of a voltage regulator is eas-
ily 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 guaranteed unless the continuous dissipation rat-
ing of the pass transistor is adequate to supply the load cur-
rent 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.
POWER LIMITING
Should the power ratings of the LM12 be exceeded, dynamic
safe-area protection is activated. Waveforms with this power
limiting are shown for the LM12 driving
3 in series with 24 mH ( = 45˚). With an inductive load, the
output clamps to the supplies in power limit, as above. With
resistive loads, the output voltage drops in limit. Behavior
with 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 IC
completely (open output) until the case temperature drops to
about 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-line
supply voltage in the ripple trough. The worst-case power
dissipation is established by the average supply voltage with
high-line conditions. The loss in power output that can be
guaranteed 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 provide
a substantial increase in the power output that can be guar-
in
even
higher
peak
dissipation
DS008704-27
±
26V at 30 Hz into
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than
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