LM12CL National Semiconductor, LM12CL Datasheet - Page 12

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

www.national.com
Application Information
anteed under worst-case conditions. In addition, switching
power supplies can convert low-voltage power sources such
as automotive batteries up to regulated, dual, high-voltage
supplies optimized for powering power op amps.
HEAT SINKING
A semiconductor manufacturer has no control over heat sink
design. Temperature rating can only be based upon case
temperature as measured at the center of the package bot-
tom. With power pulses of longer duration than 100 ms, case
temperature is almost entirely dependent on heat sink de-
sign and the mounting of the IC to the heat sink.
The design of heat sink is beyond the scope of this work.
Convection-cooled heat sinks are available commercially,
and their manufacturers should be consulted for ratings. The
preceding figure is a rough guide for temperature rise as a
function of fin area (both sides) available for convection cool-
ing.
Proper mounting of the IC is required to minimize the thermal
drop between the package and the heat sink. The heat sink
must also have enough metal under the package to conduct
heat from the center of the package bottom to the fins with-
out excessive temperature drop.
A thermal grease such as Wakefield type 120 or Thermalloy
Thermacote should be used when mounting the package to
the heat sink. Without this compound, thermal resistance will
be no better than 0.5˚C/W, and probably much worse. With
the compound, thermal resistance will be 0.2˚C/W or less,
assuming under 0.005 inch combined flatness runout for the
package and heat sink. Proper torquing of the mounting
bolts is important. Four to six inch-pounds is recommended.
Should it be necessary to isolate V− from the heat sink, an
insulating washer is required. Hard washers like berylium ox-
ide, anodized aluminum and mica require the use of thermal
compound on both faces. Two-mil mica washers are most
common, giving about 0.4˚C/W interface resistance with the
compound. Silicone-rubber washers are also available. A
0.5˚C/W thermal resistance is claimed without thermal com-
(Continued)
DS008704-28
12
pound. Experience has shown that these rubber washers
deteriorate and must be replaced should the IC be dis-
mounted.
“Isostrate” insulating pads for four-lead TO-3 packages are
available from Power Devices, Inc. Thermal grease is not re-
quired, and the insulators should not be reused.
Definition of Terms
Input offset voltage: The absolute value of the voltage be-
tween the input terminals with the output voltage and current
at zero.
Input bias current: The absolute value of the average of the
two input currents with the output voltage and current at
zero.
Input offset current: The absolute value of the difference in
the two input currents with the output voltage and current at
zero.
Common-mode rejection: The ratio of the input voltage
range to the change in offset voltage between the extremes.
Supply-voltage rejection: The ratio of the specified
supply-voltage change to the change in offset voltage be-
tween the extremes.
Output saturation threshold: The output swing limit for a
specified input drive beyond that required for zero output. It
is measured with respect to the supply to which the output is
swinging.
Large signal voltage gain: The ratio of the output voltage
swing to the differential input voltage required to drive the
output from zero to either swing limit. The output swing limit
is the supply voltage less a specified quasi-saturation volt-
age. A pulse of short enough duration to minimize thermal ef-
fects is used as a measurement signal.
Thermal gradient feedback: The input offset voltage
change caused by thermal gradients generated by heating of
the output transistors, but not the package. This effect is de-
layed by several milliseconds and results in increased gain
error below 100 Hz.
Output-current limit: The output current with a fixed output
voltage and a large input overdrive. The limiting current
drops with time once the protection circuitry is activated.
Power dissipation rating: The power that can be dissi-
pated for a specified time interval without activating the pro-
tection circuitry. For time intervals in excess of 100 ms, dis-
sipation capability is determined by heat sinking of the IC
package rather than by the IC itself.
Thermal resistance: The peak, junction-temperature rise,
per unit of internal power dissipation, above the case tem-
perature as measured at the center of the package bottom.
The dc thermal resistance applies when one output transis-
tor is operating continuously. The ac thermal resistance ap-
plies with the output transistors conducting alternately at a
high enough frequency that the peak capability of neither
transistor is exceeded.
Supply current: The current required from the power
source to operate the amplifier with the output voltage and
current at zero.