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-

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.

DS008704-28

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