LM341/LM78MXX

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SNVS090E – MAY 2004 – REVISED AUGUST 2005

DESIGN CONSIDERATIONS

The LM78MXX/LM341XX fixed voltage regulator series has built-in thermal overload protection which prevents

the device from being damaged due to excessive junction temperature.

The regulators also contain internal short-circuit protection which limits the maximum output current, and safe-

area protection for the pass transistor which reduces the short-circuit current as the voltage across the pass

transistor is increased.

Although the internal power dissipation is automatically limited, the maximum junction temperature of the device

must be kept below +125°C in order to meet data sheet specifications. An adequate heatsink should be provided

to assure this limit is not exceeded under worst-case operating conditions (maximum input voltage and load

current) if reliable performance is to be obtained).

HEATSINK CONSIDERATIONS

When an integrated circuit operates with appreciable current, its junction temperature is elevated. It is important

to quantify its thermal limits in order to achieve acceptable performance and reliability. This limit is determined by

summing the individual parts consisting of a series of temperature rises from the semiconductor junction to the

operating environment. A one-dimension steady-state model of conduction heat transfer is demonstrated in

Figure 14 The heat generated at the device junction flows through the die to the die attach pad, through the lead

frame to the surrounding case material, to the printed circuit board, and eventually to the ambient environment.

Below is a list of variables that may affect the thermal resistance and in turn the need for a heatsink.

Leadframe Size & Material

Mounting Pad Size, Material, & Location

No. of Conduction Pins

Placement of Mounting Pad

Die Size

PCB Size & Material

Die Attach Material

Traces Length & Width

Molding Compound Size and Material

Adjacent Heat Sources

Volume of Air

Air Flow

Ambient Temperature

Shape of Mounting Pad

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