Design Considerations

The LM78MXX/LM341XX fixed voltage regulator series has

built-in thermal overload protection which prevents the de-

vice from being damaged due to excessive junction tem-

perature.

The regulators also contain internal short-circuit protection

which limits the maximum output current, and safe-area pro-

tection for the pass transistor which reduces the short-circuit

current as the voltage across the pass transistor is in-

creased.

Although the internal power dissipation is automatically lim-

ited, the maximum junction temperature of the device must

be kept below +125˚C in order to meet data sheet specifica-

tions. An adequate heatsink should be provided to assure

this limit is not exceeded under worst-case operating condi-

tions (maximum input voltage and load current) if reliable

performance is to be obtained).

1.0 Heatsink Considerations

When an integrated circuit operates with appreciable cur-

rent, its junction temperature is elevated. It is important to

quantify its thermal limits in order to achieve acceptable per-

formance 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 envi-

ronment. A one-dimension steady-state model of conduction

heat transfer is demonstrated in 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 envi-

ronment. Below is a list of variables that may affect the ther-

mal 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

Quiescent Current

DS010484-16

Output Impedance

DS010484-17

Line Transient Response

DS010484-7

Load Transient Response

DS010484-8

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