Design Considerations

The

LM79MXX

fixed

voltage

regulator

series

have

thermal-overload protection from excessive power, internal

short-circuit protection which limits the circuit’s maximum

current, and output transistor safe-area compensation for

reducing the output current as the voltage across the pass

transistor is increased.

Although the internal power dissipation is limited, the junc-

tion temperature must be kept below the maximum specified

temperature in order to meet data sheet specifications. To

calculate the maximum junction temperature or heat sink

required, the following thermal resistance values should be

used:

Package

θ

JC

θ

JA

(˚C/W)

(˚C/W)

TO-220

3

40

(1)

θ

CA

=

θ

Solving for T

T

J

=T

=T

Where

T

J

= Junction Temperature

T

A

= Ambient Temperature

P

D

= Power Dissipation

θ

JC

= Junction-to-Case Thermal Resistance

θ

CA

= Case-to-Ambient Thermal Resistance

θ

CS

= Case-to-Heat Sink Thermal Resistance

θ

SA

= Heat Sink-to-Ambient Thermal Resistance

θ

JA

= Junction-to-Ambient Thermal Resistance

Typical Applications

Bypass capacitors are necessary for stable operation of the

LM79MXX series of regulators over the input voltage and

output current ranges. Output bypass capacitors will improve

the transient response of the regulator.

The bypass capacitors (2.2µF on the input, 1.0µF on the

output), should be ceramic or solid tantalum which have

good high frequency characteristics. If aluminum electrolyt-

ics are used, their values should be 10µF or larger. The

bypass capacitors should be mounted with the shortest

leads, and if possible, directly across the regulator terminals.

Fixed Regulator

DS010483-2

*Required if regulator is separated from filter capacitor by more than 3".

For value given, capacitor must be solid tantalum. 25µF aluminum

electrolytic may be substituted.

†Required for stability. For value given, capacitor must be solid tantalum.

25µF aluminum electrolytic may be substituted. Values given may be

increased without limit.

For output capacitance in excess of 100µF, a high current diode from input

to output (1N4001, etc.) will protect the regulator from momentary input

shorts.

Variable Output

DS010483-3

*Improves transient response and ripple rejection.

Do not increase beyond 50µF.

Select R2 as follows:

LM79M05C

300

Ω

LM79M12C

750

Ω

LM79M15C

1k

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