LM7905, LM7912, LM7915

SNOSBQ7C – JUNE 1999 – REVISED MAY 2013

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DESIGN CONSIDERATIONS

The LM79XX fixed voltage regulator series has thermal overload protection from excessive power dissipation,

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 junction temperature must be kept below the maximum

specified temperature (125°C) 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:

Typ

Max

Typ

Max

Package

θJC

θJC

θJA

θJA

°C/W

°C/W

°C/W

°C/W

TO-220

3.0

5.0

60

40

(1)

Solving for TJ:

TJ = TA + PD (θJC + θCA)

or

= TA + PDθJA (without heat sink)

where

•

TJ = Junction Temperature

•

TA = Ambient Temperature

•

PD = Power Dissipation

•

θJA = Junction-to-Ambient Thermal Resistance

•

θJC = Junction-to-Case Thermal Resistance

•

θCA = Case-to-Ambient Thermal Resistance

•

θCS = Case-to-Heat Sink Thermal Resistance

•

θSA = Heat Sink-to-Ambient Thermal Resistance

Typical Applications

Bypass capacitors are necessary for stable operation of the LM79XX series of regulators over the input voltage

and output current ranges. Output bypass capacitors will improve the transient response by the regulator.

The bypass capacitors, (2.2

μF on the input, 1.0μF on the output) should be ceramic or solid tantalum which have

good high frequency characteristics. If aluminum electrolytics are used, their values should be 10

μF or larger.

The bypass capacitors should be mounted with the shortest leads, and if possible, directly across the regulator

terminals.

4

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