LM7905C datasheet(4/10 Pages) NSC

4 / 10 page

Design Considerations

The LM79XX fixed voltage regulator series has thermal

overload protection from excessive power dissipation inter-

nal short circuit protection which limits the circuit’s maxi-

mum 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 speci-

fied temperature (125 C) in order to meet data sheet specifi-

cations To calculate the maximum junction temperature or

heat sink required the following thermal resistance values

should be used

Typ

Max

Typ

Max

Package

iJC

iJA

TO-220

PD MAX e

TJ Max b TA

iJC a iCA

TJ Max TA

iJA

iCA e iCS a iSA (without heat sink)

Solving for TJ

TJ e TA a PD (iJC a iCA)or

TA a PDiJA (without heat sink)

Where

Junction Temperature

Ambient Temperature

Power Dissipation

iJA e Junction-to-Ambient Thermal Resistance

iJC e Junction-to-Case Thermal Resistance

iCA e Case-to-Ambient Thermal Resistance

iCS e Case-to-Heat Sink Thermal Resistance

iSA e Heat Sink-to-Ambient Thermal Resistance

Typical Applications (Continued)

Bypass capacitors are necessary for stable operation of the

LM79XX series of regulators over the input voltage and out-

put current ranges Output bypass capacitors will improve

the transient response by the regulator

The bypass capacitors (22 mF on the input 10 mFonthe

output) should be ceramic or solid tantalum which have

good high frequency characteristics If aluminum electrolyt-

ics are used their values should be 10 mF or larger The

bypass capacitors should be mounted with the shortest

leads and if possible directly across the regulator termi-

nals

High Stability 1 Amp Regulator

TLH7340 – 5

Load and line regulation k 001% temperature stability s 02%

Determine Zener current

Solid tantalum

Select resistors to set output voltage 2 ppm C tracking suggested

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