LM78L00 datasheet(7/10 Pages) NSC

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Design Considerations

The LM78L series regulators have thermal overload protec-

tion from excessive power internal short-circuit protection

which limits each circuit’s maximum current and output

transistor safe-area protection for reducing the output cur-

rent as the voltage across each 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

Package

Typ

Max

Typ

Max

iJC

iJA

TO-92

160

Thermal Considerations

The TO-92 molded package is capable of unusually high

power dissipation due to the lead frame design However its

thermal capabilities are generally overlooked because of a

lack of understanding of the thermal paths from the semi-

conductor junction to ambient temperature While thermal

resistance is normally specified for the device mounted

1 cm above an infinite heat sink very little has been men-

tioned of the options available to improve on the conserva-

tively rated thermal capability

An explanation of the thermal paths of the TO-92 will allow

the designer to determine the thermal stress he is applying

in any given application

The TO-92 Package

The TO-92 package thermal paths are complex In addition

to the path through the molding compound to ambient tem-

perature there is another path through the leads in parallel

with the case path to ambient temperature as shown in

Figure 1

The total thermal resistance in this model is then

iJA e

(iJC a iCA)(iJL a iLA)

iJC a iCA a iJL a iLA

(1)

Where

iJCe thermal resistance of the case between the regu-

lator die and a point on the case directly above

the die location

iCAe thermal resistance between the case and air at

ambient temperature

iJLe thermal resistance from regulator die through the

input lead to a point

inch below the regulator

case

iLAe total thermal resistance of the inputoutput

ground leads to ambient temperature

iJAe junction to ambient thermal resistance

TLH10051 – 4

FIGURE 1 TO-92 Thermal Equivalent Circuit

Methods of Heat Sinking

With two external thermal resistances in each leg of a paral-

lel network available to the circuit designer as variables he

can choose the method of heat sinking most applicable to

his particular situation To demonstrate consider the effect

of placing a small 72 CW flag type heat sink such as the

Staver F1-7D-2 on the LM78L00 molded case The heat

sink effectively replaces the iCA (Figure 2) and the new

thermal resistance i JA equals 145 CW (assuming 0125

inch lead length)

The net change of 15 CW increases the allowable power

dissipation to 086W with a minimal inserted cost A still

further decrease in iJA could be achieved by using a heat

sink rated at 46 CW such as the Staver FS-7A Also if the

case sinking does not provide an adequate reduction in total

iJA the other external thermal resistance iLA may be re-

duced by shortening the lead length from package base to

mounting medium However one point must be kept in

mind The lead thermal path includes a thermal resistance

iSA from the leads at the mounting point to ambient that is

the mounting medium iLA is then equal to iLS a iSA The

new model is shown in

Figure 2

In the case of a socket iSA could be as high as 270 CW

thus causing a net increase in iJA and a consequent de-

crease in the maximum dissipation capability Shortening

the lead length may return the net iJA to the original value

but lead sinking would not be accomplished

In those cases where the regulator is inserted into a copper

clad printed circuit board it is advantageous to have a maxi-

mum area of copper at the entry points of the leads While it

would be desirable to rigorously define the effect of PC

board copper the real world variables are too great to allow

anything more than a few general observations

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