9

LT1083/84/85 Fixed

thermal resistance specifications have been developed to

more accurately reflect device temperature and ensure

safe operating temperatures. The data section for these

new regulators provides a separate thermal resistance and

maximum junction temperature for both the

Control Sec-

tion and the Power Section. Previous regulators, with a

single junction-to-case thermal resistance specification,

used an average of the two values provided here and

therefore could allow excessive junction temperatures

under certain conditions of ambient temperature and heat

sink resistance. To avoid this possibility, calculations

should be made for both sections to ensure that both

thermal limits are met.

Junction-to-case thermal resistance is specified from the

IC junction to the bottom of the case directly below the die.

This is the lowest resistance path for heat flow. Proper

mounting is required to ensure the best possible thermal

flow from this area of the package to the heat sink. Thermal

compound at the case-to-heat sink interface is strongly

recommended. If the case of the device must be electroni-

cally isolated, a thermally conductive spacer can be used

as long as its added contribution to thermal resistance is

considered. Note that the case of all devices in this series

is electronically connected to the ouput.

For example, using a LT1083-5CK (TO-3, Commercial)

and assuming:

thermal compound.

Power dissipation under these conditions is equal to:

Junction temperature will be equal to:

For the Control Section:

118

°C

118

cial Range)

For the Power Transistor:

142

°C

142

cial Range)

In both cases the junction temperature is below the

maximum rating for the respective sections, ensuring

reliable operation.

High Efficiency Regulator

+

–

+

28V

INPUT

28V

470

Ω

28V

4N28

1N914

1N914

LT1011

10k

10k

10k

1k

1M

MR1122

1mH

10,000µF

OUTPUT

LT1083-5 OUT

IN

GND

LT1083/4/5 TA03