9

LT1083/LT1084/LT1085 Fixed

1083ffd

case- to-heat sink interface, and heat sink resistance itself.

New thermal resistance specifications have been devel-

oped to more accurately reflect device temperature and

ensure safe operating temperatures. The data section for

these new regulators provides a separate thermal resis-

tance and maximum junction temperature for both the

Control Section and the Power Section. Previous regula-

tors, with a single junction-to-case thermal resistance

specification, used an average of the two values provided

here and therefore could allow excessive junction tem-

peratures under certain conditions of ambient tempera-

ture 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 LT1083-5CP (TO-3P, 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

10,000

μF

OUTPUT

LT1083-5 OUT

IN

GND

LT1083/4/5 TA03

+

APPLICATIO S I FOR ATIO

TYPICAL APPLICATIO S

Paralleling Regulators

LT1083-5 OUT

IN

GND

LT1083-5

0.015

Ω

OUT

2 FEET #18 WIRE*

IN

GND

*THE #18 WIRE ACTS

AS BALLAST RESISTANCE

INSURING CURRENT SHARING

BETWEEN BOTH DEVICES

LT1083/4/5 TA04