10

LT1083/LT1084/LT1085

108345fe

This translates to 4mV/ft at 1A load current, so it is

important to keep the positive lead between regulator and

load as short as possible and use large wire or PC board

traces.

Thermal Considerations

The LT1083 series of regulators have internal power and

thermal limiting circuitry designed to protect the device

under overload conditions. For continuous normal load

conditions however, maximum junction temperature rat-

ings must not be exceeded. It is important to give careful

consideration to all sources of thermal resistance from

junction to ambient. This includes junction-to-case, case-

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

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 Transistor. 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 electri-

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 electrically connected to the output.

For example, using an LT1083CK (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

Commercial Range)

For the Power Transistor:

142

Commercial Range)

In both cases the junction temperature is below the

maximum rating for the respective sections, ensuring

reliable operation.