10

LT1581/LT1581-2.5

Characteristics curves.

The power in the output transistor is equal to:

The total power is equal to:

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 the added contribution to thermal resistance is

considered. Please consult Linear Technology’s “Mount-

ing Considerations for Power Semiconductors,”

1990

Linear Applications Handbook, Volume 1, Pages RR3-1 to

RR3-20. Note that the case of the LT1581 is electrically

connected to the output.

The following example illustrates how to calculate

maximum junction temperature. Using an LT1581 and

assuming:

Power dissipation under these conditions is equal to:

= ( 3.465V – 2.5V)(4A) = 3.9W

Total Power Dissipation = 4.09W

Junction temperature will be equal to:

For the Control section:

93

For the Power section:

101

In both cases the junction temperature is below the

maximum rating for the respective sections, ensuring

reliable operation.