6

LT1587-1.5

regulation is limited by the resistance of the wire connect-

ing the regulators to the load. Load regulation per the data

sheet specification is measured at the bottom of the

package.

For fixed voltage devices, negative side sensing is a true

Kelvin connection with the Ground pin of the device re-

turned to the negative side of the load. This is illustrated in

Figure 3.

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 ensures the best thermal flow from this area of

the package to the heat sink. Linear Technology strongly

recommends thermal compound at the case-to-heat sink

interface. Use a thermally conductive spacer if the case of

the device must be electrically isolated and include its

contribution to the total thermal resistance. Please consult

“Mounting Considerations for Power Semiconductors”

1990 Linear Applications Handbook, Volume I, Pages

RR3-1 to RR3-20. The output connects to the case of the

device in the LT1587-1.5.

For example, using an LT1587CT-1.5 (TO-220, commer-

cial) and assuming:

Power dissipation under these conditions is equal to:

Junction temperature will be equal to:

For the Control Section:

121.3

Range)

For the Power Transistor:

134.8

Range)

In both cases the junction temperature is below the maxi-

mum rating for the respective sections, ensuring reliable

operation.

Thermal Considerations

The LT1587-1.5 protects the device under overload condi-

tions with internal power and thermal limiting circuitry.

However, for normal continuous load conditions, do not

exceed maximum junction temperature ratings. It is impor-

tant to consider all sources of thermal resistance from

junction-to-ambient. These sources include the junction-

to-case resistance, the case-to-heat sink interface resis-

tance, and the heat sink resistance. Thermal resistance

specifications have been developed to more accurately

reflect device temperature and ensure safe operating tem-

peratures. The electrical characteristics section provides a

separate thermal resistance and maximum junction tem-

perature for both the control circuitry and the power

transistor. Older regulators with a single junction-to-case

thermal resistance specification, use an average of the two

values provided here and allow excessive junction tem-

peratures under certain conditions of ambient temperature

and heat sink resistance. Calculate the maximum junction

temperature for both sections to ensure that both thermal

limits are met.

LT1587-1.5

PARASITIC

LINE RESISTANCE

GND

IN

OUT

LT1587 • F03

Figure 3. Connection for Best Load Regulation