LT1963 datasheet(11/18 Pages) LINER

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LT1083/LT1084/LT1085 Fixed

1083ffe

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applicaTions inForMaTion

Load Regulation

BecausetheLT1083isathree-terminaldevice, itisnotpos-

sible to provide true remote load sensing. Load regulation

will not be limited by the resistance of the wire connect-

ing the regulator to the load. The data sheet specification

for the load regulation is measured at the bottom of the

package.NegativesidesensingisatrueKelvinconnection,

with the ground pin of the device returned to the negative

side of the load.

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

ratings 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.

Newthermalresistancespecificationshavebeendeveloped

to more accurately reflect device temperature and ensure

safe operating temperatures. The data section for these

newregulatorsprovidesaseparatethermalresistanceand

maximumjunctiontemperatureforboththeControlSection

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

couldallowexcessivejunctiontemperaturesundercertain

conditionsofambienttemperatureandheatsinkresistance.

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 electronically 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 output.

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

and assuming:

VIN(max continuous) = 9V, VOUT = 5V, IOUT = 6A,

TA = 75°C, θHEAT SINK = 1°C/W,

θCASE-TO-HEAT SINK = 0.2°C/W for P package with

thermal compound.

Power dissipation under these conditions is equal to:

PD = (VIN – VOUT) (IOUT) = 24W

Junction temperature will be equal to:

TJ = TA + PD (θHEAT SINK +θCASE-TO-HEAT SINK + θJC)

For the Control Section:

TJ = 75°C + 24W (1°C/W + 0.2°C/W + 0.5°C/W) =

118°C

118°C < 125°C = TJMAX (Control Section Commer-

cial Range)

For the Power Transistor:

TJ = 75°C + 24W (1°C/W + 0.2°C/W + 1.6°C/W)=

142°C

142°C < 150°C = TJMAX (Power Transistor Commer-

cial Range)

In both cases the junction temperature is below the maxi-

mum rating for the respective sections, ensuring reliable

operation.

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