LT1083/LT1084/LT1085
applicaTions inForMaTion
Connected as shown, R is not multiplied by the divider
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.
P
ratio.R isabout0.004Ωperfootusing16-gaugewire.This
P
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
For example, using an LT1083CK (TO-3, Commercial)
and assuming:
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
newregulatorsprovidesaseparatethermalresistanceand
maximumjunctiontemperatureforboththeControlSection
andthePowerTransistor.Previousregulators,withasingle
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.
V (Max Continuous) = 9V, V
= 5V, I
= 6A,
OUT
IN
OUT
T = 75°C, θ
= 1°C/W,
A
HEAT SINK
θ
= 0.2°C/W for K package with
CASE-TO-HEAT SINK
thermal compound.
Power dissipation under these conditions is equal to:
P = (V – V )(I ) = 24W
D
IN
OUT OUT
Junction temperature will be equal to:
T = T + P (θ + θ
+ θ )
J
A
D
HEAT SINK
CASE-TO-HEAT SINK
JC
For the Control Section:
T = 75°C + 24W (1°C/W + 0.2°C/W + 0.6°C/W) = 118°C
J
118°C < 125°C = T
Commercial Range)
(Control Section
JMAX
For the Power Transistor:
T = 75°C + 24W (1°C/W + 0.2°C/W + 1.6°C/W) = 142°C
J
142°C < 150°C = T
Commercial Range)
(Power Transistor
JMAX
Junction-to-case thermal resistance is specified from the
IC junction to the bottom of the case directly below the
die. Thisis thelowestresistancepath forheatflow. Proper
mounting is required to ensure the best possible thermal
flowfromthisareaofthepackagetotheheatsink.Thermal
In both cases the junction temperature is below the maxi-
mum rating for the respective sections, ensuring reliable
operation.
108345fh
13
For more information www.linear.com/LT1083
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