Datasheet

LT1083/LT1084/LT1085 Fixed
11
1083ffe
For more information www.linear.com/LT1083
applicaTions inForMaTion
Load Regulation
Because the LT1083 is a three-terminal device, it is not pos-
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
. Negative side sensing is a true Kelvin connection,
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.
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 Section
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
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 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:
V
IN
(max continuous) = 9V, V
OUT
= 5V, I
OUT
= 6A,
T
A
= 75°C, θ
HEAT SINK
= 1°C/W, 
θ
CASE-TO-HEAT SINK
= 0.2°C/W for P package with
thermal compound.
Power dissipation under these conditions is equal to:
P
D
= (V
IN
– V
OUT
) (I
OUT
) = 24W
Junction temperature will be equal to:
T
J
= T
A
+ P
D
(θ
HEAT SINK
+θ
CASE-TO-HEAT SINK
+ θ
JC
)
For the Control Section:
T
J
= 75°C + 24W (1°C/W + 0.2°C/W + 0.5°C/W) =
118°C
118°C < 125°C = T
JMAX
(Control Section Commer-
cial Range)
For the Power Transistor:
T
J
= 75°C + 24W (1°C/W + 0.2°C/W + 1.6°C/W)=
142°C
142°C < 150°C = T
JMAX
(Power Transistor Commer-
cial Range)
In both cases the junction temperature is below the maxi-
mum rating for the respective sections, ensuring reliable
operation.