LT1083/LT1084/LT1085 Fixed
APPLICATIO S I FOR ATIO
case- to-heat sink interface, and heat sink resistance itself.
New thermal resistance specifications have been devel-
oped to more accurately reflect device temperature and
ensure safe operating temperatures. The data section for
these new regulators provides a separate thermal resis-
tance and maximum junction temperature for both the
Control Section
and the
Power Section.
Previous regula-
tors, with a single junction-to-case thermal resistance
specification, used an average of the two values provided
here and therefore could allow excessive junction tem-
peratures under certain conditions of ambient tempera-
ture 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 electroni-
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 electronically connected to the ouput.
TYPICAL APPLICATIO S
High Efficiency Regulator
1mH
IN
LT1083-5 OUT
GND
470Ω
1N914
OUTPUT
28V
INPUT
10k
+
10,000μF
MR1122
28V
1k
1M
4N28
+
LT1011
10k
–
1N914
10k
28V
U
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
maximum rating for the respective sections, ensuring
reliable operation.
Paralleling Regulators
2 FEET #18 WIRE*
V
IN
IN
LT1083-5 OUT
GND
V
OUT
= 5V
I
OUT
= 0A TO 15A
0.015Ω
IN
LT1083-5 OUT
GND
*THE #18 WIRE ACTS
AS BALLAST RESISTANCE
INSURING CURRENT SHARING
BETWEEN BOTH DEVICES
LT1083/4/5 TA04
LT1083/4/5 TA03
W
U
U U
1083ffd
9
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
