LT1083/LT1084/LT1085 Fixed
W U U
APPLICATIO S I FOR ATIO
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case-to-heatsinkinterface,andheatsinkresistanceitself.
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.
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,
θ
CASE-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
Junction-to-case thermal resistance is specified from the
ICjunctiontothebottomofthecasedirectlybelowthedie.
This is the lowest resistance path for heat flow. Proper
mounting is required to ensure the best possible thermal
flowfromthisareaofthepackagetotheheatsink.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.
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
maximum rating for the respective sections, ensuring
reliable operation.
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TYPICAL APPLICATIO S
High Efficiency Regulator
Paralleling Regulators
1mH
2 FEET #18 WIRE*
28V
INPUT
OUTPUT
IN
LT1083-5 OUT
GND
V
IN
IN
LT1083-5 OUT
GND
IN
+
10,000μF
MR1122
V
= 5V
= 0A TO 15A
470Ω
1N914
OUT
OUT
10k
I
0.015Ω
28V
LT1083-5 OUT
GND
*THE #18 WIRE ACTS
AS BALLAST RESISTANCE
INSURING CURRENT SHARING
BETWEEN BOTH DEVICES
1k
1M
4N28
10k
LT1083/4/5 TA04
LT1083/4/5 TA03
+
–
LT1011
10k
28V
1N914
1083ffd
9
Linear Systems [ Linear Systems ]
