Datasheet

9
LT1585A/LT1585A-3.3
1585afa
Thermal Considerations
The LT1585A/LT1585A-3.3 family protects the device
under overload conditions with internal power and ther-
mal limiting circuitry. However, for normal continuous
load conditions, do not exceed maximum junction tem-
perature ratings. It is important to consider all sources of
thermal resistance from junction-to-ambient. These
sources include the junction-to-case resistance, the case-
to-heat sink interface resistance and the heat sink resis-
tance. Thermal resistance specifications have been devel-
oped to more accurately reflect device temperature and
ensure safe operating temperatures. The Electrical Char-
acteristics section provides a separate thermal resistance
and maximum junction temperature for both the control
circuitry and the power transistor. Older regulators, with
a single junction-to-case thermal resistance specifica-
tion, use an average of the two values provided here and
allow excessive junction temperatures under certain con-
ditions of ambient temperature and heat sink resistance.
Calculate the maximum junction temperature 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 ensures the best thermal flow from this area of
the package to the heat sink. Linear Technology strongly
recommends thermal compound at the case-to-heat sink
interface. Use a thermally conductive spacer if the case of
the device must be electrically isolated and include its
contribution to the total thermal resistance. Please con-
sult “Mounting Considerations for Power Semiconduc-
tors”
1990 Linear Applications Handbook, Volume I ,
Pages RR3-1 to RR3-20. The output connects to the case
of both the LT1585A and the LT1585A-3.3.
For example, using an LT1585ACT-3.3 (TO-220, com-
mercial) and assuming:
V
IN
(Max Continuous) = 5.25V (5V + 5%), V
OUT
= 3.3V,
I
OUT
= 5A
T
A
= 70°C, θ
HEAT SINK
= 3°C/W
θ
CASE-TO-HEAT SINK
= 1°C/W (with Thermal Compound)
Power dissipation under these conditions is equal to:
P
D
= (V
IN
– V
OUT
)(I
OUT
) = (5.25 – 3.3)(5) = 9.75W
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
= 70°C + 9.75W (3°C/W + 1°C/W + 0.7°C/W)
= 115.8°C
115.8°C < 125°C = T
JMAX
(Control Section Commer-
cial range)
For the Power Transistor:
T
J
= 70°C + 9.75W (3°C/W + 1°C/W + 3°C/W)
= 138.3°C
138.3°C < 150°C = T
JMAX
(Power Transistor Com-
mercial Range)
In both cases the junction temperature is below the
maximum rating for the respective sections, ensuring
reliable operation.
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