10
LT1581/LT1581-2.5
APPLICATIONS INFORMATION
WUU
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where I
CONTROL
is equal to between I
OUT
/100 (typ) and
I
OUT
/58 (max).
I
CONTROL
is a function of output current. A curve of
I
CONTROL
vs I
OUT
can be found in the Typical Performance
Characteristics curves.
The power in the output transistor is equal to:
P
OUTPUT
= (V
POWER
– V
OUT
)(I
OUT
)
The total power is equal to:
P
TOTAL
= P
DRIVE
+ P
OUTPUT
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 the added contribution to thermal resistance is
considered. Please consult Linear Technology’s “Mount-
ing Considerations for Power Semiconductors,”
1990
Linear Applications Handbook, Volume 1
, Pages RR3-1 to
RR3-20. Note that the case of the LT1581 is electrically
connected to the output.
The following example illustrates how to calculate
maximum junction temperature. Using an LT1581 and
assuming:
V
CONTROL
(max continuous) = 5.25V (5V + 5%),
V
POWER
(max continuous) = 3.465V (3.3V + 5%),
V
OUT
= 2.5V, Iout = 4A,
T
A
= 70°C, θ
HEATSINK
= 4°C/W,
θ
CASE-HEATSINK
= 1°C/W (with thermal compound)
Power dissipation under these conditions is equal to:
Total Power Dissipation = P
DRIVE
+ P
OUTPUT
P
DRIVE
= (V
CONTROL
– V
OUT
) (I
CONTROL
)
I
CONTROL
= I
OUT
/58 = 4A/58 = 69mA
P
DRIVE
= (5.25V – 2.5V)(69mA) = 190mW
P
OUTPUT
= (V
POWER
– V
OUT
)(I
OUT
)
= ( 3.465V – 2.5V)(4A) = 3.9W
Total Power Dissipation = 4.09W
Junction temperature will be equal to:
T
J
= T
A
+ P
TOTAL
(θ
HEATSINK
+ θ
CASE-HEATSINK
+ θ
JC
)
For the Control section:
T
J
= 70°C + 4.09W(4°C/W + 1°C/W + 0.65°C/W) = 93°C
93°C < 125°C = T
JMAX
for Control Section
For the Power section:
T
J
= 70°C + 4.09W (4°C/W + 1°C/W + 2.5°C/W) = 101°C
101°C < 150°C = T
JMAX
for Power Section
In both cases the junction temperature is below the
maximum rating for the respective sections, ensuring
reliable operation.
