Datasheet

LT3085
16
3085fb
APPLICATIONS INFORMATION
Figure 8. Reducing Power Dissipation Using a Series Resistor
+
–
LT3085
IN
V
CONTROL
OUT
V
OUT
V
IN
a
V
IN
C2
3085 F08
SET
R
SET
R
S
C1
The power in the output transistor equals:
P
OUTPUT
= (V
IN
– V
OUT
)(I
OUT
)
The total power equals:
P
TOTAL
= P
DRIVE
+ P
OUTPUT
The current delivered to the SET pin is negligible and can
be ignored.
V
CONTROL(MAX CONTINUOUS)
= 3.630V (3.3V + 10%)
V
IN(MAX CONTINUOUS)
= 1.575V (1.5V + 5%)
V
OUT
= 0.9V, I
OUT
= 0.5A, T
A
= 50°C
Power dissipation under these conditions is equal to:
P
DRIVE
= (V
CONTROL
– V
OUT
)(I
CONTROL
)
I
CONTROL
=
I
OUT
60
=
0.5A
60
= 8.3mA
P
DRIVE
= (3.630V – 0.9V)(8.3mA) = 23mW
P
OUTPUT
= (V
IN
– V
OUT
)(I
OUT
)
P
OUTPUT
= (1.575V – 0.9V)(0.5A) = 337mW
Total Power Dissipation = 360mW
Junction Temperature will be equal to:
T
J
= T
A
+ P
TOTAL
• θ
JA
(approximated using tables)
T
J
= 50°C + 360mW • 73°C/W = 76°C
In this case, the junction temperature is below the maximum
rating, ensuring reliable operation.
Reducing Power Dissipation
In some applications it may be necessary to reduce
the power dissipation in the LT3085 package without
sacrifi cing output current capability. Two techniques are
available. The fi rst technique, illustrated in Figure 8, em-
ploys a resistor in series with the regulator’s input. The
voltage drop across R
S
decreases the LT3085’s IN-to-OUT
differential voltage and correspondingly decreases the
LT3085’s power dissipation.
As an example, assume: V
IN
= V
CONTROL
= 5V, V
OUT
= 3.3V
and I
OUT(MAX)
= 0.5A. Use the formulas from the Calculating
Junction Temperature section previously discussed.