Datasheet

LT3080
15
3080fc
The power in the drive circuit equals:
P
DRIVE
= (V
CONTROL
– V
OUT
)(I
CONTROL
)
where I
CONTROL
is equal to I
OUT
/60. 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 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
= 1A, T
A
= 50°C
Power dissipation under these conditions is equal to:
PDRIVE = (V
CONTROL
– V
OUT
)(I
CONTROL
)
I
CONTROL
=
I
OUT
60
=
1A
60
= 17mA
P
DRIVE
= (3.630V – 0.9V)(17mA) = 46mW
P
OUTPUT
= (V
IN
– V
OUT
)(I
OUT
)
P
OUTPUT
= (1.575V – 0.9V)(1A) = 675mW
Total Power Dissipation = 721mW
Junction Temperature will be equal to:
T
J
= T
A
+ P
TOTAL
• θ
JA
(approximated using tables)
T
J
= 50°C + 721mW • 64°C/W = 96°C
In this case, the junction temperature is below the maxi-
mum rating, ensuring reliable operation.
Reducing Power Dissipation
In some applications it may be necessary to reduce
the power dissipation in the LT3080 package without
sacrificing output current capability. Two techniques are
available. The first technique, illustrated in Figure 8, em-
ploys a resistor in series with the regulator’s input. The
voltage drop across R
S
decreases the LT3080’s IN-to-OUT
differential voltage and correspondingly decreases the
LT3080’s power dissipation.
As an example, assume: V
IN
= V
CONTROL
= 5V, V
OUT
= 3.3V
and I
OUT(MAX)
= 1A. Use the formulas from the Calculating
Junction Temperature section previously discussed.
Without series resistor R
S
, power dissipation in the LT3080
equals:
P
TOTAL
= 5V – 3.3V
( )
•
1A
60






+ 5V – 3.3V
( )
• 1A
= 1.73W
If the voltage differential (V
DIFF
) across the NPN pass
transistor is chosen as 0.5V, then R
S
equals:
R
S
=
5V – 3.3V
−
0.5V
1A
= 1.2Ω
Power dissipation in the LT3080 now equals:
P
TOTAL
= 5V – 3.3V
( )
•
1A
60






+ 0.5V
( )
• 1A = 0.53W
The LT3080’s power dissipation is now only 30% compared
to no series resistor. R
S
dissipates 1.2W of power. Choose
appropriate wattage resistors to handle and dissipate the
power properly.
Figure 8. Reducing Power Dissipation Using a Series Resistor
+
–
LT3080
IN
V
CONTROL
OUT
V
OUT
V
IN
ʹ
V
IN
C2
3080 F08
SET
R
SET
R
S
C1
applicaTions inForMaTion