LT3081
20
3081fc
applicaTions inForMaTion
T7 Package, 7-Lead TO-220
Thermal Resistance (Junction-to-Case) = 3°C/W
For further information on thermal resistance and using
thermal information, refer to JEDEC standard JESD51,
notably JESD51-12.
PCB layers, copper weight, board layout and thermal vias
affect the resultant thermal resistance. Tables 3 through 5
provide thermal resistance numbers for best-case 4-layer
boards with 1oz internal and 2oz external copper. Modern,
multilayer PCBs may not be able to achieve quite the same
level performance as found in these tables. Demo circuit
1870A’s board layout using multiple inner V
OUT
planes
and multiple thermal vias achieves 16°C/W performance
for the FE package.
Calculating Junction Temperature
Example: Given an output voltage of 0.9V, an IN voltage
of 2.5V ±5%, output current range from 10mA to 1A
and a maximum ambient temperature of 50°C, what is
the maximum junction temperature for the DD-Pak on a
2500mm2 board with topside copper of 1000mm
2
?
The power in the circuit equals:
P
TOTAL
= (V
IN
– V
OUT
)(I
OUT
)
The current delivered to the SET pin is negligible and can
be ignored.
V
IN(MAX_CONTINUOUS)
= 2.625V (2.5V + 5%)
V
OUT
= 0.9V, I
OUT
= 1A, T
A
= 50°C
Power dissipation under these conditions equals:
P
TOTAL
= (V
IN
– V
OUT
)(I
OUT
)
P
TOTAL
= (2.625V – 0.9V)(1A) = 1.73W
Junction Temperature equals:
T
J
= T
A
+ P
TOTAL
•
θ
JA
(using tables)
T
J
= 50°C + 1.73W
• 14°
C/W = 74.2°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 LT3081 package without sacrificing
output current capability. Two techniques are available. The
first technique, illustrated in Figure 12, employs a resis-
tor in series with the regulator’s input. The voltage drop
across R
S
decreases the LT3081’s IN-to-OUT differential
voltage and correspondingly decreases the LT3081’s
power dissipation.
As an example, assume: V
IN
= 7V, V
OUT
= 3.3V and I
OUT(MAX)
= 1.5A. Use the formulas from the Calculating Junction
Temperature section previously discussed.
Without series resistor R
S
, power dissipation in the
LT3081 equals:
P
TOTAL
= (7V – 3.3V
) • 1.5
A = 5.55W
If the voltage differential (V
DIFF
) across the LT3081 is
chosen as 1.5V, then R
S
equals:
R
S
=
7V – 3.3V – 1.5V
1.5A
=
1.5
Ω
Power dissipation in the LT3081 now equals:
P
TOTAL
= 1.5V
• 1.5
A = 2.25W
The LT3081’s power dissipation is now only 40% compared
to no series resistor. R
S
dissipates 3.3W of power. Choose
appropriate wattage resistors or use multiple resistors in
parallel to handle and dissipate the power properly.
3081 F12
IN
V
IN
′
SET
OUT
+
–
LT3081
50µA
R
SET
R
S
V
OUT
V
IN
C2
C1
Figure 12. Reducing Power Dissipation Using a Series Resistor
