Datasheet

LT3080-1
14
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The following tables list thermal resistance for several
different copper areas given a fixed board size. All mea-
surements were taken in still air on two-sided 1/16" FR-4
board with one ounce copper.
Table 1. MSE Package, 8-Lead MSOP
COPPER AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
TOPSIDE* BACKSIDE BOARD AREA
2500mm
2
2500mm
2
2500mm
2
55°C/W
1000mm
2
2500mm
2
2500mm
2
57°C/W
225mm
2
2500mm
2
2500mm
2
60°C/W
100mm
2
2500mm
2
2500mm
2
65°C/W
*Device is mounted on topside
Table 2. DD Package, 8-Lead DFN
COPPER AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
TOPSIDE* BACKSIDE BOARD AREA
2500mm
2
2500mm
2
2500mm
2
60°C/W
1000mm
2
2500mm
2
2500mm
2
62°C/W
225mm
2
2500mm
2
2500mm
2
65°C/W
100mm
2
2500mm
2
2500mm
2
68°C/W
*Device is mounted on topside
PCB layers, copper weight, board layout and thermal vias
affect the resultant thermal resistance. Although Tables 1
and 2 provide thermal resistance numbers for a 2-layer
board with 1 ounce copper, modern multilayer PCBs pro-
vide better performance than found in these tables. For
example, a 4-layer, 1 ounce copper PCB board with five
thermal vias from the DFN or MSOP exposed backside pad
to inner layers (connected to V
OUT
) achieves 40°C/W ther-
mal resistance. Demo circuit 995A’s board layout achieves
this 40°C/W performance. This is approximately a 33%
improvement over the numbers shown in Tables 1 and 2.
Calculating Junction Temperature
Example: Given an output voltage of 0.9V, a V
CONTROL
voltage of 3.3V ±10%, an IN voltage of 1.5V ±5%, output
current range from 1mA to 1A and a maximum ambient
temperature of 50°C, what will the maximum junction
temperature be for the DFN package on a 2500mm
2
board
with topside copper area of 500mm
2
?
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 func-
tion 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.
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