lt3080-1 Linear Technology Corporation, lt3080-1 Datasheet - Page 14

• Manufacturer Part Number: lt3080-1
• Description: Parallelable 1.1a Adjustable Single Resistor Low Dropout Regulator
• Manufacturer: Linear Technology Corporation

LT3080-1
to give consideration to all sources of thermal resistance
from junction to ambient. This includes junction-to-case,
case-to-heat sink interface, heat sink resistance or circuit
board-to-ambient as the application dictates. Additional
heat sources nearby must also be considered.
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Surface mount heat sinks and plated
through-holes can also be used to spread the heat gener-
ated by power devices.
Junction-to-case thermal resistance is specifi ed from the
IC junction to the bottom of the case directly below the
die. This is the lowest resistance path for heat fl ow. Proper
mounting is required to ensure the best possible thermal
fl ow from this area of the package to the heat sinking
material. Note that the Exposed Pad is electrically con-
nected to the output.
The following tables list thermal resistance for several
different copper areas given a fi xed 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
*Device is mounted on topside
Table 2. DD Package, 8-Lead DFN
APPLICATIONS INFORMATION
14
*Device is mounted on topside
TOPSIDE*
TOPSIDE*
2500mm
1000mm
2500mm
1000mm
225mm
100mm
225mm
100mm
COPPER AREA
COPPER AREA
2
2
2
2
2
2
2
2
BACKSIDE
BACKSIDE
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2
2
2
2
2
2
2
2
BOARD AREA
BOARD AREA
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2
2
2
2
2
2
2
2
(JUNCTION-TO-AMBIENT)
(JUNCTION-TO-AMBIENT)
THERMAL RESISTANCE
THERMAL RESISTANCE
55°C/W
57°C/W
60°C/W
65°C/W
60°C/W
62°C/W
65°C/W
68°C/W
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 provide
better performance than found in these tables. For example,
a 4-layer, 1 ounce copper PCB board with fi ve thermal vias
from the DFN or MSOP exposed backside pad to inner layers
(connected to V
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
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
with topside copper area of 500mm
The power in the drive circuit equals:
where I
of output current. A curve of I
in the Typical Performance Characteristics curves.
The power in the output transistor equals:
The total power equals:
The current delivered to the SET pin is negligible and can
be ignored.
V
V
V
CONTROL(MAX CONTINUOUS)
IN(MAX CONTINUOUS)
OUT
P
P
P
TOTAL
DRIVE
OUTPUT
= 0.9V, I
CONTROL
= (V
= P
= (V
DRIVE
OUT
CONTROL
OUT
IN
is equal to I
= 1A, T
– V
) achieves 40°C/W thermal resistance.
+ P
= 1.575V (1.5V + 5%)
OUT
OUTPUT
– V
A
)(I
OUT
= 50°C
OUT
= 3.630V (3.3V + 10%)
OUT
CONTROL
)(I
/60. I
)
CONTROL
2
CONTROL
vs I
?
OUT
)
can be found
is a function
CONTROL
2
board
30801f