lt3085ems8e Linear Technology Corporation, lt3085ems8e Datasheet - Page 16

• Manufacturer Part Number: lt3085ems8e
• Description: Adjustable 500ma Single Resistor Low Dropout Regulator
• Manufacturer: Linear Technology Corporation

LT3085
APPLICATIONS INFORMATION
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
generated by power devices. Boards specifi ed in thermal
resistance tables have no vias on plated through-holes
from topside to backside.
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
connected 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.
PCB layers, copper weight, board layout and thermal vias
affect the resultant thermal resistance. Although Tables
2 and 3 provide thermal resistance numbers for 2-layer
board with 1 ounce copper, modern multi-layer PCBs
provide better performance than found in these tables.
For example, a 4-layer, 1 ounce copper PCB board with
5 thermal vias from the DFN or MSOP exposed backside
pad to inner layers (connected to V
thermal resistance. Demo circuit 1401A’s board layout
achieves this 40 °C/W performance. This is approximately
a 45% improvement over the numbers shown in Tables
2 and 3.
16
OUT
) achieves 40 °C/W
Table 2. MSE Package, 8-Lead MSOP
*Device is mounted on topside
Table 3. DCB Package, 6-Lead DFN
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 0.5A and a maximum ambi-
ent 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.
*Device is mounted on topside
For future information on the thermal resistance and using thermal
information, refer to JEDEC standard JESD51, notably JESD51-12.
TOPSIDE*
TOPSIDE*
2500mm
1000mm
2500mm
1000mm
225mm
100mm
225mm
100mm
P
DRIVE
COPPER AREA
COPPER AREA
CONTROL
2
2
2
2
2
2
2
2
= (V
BACKSIDE
BACKSIDE
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
CONTROL
is equal to I
2
2
2
2
2
2
2
2
BOARD AREA
BOARD AREA
– V
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
OUT
OUT
CONTROL
)(I
2
2
2
2
2
2
2
2
/60. I
CONTROL
(JUNCTION-TO-AMBIENT)
(JUNCTION-TO-AMBIENT)
2
CONTROL
THERMAL RESISTANCE
THERMAL RESISTANCE
vs I
?
OUT
)
55°C/W
57°C/W
60°C/W
65°C/W
68°C/W
70°C/W
73°C/W
78°C/W
can be found
is a function
CONTROL
2
board
3085f