Datasheet

LT3085
15
3085fb
If the LT3085 is confi gured as a three-terminal (single supply)
regulator with IN and V
CONTROL
shorted together, the internal
diode of the IN pin will protect the V
CONTROL
pin.
Like any other regulator, exceeding the maximum input-
to-output differential causes internal transistors to break
down and then none of the internal protection circuitry
is functional.
Thermal Considerations
The LT3085 has internal power and thermal limiting cir-
cuitry designed to protect it under overload conditions.
For continuous normal load conditions, maximum junc-
tion temperature must not be exceeded. It is important
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
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
APPLICATIONS INFORMATION
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
OUT
) achieves 40 °C/W
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.
Table 2. 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 3. DCB Package, 6-Lead DFN
COPPER AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
TOPSIDE* BACKSIDE BOARD AREA
2500mm
2
2500mm
2
2500mm
2
68°C/W
1000mm
2
2500mm
2
2500mm
2
70°C/W
225mm
2
2500mm
2
2500mm
2
73°C/W
100mm
2
2500mm
2
2500mm
2
78°C/W
*Device is mounted on topside
For future information on the thermal resistance and using thermal
information, refer to JEDEC standard JESD51, notably JESD51-12.
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 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
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 function
of output current. A curve of I
CONTROL
vs I
OUT
can be found
in the Typical Performance Characteristics curves.