Linear Technology LT3081, Datasheet, Page: 19 / 32

LT3081
19
3081fc
For more information www.linear.com/LT3081
Connecting a resistor from I
MON
to ground converts the
I
MON
pin current into a voltage to allow for monitoring by
an ADC. With a 1k resistor, 0mV to 300mV indicates 0A
to 1.5A of load current.
Compensating for Cable Drops with I
MON
The I
MON
pin can compensate for resistive drops in wires
or cables between the LT3081 and the load. Breaking the
SET resistor into two pieces adjusts the output voltage as a
function of load current. The ratio of the output wire/cable
impedance to the bottom resistor should be 1:5000. The
sum total of the two SET resistor values determines the
initial output voltage. Figure 11 shows a typical application
and formulas for calculating resistor values.
PC board, copper traces and planes. Surface mount heat
sinks, plated through-holes and solder-filled vias can also
spread the heat generated by power devices.
Junction-to-case thermal resistance is specified from the
IC junction to the bottom of the case directly, or the bot-
tom of the pin most directly in the heat path. This is the
lowest thermal resistance path for heat flow. Only proper
device mounting ensures the best possible thermal flow
from this area of the packages to the heat sinking material.
Note that the exposed pad of the DFN and TSSOP pack-
ages and the tab of the DD-Pak and TO-220 packages
are electrically connected to the output (V
OUT
).
Tables 3 through 5 list thermal resistance as a function
of copper areas on a fixed board size. All measurements
were taken in still air on a 4-layer FR-4 board with 1oz
solid internal planes and 2oz external trace planes with a
total finished board thickness of 1.6mm.
Table 3. DF Package, 12-Lead DFN
COPPER AREA
BOARD AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT) TOPSIDE* BACKSIDE
2500mm
2
2500mm
2
2500mm
2
18°C/W
1000mm
2
2500mm
2
2500mm
2
22°C/W
225mm
2
2500mm
2
2500mm
2
29°C/W
100mm
2
2500mm
2
2500mm
2
35°C/W
*Device is mounted on topside
Table 4. FE Package, 16-Lead TSSOP
COPPER AREA
BOARD AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT) TOPSIDE* BACKSIDE
2500mm
2
2500mm
2
2500mm
2
16°C/W
1000mm
2
2500mm
2
2500mm
2
20°C/W
225mm
2
2500mm
2
2500mm
2
26°C/W
100mm
2
2500mm
2
2500mm
2
32°C/W
*Device is mounted on topside
Table 5. R Package, 7-Lead DD-Pak
COPPER AREA
BOARD AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT) TOPSIDE* BACKSIDE
2500mm2 2500mm2 2500mm2 13°C/W
1000mm2 2500mm2 2500mm2 14°C/W
225mm2 2500mm2 2500mm2 16°C/W
*Device is mounted on topside
applicaTions inForMaTion
Figure 11. Using I
MON
to Compensate for Cable Drops
Thermal Considerations
The LT3081’s internal power and thermal limiting circuitry
protects itself under overload conditions. For continuous
normal load conditions, do not exceed the 125°C (E- and
I-grades) or 150°C (H- and MP-grades) maximum junc-
tion temperature. Carefully consider all sources of thermal
resistance from junction-to-ambient. This includes (but is
not limited to) junction-to-case, case-to-heat sink inter-
face, heat sink resistance or circuit board-to-ambient as
the application dictates. Consider all additional, adjacent
heat generating sources in proximity on the PCB.
Surface mount packages provide the necessary heat
sinking by using the heat spreading capabilities of the
LT3081
IN
C
IN
1µF
C
OUT
10µF
3081 F11
OUT
SET
R
SET
29.8k
R
COMP
= 5000 • R
CABLE(TOTAL)
V
OUT(LOAD)
= 50µA (R
SET
+ R
COMP
)
R
CABLE2
0.02Ω
R
CABLE
0.02Ω
R
COMP
200Ω
I
MON
LOAD