Datasheet

LT3045
22
Rev. B
For more information www.analog.com
Thermal Considerations
The LT3045 has internal power and thermal limiting circuits
that protect the device under overload conditions. The ther
-
mal shutdown temperature is nominally 165°C with about
8°C of hysteresis. For continuous normal load conditions,
do not exceed the maximum junction temperature (125°C
for E- and I-grades and 150°C for H- and MP-Grades). It
is important to consider 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. Additionally,
consider all heat sources in close proximity to the LT3045.
The undersides of the DFN and MSOP packages have
exposed metal from the lead frame to the die attachment.
Both packages allow heat to directly transfer from the die
junction to the PCB metal to limit maximum operating
junction temperature. The dual-in-line pin arrangement
allows metal to extend beyond the ends of the package
on the topside (component side) of the PCB.
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PCB and its
copper traces. Copper board stiffeners and plated through-
holes can also be used to spread the heat generated by
the regulator.
Tables 3 and 4 list thermal resistance as a function of copper
area 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 top/bottom planes with a total board thick
-
ness of 1.6mm. The four layers were electrically isolated
with no thermal vias present. PCB layers, copper weight,
board layout and thermal vias affect the resultant thermal
resistance. For more information on thermal resistance
and high thermal conductivity test boards, refer to JEDEC
standard JE
SD51, notably JESD51-7 and JESD51-12.
Achieving low thermal resistance necessitates attention
to detail and careful PCB layout.
Table 3. Measured Thermal Resistance for DFN Package
COPPER AREA
BOARD AREA
THERMAL
RESISTANCETOP SIDE* BOTTOM SIDE
2500mm
2
2500mm
2
2500mm
2
34°C/W
1000mm
2
2500mm
2
2500mm
2
34°C/W
225mm
2
2500mm
2
2500mm
2
35°C/W
100mm
2
2500mm
2
2500mm
2
36°C/W
*Device is mounted on topside
Table 4. Measured Thermal Resistance for MSOP Package
COPPER AREA
BOARD AREA
THERMAL
RESISTANCETOP SIDE* BOTTOM SIDE
2500mm
2
2500mm
2
2500mm
2
33°C/W
1000mm
2
2500mm
2
2500mm
2
33°C/W
225mm
2
2500mm
2
2500mm
2
34°C/W
100mm
2
2500mm
2
2500mm
2
35°C/W
*Device is mounted on topside
Calculating Junction Temperature
Example: Given an output voltage of 3.3V and input voltage
of 5V ± 5%, output current range from 1mA to 500mA,
and a maximum ambient temperature of 85°C, what is the
maximum junction temperature?
The LT3045’s power dissipation is:
I
OUT(MAX)
• (V
IN(MAX)
– V
OUT
) + I
GND
• V
IN(MAX)
where:
I
OUT(MAX)
= 500mA
V
IN(MAX)
= 5.25V
I
GND
(at I
OUT
= 500mA and V
IN
= 5.25V) = 12.5mA
thus:
P
DISS
= 0.5A • (5.25V – 3.3V) + 12.5mA • 5.25V = 1W
Using a DFN package, the thermal resistance is in the
range of 34°C/W to 36°C/W depending on the copper area.
Therefore, the junction temperature rise above ambient
approximately equals:
1W • 35°C/W = 35°C
The maximum junction temperature equals the maxi-
mum ambient temperature plus the maximum junction
temperature rise above ambient:
T
JMAX
= 85°C + 35°C = 120°C
APPLICATIONS INFORMATION
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