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LM3404 Datasheet(PDF) 3 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part # LM3404
Description  Thermal Performance of the LM3404/04HV in SO-8 and PSOP-8 Packages
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Manufacturer  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM3404 Datasheet(HTML) 3 Page - National Semiconductor (TI)

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Predicting Thermals with FR4
30019203
FIGURE 3. System Diagram 1
Using finite element analysis, the thermal performance of the
LM3404HV in PSOP-8 and SO-8 was simulated with three
different sized PCBs. These simulations assumed an ambient
temperature of 22°C, a power dissipation of 1W, and a 2-layer
PCB comprised of 1oz copper on top and bottom, separated
by 62mil of FR4. The results are listed below:
θ
JA (°C/W)
PSOP-8
SO-8
1.25" x 1.95" (Eval Board)
55.1
104.8
1" x 1" (Typical Case)
73.0
120.8
0.5" x 0.5" (Worst Case)
240.2
263.7
There is good correlation between the test results and simu-
lation results for the Evaluation Board Case. The strong in-
fluence of the PCB size can be seen by comparing the results
from the demo board to those of the Typical and Worst Cases.
Without enough copper area to spread and dissipate heat, the
advantages of the exposed pad are reduced, and even the
PSOP-8 package will be limited to low power, low ambient
temperature applications.
Predicting Thermals with Metal-
Core PCB
30019204
FIGURE 4. System Diagram 2
Finite element analysis was again used to simulate the per-
formance of the LM3404HV when dissipating 1W in PSOP-8
and SO-8 packages on a 1” square section of MCPCB with
the following composition:
Material
Thickness (mil)
Thermal
Conductivity
(W/m-K)
Metal Core (6061-T6
aluminum)
40
167
Insulator Material
3
2.4
Copper Traces (75%
coverage)
1.4
377
To attain typical operating conditions, the 1” square section
containing the LM3404HV was assumed to be part of a larger
MCPCB measuring 1” x 6”. The other 5 square inches con-
tained 5 high power LEDs, and it was assumed that they
heated the entire board to a uniform temperature, TBD, of 50°
C (representing a large heatsink) and 75°C (representing a
smaller heatsink.) The results for thermal impedance from the
board to the LM3404HV die,
, are listed below:
(°C/W)
T
BD = 75°C
T
A = 60°C
T
BD = 75°C
T
A = 45°C
T
BD = 50°C
T
A = 40°C
T
BD = 50°C
T
A = 30°C
SO-8
70.5
69.2
71.4
70.5
PSOP-
8
9.8
9.2
10.5
10.1
The parameter
can be used to calculate the LM3404HV
junction temperature using the following equation:
Alternatively,
can be used to determine the maximum tol-
erable board temperature by working back from a T
J-MAX of
125°C:
Two main conclusions can be drawn from the results. First,
the thermal impedance has little dependence on the board
temperature or the ambient temperature, and instead de-
pends highly on the package used. Second, the PSOP-8
package is again far superior to the SO-8 package when the
DAP is soldered to a mass with low thermal impedance, such
as an MCPCB.
3
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