SI6925ADQ_08 Datasheet, PDF (8/11 Pages) Vishay Siliconix – Dual N-Channel 2.5-V (G-S) MOSFET

AN1001
Vishay Siliconix
Because the TSSOP has a fine pitch foot print, the pad layout
is somewhat more demanding than the layout of the SOIC.
Careful attention must be paid to silkscreen-to-pad and
soldermask-to-pad clearances. Also, fiduciary marks may be
required. The design and spacing of the pads must be dealt
with carefully. The pads must be sized to hold enough solder
paste to form a good joint, but should not be so large or so
placed as to extend under the body, increasing the potential for
solder bridging. The pad pattern should allow for typical pick
and place errors of 0.25 mm. See Application Note 826,
Recommended Minimum Pad Patterns With Outline
Drawing Access for Vishay Siliconix MOSFETs,
(http://www.vishay.com/doc?72286), for the recommended
pad pattern for PC board layout.
THERMAL ISSUES
LITTLE FOOT TSSOP MOSFETs have been given thermal
ratings using the same methods used for LITTLE FOOT. The
maximum thermal resistance junction-to-ambient is 83_C/W
for the single die and 125_C/W for dual-die parts. TSSOP relies
on a leadframe similar to LITTLE FOOT to remove heat from
the package. The single- and dual-die leadframes are shown
in Figure 4.
a) 8-Pin Single-Pad TSSOP
b) 8-Pin Dual-Pad TSSOP
Figure 4. Leadframe
The MOSFETs are characterized using a single pulse power
test. For this test the device mounted on a one-square-inch
piece of copper clad FR-4 PC board, such as those shown in
Figure 5. The single pulse power test determines the
maximum amount of power the part can handle for a given
pulse width and defines the thermal resistance
junction-to-ambient. The test is run for pulse widths ranging
from approximately 10 ms to 100 seconds. The thermal
resistance at 30 seconds is the rated thermal resistance for the
part. This rating was chosen to allow comparison of packages
and leadframes. At longer pulse widths, the PC board thermal
charateristics become dominant, making all parts look the
same.
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Figure 5.
The actual test is based on dissipating a known amount of
power in the device for a known period of time so the junction
temperature is raised to 150_C. The starting and ending
junction temperatures are determined by measuring the
forward drop of the body diode. The thermal resistance for that
pulse width is defined by the temperature rise of the junction
above ambient and the power of the pulse, DTja/P.
Figure 6 shows the single pulse power curve of the Si6436DQ
laid over the curve of the Si9936DY to give a comparison of the
thermal performance. The die in the two devices have
equivalent die areas, making this a comparison of the
packaging. This comparison shows that the TSSOP package
performs as well as the SOIC out to 150 ms, with long-term
performance being 0.5 W less. Although the thermal
performance is less, LITTLE FOOT TSSOP will operate in a
large percentage of applications that are currently being
served by LITTLE FOOT.
14.0
12.0
10.0
8.0
6.0
4.0
Si9936
2.0
Si6436
0.0
0.1
1
10
100
Time (Sec.)
Figure 6. Comparison of Thermal Performance
CONCLUSION
TSSOP power MOSFETs provide a significant reduction in PC
board footprint and package height, allowing reduction in
board size and application where SOICs will not fit. This is
accomplished using a standard IC package and a custom
leadframe, combining small size with good power handling
capability.
For the TSSOP-8 package outline visit:
http://www.vishay.com/doc?71201
For the SOIC-8 package outline visit:
http://www.vishay.com/doc?71192
Document Number: 70571
12-Dec-03