AN1001 [VISHAY]
The Next Step in Surface-Mount Power MOSFETs; 下一步在表面贴装功率MOSFET
| 型号： | AN1001 |
| 厂家： | 
VISHAY |
| 描述： | The Next Step in Surface-Mount Power MOSFETs |
| 文件： | 总2页 (文件大小：50K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
AN1001
Vishay Siliconix
LITTLE FOOTR TSSOP-8
The Next Step in Surface-Mount Power MOSFETs
Wharton McDaniel and David Oldham
When Vishay Siliconix introduced its LITTLE FOOT
MOSFETs, it was the first time that power MOSFETs had been
offered in a true surface-mount package, the SOIC. LITTLE
FOOT immediately found a home in new small form factor disk
drives, computers, and cellular phones.
This is the low profile demanded by applications such as
PCMCIA cards.
It reduces the power package to the same height as many
resistors and capacitors in 0805 and 0605 sizes. It also allows
placement on the “passive” side of the PC board.
The new LITTLE FOOT TSSOP-8 power MOSFETs are the
natural evolutionary response to the continuing demands of
many markets for smaller and smaller packages. LITTLE
FOOT TSSOP-8 MOSFETs have a smaller footprint and a
lower profile than LITTLE FOOT SOICs, while maintaining low
rDS(on) and high thermal performance. Vishay Siliconix has
accomplished this by putting one or two high-density MOSFET
die in a standard 8-pin TSSOP package mounted on a custom
leadframe.
The standard pinouts of the LITTLE FOOT TSSOP-8
packages have been changed from the standard established
by LITTLE FOOT. This change minimizes the contribution of
interconnection resistance to rDS(on) and maximizes the
transfer of heat out of the package.
Figure 2 shows the pinouts for a single-die TSSOP. Notice that
both sides of the package have Source and Drain
connections, whereas LITTLE FOOT has the Source and Gate
connections on one side of the package, and the Drain
connections are on the opposite side.
THE TSSOP-8 PACKAGE
LITTLE FOOT TSSOP-8 power MOSFETs require
approximately half the PC board area of an equivalent LITTLE
FOOT device (Figure 1). In addition to the reduction in board
area, the package height has been reduced to 1.1 mm.
Drain
Source
Source
Gate
Drain
Source
Source
Drain
Figure 2. Pinouts for Single Die TSSOP
Figure 3 shows the standard pinouts for a dual-die TSSOP-8.
In this case, the connections for each individual MOSFET
occupy one side.
Top View
Drain 1
Source 1
Source 1
Gate 1
Drain 2
Source 2
Source 2
Gate 2
Side View
Figure 1. An TSSOP-8 Package Next to a SOIC-8 Package
with Views from Both Top and Side
Figure 3. Pinouts for Dual-Die TSSOP
Document Number: 70571
12-Dec-03
www.vishay.com
1
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
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.
Drawing Access
for
Vishay
Siliconix
MOSFETs,
(http://www.vishay.com/doc?72286), for the recommended
pad pattern for PC board layout.
THERMAL ISSUES
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.
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.
14.0
12.0
10.0
8.0
a) 8-Pin Single-Pad TSSOP
6.0
Si9936
4.0
2.0
0.0
Si6436
1
0.1
10
100
Time (Sec.)
b) 8-Pin Dual-Pad TSSOP
Figure 6. Comparison of Thermal Performance
Figure 4. Leadframe
CONCLUSION
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.
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
www.vishay.com
2
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