Datasheet Si2399DS (Vishay) - 8

AN807 Vishay Siliconix Mounting LITTLE FOOT
R
SOT-23 Power MOSFETs
Wharton McDaniel Surface-mounted LITTLE FOOT power MOSFETs use integrated ambient air. This pattern uses al the available area underneath the circuit and small-signal packages which have been been modified body for this purpose. to provide the heat transfer capabilities required by power devices. Leadframe materials and design, molding compounds, and die attach materials have been changed, while the footprint of the packages remains the same. 0.114 2.9 0.081 See Application Note 826, Recommended Minimum Pad 2.05 Patterns With Outline Drawing Access for Vishay Siliconix 0.150 3.8 MOSFETs, (http://www.vishay.com/doc?72286), for the basis of the pad design for a LITTLE FOOT SOT-23 power MOSFET 0.059 footprint . In converting this footprint to the pad set for a power 1.5 device, designers must make two connections: an electrical connection and a thermal connection, to draw heat away from the package. 0.0394 0.037 1.0 0.95
FIGURE 1.
Footprint With Copper Spreading The electrical connections for the SOT-23 are very simple. Pin 1 is the gate, pin 2 is the source, and pin 3 is the drain. As in the other LITTLE FOOT packages, the drain pin serves the additional Since surface-mounted packages are small, and reflow soldering function of providing the thermal connection from the package to is the most common way in which these are affixed to the PC the PC board. The total cross section of a copper trace connected board, “thermal” connections from the planar copper to the pads to the drain may be adequate to carry the current required for the have not been used. Even if additional planar copper area is used, application, but it may be inadequate thermally. Also, heat spreads there should be no problems in the soldering process. The actual in a circular fashion from the heat source. In this case the drain pin solder connections are defined by the solder mask openings. By is the heat source when looking at heat spread on the PC board. combining the basic footprint with the copper plane on the drain pins, the solder mask generation occurs automatically. Figure 1 shows the footprint with copper spreading for the SOT-23 A final item to keep in mind is the width of the power traces. The package. This pattern shows the starting point for utilizing the absolute minimum power trace width must be determined by the board area available for the heat spreading copper. To create this amount of current it has to carry. For thermal reasons, this pattern, a plane of copper overlies the drain pin and provides minimum width should be at least 0.020 inches. The use of wide planar copper to draw heat from the drain lead and start the traces connected to the drain plane provides a low-impedance process of spreading the heat so it can be dissipated into the path for heat to move away from the device. Document Number: 70739 www.vishay.com 26-Nov-03
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