Datasheet Si7611DN (Vishay) - 10

AN822
Vishay Siliconix
TABLE 1: EQIVALENT STEADY STATE PERFORMANCE Package SO-8 TSSOP-8 TSOP-8 PPAK 1212 PPAK SO-8 Configuration Single Dual Single Dual Single Dual Single Dual Single Dual
Thermal Resiatance RthJC(C/W) 20 40 52 83 40 90 2.4 5.5 1.8 5.5 PowerPAK 1212 Standard SO-8 Standard TSSOP-8 TSOP-6 49.8 °C 85 °C 149 °C 125 °C 2.4 °C/W 20 °C/W 52 °C/W 40 °C/W PC Board at 45 °C
Figure 4.
Temperature of Devices on a PC Board
THERMAL PERFORMANCE Introduction Spreading Copper
A basic measure of a device’s thermal performance is Designers add additional copper, spreading copper, to the junction-to-case thermal resistance, Rθjc, or the the drain pad to aid in conducting heat from a device. It junction to- foot thermal resistance, Rθjf. This parameter is helpful to have some information about the thermal is measured for the device mounted to an infinite heat performance for a given area of spreading copper. sink and is therefore a characterization of the device Figure 5 and Figure 6 show the thermal resistance of a only, in other words, independent of the properties of the PowerPAK 1212-8 single and dual devices mounted on object to which the device is mounted. Table 1 shows a a 2-in. x 2-in., four-layer FR-4 PC boards. The two inter- comparison of the PowerPAK 1212-8, PowerPAK SO-8, nal layers and the backside layer are solid copper. The standard TSSOP-8 and SO-8 equivalent steady state internal layers were chosen as solid copper to model the performance. large power and ground planes common in many appli- By minimizing the junction-to-foot thermal resistance, the cations. The top layer was cut back to a smaller area and MOSFET die temperature is very close to the tempera- at each step junction-to-ambient thermal resistance ture of the PC board. Consider four devices mounted on measurements were taken. The results indicate that an a PC board with a board temperature of 45 °C (Figure 4). area above 0.2 to 0.3 square inches of spreading copper Suppose each device is dissipating 2 W. Using the junc- gives no additional thermal performance improvement. tion-to-foot thermal resistance characteristics of the A subsequent experiment was run where the copper on PowerPAK 1212-8 and the other SMT packages, die the back-side was reduced, first to 50 % in stripes to temperatures are determined to be 49.8 °C for the Pow- mimic circuit traces, and then totally removed. No signif- erPAK 1212-8, 85 °C for the standard SO-8, 149 °C for icant effect was observed. standard TSSOP-8, and 125 °C for TSOP-6. This is a 4.8 °C rise above the board temperature for the Power- PAK 1212-8, and over 40 °C for other SMT packages. A 4.8 °C rise has minimal effect on rDS(ON) whereas a rise of over 40 °C will cause an increase in rDS(ON) as high as 20 %. Document Number 71681 www.vishay.com 03-Mar-06 3