ZHCSHW2I June 2007 – September 2018 LM2735
PRODUCTION DATA.
The PCB design is a very important step in the thermal design procedure. The LM2735 is available in three package options (5-pin SOT-23, 8-pin MSOP-PowerPAD, and 6-pin WSON). The options are electrically the same, but difference between the packages is size and thermal performance. The WSON and MSOP-PowerPAD have thermal Die Attach Pads (DAP) attached to the bottom of the packages, and are therefore capable of dissipating more heat than the SOT-23 package. It is important that the correct package for the application is chosen. A detailed thermal design procedure has been included in this data sheet. This procedure will help determine which package is correct, and common applications will be analyzed.
There is one significant thermal PCB layout design consideration that contradicts a proper electrical PCB layout design consideration. This contradiction is the placement of external components that dissipate heat. The greatest external heat contributor is the external Schottky diode. It would be ideal to be able to separate by distance the LM2735 from the Schottky diode, and thereby reducing the mutual heating effect, however, this will create electrical performance issues. It is important to keep the LM2735, the output capacitor, and Schottky diode physically close to each other (see Figure 44). The electrical design considerations outweigh the thermal considerations. Other factors that influence thermal performance are thermal vias, copper weight, and number of board layers.