ZHCSIR2 September 2018 LM2735-Q1
PRODUCTION DATA.
The PCB design is a very important step in the thermal design procedure. The LM2735-Q1 is available in 2 package options (5-pin SOT-23 and 6-pin WSON). The options are electrically the same, but difference between the packages is size and thermal performance. The WSON has 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 helps determine which package is correct, and common applications are 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-Q1 from the Schottky diode, and thereby reducing the mutual heating effect, however, this creates electrical performance issues. It is important to keep the LM2735-Q1, the output capacitor, and Schottky diode physically close to each other (see Figure 42). The electrical design considerations outweigh the thermal considerations. Other factors that influence thermal performance are thermal vias, copper weight, and number of board layers.