Datasheet

GND

PCB

GND

OUT

LM2735

SNVS485F –JUNE 2007–REVISED APRIL 2013

www.ti.com

Below is an example of a good thermal & electrical PCB design. This is very similar to our LM2735

demonstration boards that are obtainable via the Texas Instruments website. The demonstration board consists

of a two layer PCB with a common input and output voltage application. Most of the routing is on the top layer,

with the bottom layer consisting of a large ground plane. The placement of the external components satisfies the

electrical considerations, and the thermal performance has been improved by adding thermal vias and a top layer

“Dog-Bone”.

Figure 26. Example of Proper PCB Layout

Thermal Design

When designing for thermal performance, one must consider many variables:

Ambient Temperature: The surrounding maximum air temperature is fairly explanatory. As the temperature

increases, the junction temperature will increase. This may not be linear though. As the surrounding air

temperature increases, resistances of semiconductors, wires and traces increase. This will decrease the

efficiency of the application, and more power will be converted into heat, and will increase the silicon junction

temperatures further.

Forced Airflow: Forced air can drastically reduce the device junction temperature. Air flow reduces the hot spots

within a design. Warm airflow is often much better than a lower ambient temperature with no airflow.

External Components: Choose components that are efficient, and you can reduce the mutual heating between

devices.

PCB design with thermal performance in mind:

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 & 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 customer choose the correct package for the

application. 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 nice if you were able to separate by

distance the LM2735 from the Schottky diode, and thereby reducing the mutual heating effect. This will however

create electrical performance issues. It is important to keep the LM2735, the output capacitor, and Schottky

diode physically close to each other (see Figure 26). The electrical design considerations outweigh the thermal

considerations. Other factors that influence thermal performance are thermal vias, copper weight, and number of

board layers.

Product Folder Links: LM2735