LM2735XMFSLASHNOPB datasheet(41/59 Pages) TI

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LM2735, LM2735-Q1

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SNVS485G – JUNE 2007 – REVISED AUGUST 2015

Thermal Considerations (continued)

RθJA [Variables]:

•

Input voltage, output voltage, output current, RDSon.

•

Ambient temperature and air flow.

•

Internal and external components power dissipation.

•

Package thermal limitations.

•

PCB variables (copper weight, thermal vias, layers component placement).

It is incorrect to assume that the top case temperature is the proper temperature when calculating RθJC value.

The RθJC value represents the thermal impedance of all six sides of a package, not just the top side. This

document will refer to a thermal impedance called RψJC. RψJC represents a thermal impedance associated with

just the top case temperature. This will allow one to calculate the junction temperature with a thermal sensor

connected to the top case.

10.3.2 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, 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.

10.3.3 LM2735 Thermal Models

Heat is dissipated from the LM2735 and other devices. The external loss elements include the Schottky diode,

inductor, and loads. All loss elements will mutually increase the heat on the PCB, and therefore increase each

other’s temperatures.

Figure 47. Thermal Schematic

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