LM2735XMFSLASHNOPB datasheet(40/59 Pages) TI

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Dissipatio

- A

Internal

DISS

C -

CASE

J -

CASE

J-

C -

LM2735, LM2735-Q1

SNVS485G – JUNE 2007 – REVISED AUGUST 2015

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Thermal Considerations (continued)

10.3.1 Definitions

Heat energy is transferred from regions of high temperature to regions of low temperature through three basic

mechanisms: radiation, conduction and convection.

Radiation

Electromagnetic transfer of heat between masses at different temperatures.

Conduction Transfer of heat through a solid medium.

Convection Transfer of heat through the medium of a fluid; typically air.

Conduction & Convection will be the dominant heat transfer mechanism in most applications.

RθJC

Thermal impedance from silicon junction to device case temperature.

RθJA

Thermal impedance from silicon junction to ambient air temperature.

CθJC

Thermal Delay from silicon junction to device case temperature.

CθCA

Thermal Delay from device case to ambient air temperature.

RθJA & RθJC These two symbols represent thermal impedances, and most data sheets contain associated

values for these two symbols. The units of measurement are °C/Watt.

RθJAis the sum of smaller thermal impedances (see Figure 46). The capacitors represent delays that are present

from the time that power and its associated heat is increased or decreased from steady state in one medium until

the time that the heat increase or decrease reaches steady state on the another medium.

Figure 46. Simplified Thermal Impedance Model

The datasheet values for these symbols are given so that one might compare the thermal performance of one

package against another. In order to achieve a comparison between packages, all other variables must be held

constant in the comparison (PCB size, copper weight, thermal vias, power dissipation, VIN, VOUT, Load Current,

and so forth). This does shed light on the package performance, but it would be a mistake to use these values to

calculate the actual junction temperature in your application.

(29)

We will talk more about calculating the variables of this equation later, and how to eventually calculate a proper

junction temperature with relative certainty. For now we need to define the process of calculating the junction

temperature and clarify some common misconceptions.

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