Datasheet

=
TJA
R
P
nDissipatio
-
A
T
J
T
Internal
R
A
T
C
T
DISS
P-
C
A
TC
-
R
CASE
TJ
-
C
CASE
TJ
-
A
TC
-
J
T
LM2735
www.ti.com SNVS485F –JUNE 2007–REVISED APRIL 2013
Definitions
Heat energy is transferred from regions of high temperature to regions of low temperature via 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.
R
θJC
Thermal impedance from silicon junction to device case temperature.
R
θJA
Thermal impedance from silicon junction to ambient air temperature.
C
θJC
Thermal Delay from silicon junction to device case temperature.
C
θCA
Thermal Delay from device case to ambient air temperature.
R
θJA
& R
θJC
These two symbols represent thermal impedances, and most data sheets contain associated values
for these two symbols. The units of measurement are °C/Watt.
R
θJA
is the sum of smaller thermal impedances (see Figure 27). 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 27. 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, V
IN
, V
OUT
, Load Current
etc). 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.
(17)
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.
R
θJA
[Variables]:
• Input Voltage, Output Voltage, Output Current, RDSon.
• Ambient temperature & air flow.
• Internal & External components power dissipation.
• Package thermal limitations.
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