Datasheet

R
T
JC

=
T
J
- T
C
Power
R
T
JA

=
T
J
- T
A
Power
R
T
=
'T
Power
LMR10510
SNVS727B –OCTOBER 2011–REVISED APRIL 2013 www.ti.com
Thermal Definitions
T
J
= Chip junction temperature
T
A
= Ambient temperature
R
θJC
= Thermal resistance from chip junction to device case
R
θJA
= Thermal resistance from chip junction to ambient air
Heat in the LMR10510 due to internal power dissipation is removed through conduction and/or convection.
Conduction: Heat transfer occurs through cross sectional areas of material. Depending on the material, the
transfer of heat can be considered to have poor to good thermal conductivity properties (insulator vs. conductor).
Heat Transfer goes as:
Silicon → package → lead frame → PCB
Convection: Heat transfer is by means of airflow. This could be from a fan or natural convection. Natural
convection occurs when air currents rise from the hot device to cooler air.
Thermal impedance is defined as:
Thermal impedance from the silicon junction to the ambient air is defined as:
The PCB size, weight of copper used to route traces and ground plane, and number of layers within the PCB can
greatly effect R
θJA
. The type and number of thermal vias can also make a large difference in the thermal
impedance. Thermal vias are necessary in most applications. They conduct heat from the surface of the PCB to
the ground plane. Four to six thermal vias should be placed under the exposed pad to the ground plane if the
WSON package is used.
Thermal impedance also depends on the thermal properties of the application operating conditions (Vin, Vo, Io
etc), and the surrounding circuitry.
Silicon Junction Temperature Determination Method 1:
To accurately measure the silicon temperature for a given application, two methods can be used. The first
method requires the user to know the thermal impedance of the silicon junction to case temperature.
R
θJC
is approximately 18°C/Watt for the 6-pin WSON package with the exposed pad. Knowing the internal
dissipation from the efficiency calculation given previously, and the case temperature, which can be empirically
measured on the bench we have:
where T
C
is the temperature of the exposed pad and can be measured on the bottom side of the PCB.
Therefore:
T
j
= (R
θJC
x P
LOSS
) + T
C
From the previous example:
T
j
= (R
θJC
x P
INTERNAL
) + T
C
T
j
= 18°C/W x 0.149W + T
C
The second method can give a very accurate silicon junction temperature.
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