Datasheet

LM2738
www.ti.com SNVS556B –APRIL 2008–REVISED APRIL 2013
Heat in the LM2738 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:
(42)
Thermal impedance from the silicon junction to the ambient air is defined as:
(43)
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 due to the application's 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 top case temperature.
Some clarification needs to be made before we go any further.
R
θJC
is the thermal impedance from all six sides of an IC package to silicon junction.
R
ΦJC
is the thermal impedance from top case to the silicon junction.
In this data sheet we will use R
ΦJC
so that it allows the user to measure top case temperature with a small
thermocouple attached to the top case.
R
ΦJC
is approximately 30°C/Watt for the 8-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:
(44)
Therefore:
T
j
= (R
ΦJC
x P
LOSS
) + T
C
(45)
From the previous example:
T
j
= (R
ΦJC
x P
INTERNAL
) + T
C
(46)
T
j
= 30°C/W x 0.207W + T
C
(47)
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