Datasheet

7.3 General notes for specifications at maximum junction
temperature
An estimation of the chip junction temperature, T
J
, can be obtained from this equation:
where:
• T
A
= ambient temperature for the package (°C)
• R
θJA
= junction to ambient thermal resistance (°C/W)
• P
D
= power dissipation in the package (W)
The junction to ambient thermal resistance is an industry standard value that provides a
quick and easy estimation of thermal performance. Unfortunately, there are two values in
common usage: the value determined on a single layer board and the value obtained on a
board with two planes. For packages such as the PBGA, these values can be different by
a factor of two. Which value is closer to the application depends on the power dissipated
by other components on the board. The value obtained on a single layer board is
appropriate for the tightly packed printed circuit board. The value obtained on the board
with the internal planes is usually appropriate if the board has low power dissipation and
the components are well separated.
When a heat sink is used, the thermal resistance is expressed in the following equation as
the sum of a junction-to-case thermal resistance and a case-to-ambient thermal resistance:
where:
• R
θJA
= junction to ambient thermal resistance (°C/W)
• R
θJC
= junction to case thermal resistance (°C/W)
• R
θCA
= case to ambient thermal resistance (°C/W)
R
θJC
is device related and cannot be influenced by the user. The user controls the thermal
environment to change the case to ambient thermal resistance, R
θCA
. For instance, the
user can change the size of the heat sink, the air flow around the device, the interface
material, the mounting arrangement on printed circuit board, or change the thermal
dissipation on the printed circuit board surrounding the device.
Thermal attributes
S32K1xx Data Sheet, Rev. 4, 06/2017
64
Preliminary
NXP Semiconductors