Stm32 Thermal Parameters; Ambient Temperature; Junction Temperature; Ambient Temperature Versus Junction Temperature - ST STM32 Application Note

3

STM32 thermal parameters

This section explains the STM32 thermal parameters as specified by their respective datasheets.
3.1

Ambient temperature

In the electronics field there is no consensus about a standard and unified definition of ambient temperature.
What is sure is that the ambient temperature is the temperature of the device surroundings. But there are many
different ways to measure it. Measurement parameters such as the distance between the measurement point and
the device, if the surrounding air is steady or not, the volume of the test environment, can severely impact later
interpretation of the results.
JEDEC provides a standard definition for ambient temperature but that definition is limited to the JEDEC test
environment used when determining package thermal characteristics.
Most of the time the application operating conditions differ significantly from the JEDEC environment, making the
JEDEC defined thermal resistance from junction to ambient temperature, Theta-JA, not practically usable for high-
performance high-power-dissipation devices.
3.2

Junction temperature

Junction temperature is the widely used term to refer to the temperature of a die. Obviously, the die temperature
is the most important temperature to consider for a device as all its electrical parameters depend on this junction
temperature. Indeed, the junction temperature is the one considered when qualifying the reliability of a device.
In theory the die temperature is not uniformly distributed across the die. There is a temperature gradient across
the die. But in practice, the die temperature is generally considered to be uniformly distributed and only a single
temperature measurement is provided. This reduction of a temperature gradient into one temperature reading
introduces a small incertitude into subsequent assumptions and computations when the die dimensions are
relatively small. This is the case for most STM32 devices.
3.3

Ambient temperature versus junction temperature

Both ambient temperature and junction temperature to specify the thermal performance of STM32 devices.
For many STM32 products, only the maximum ambient temperature is specified as their thermal performance
limit. The junction temperature is also sometimes added.
The use of the ambient temperature for thermal performance assessment is much easier than using the junction
temperature, as the die of a packaged device is not accessible. This makes the conventional temperature
measurement methods unusable for measuring the die temperature.
Measuring the junction temperature requires more advanced measurement techniques. Most STM32 embed a
junction temperature sensor, that serves as a primitive building block for thermal watchdog implementations. But
at design stage, this embedded temperature sensor cannot help in determining the thermal performance of the
whole application. Instead, the junction temperature is estimated at the design stage. It is based on many
parameters including the application power profile, the device thermal resistances, the surrounding temperature
including the board temperature and the device case temperature. For accurate estimation of the junction
temperature, modelling software tools are used when the final application has a complex thermal model and a
complex power profile.
AN5036 - Rev 3
AN5036
STM32 thermal parameters
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