Thermal management (Naon)
DM814x/AM387x thermal behaviour is strictly related to the process used to produce the die. The manufacturer chose to focus on maximizing the performance, at the expense of power consumption because the component was conceived to penetrate the high-definition video market segment, which is known to be highly demanding in terms of hardware resources. This process is known as “Hi-Performance / Hi-Leakage”.
In some cases, the working temperature of the CPU and the module itself reach values that might be critical for some particular applications. The following paragraphs describe some methods for performing a thermal analysis of the system, to collect all the information required to identify the issues and implement a proper thermal management strategy.
Estimating CPU junction temperature (Tj)
The processor doesn't allow for measurement of the junction temperature, thus use of some indirect method is required, as described in the Power (Naon) section. Here you can find useful information related to the thermal characteristics of the package - extracted from the CPU datasheet - that, together with the contents of the TI application note "Semiconductor and IC Package Thermal Metrics" (http://www.ti.com/litv/pdf/spra953c), allows for estimation of the junction temperature Tj. The PsiJT and the θjc parameters identified in the AN are the most important ones to identify the difference between the external case temperature (which is easily measurable) and the internal junction temperature. Once this value is known, it is possible to determine the highest acceptable environment temperature to avoid reaching critical junction temperature.
Using the current/power monitor
Obtaining an accurate measurement of power consumption for the CPU (required for this kind of assessment) is not easy. However, as a first estimate, you can use the current/power monitor INA226A mounted on the NaonEVB-Mid carrier board, which allows for measurement of the CPU module global absorption (we consider negligible the part due to the electronics external to the CPU). Please refer to NELK_Power_Management#Measurements_script for details on how to perform the measurement. Once you've collected the data, you can compare it with the ones that has been reported on Power consumption (Naon).
Using a spreadsheet
Another method for estimating the Tj is using the appropriate spreadsheet provided by TI: http://processors.wiki.ti.com/index.php/DM814x_AM387x_Power_Estimation.
Other useful tools and information
Using the TSC2003
Naon module mounts the TSC2003 touch screen controller. This component allows for measurement of its case temperature, accessing its register via the I2C bus. This measurement is a value closely related to the CPU temperature. For further details, please refer to the component datasheet.
Using the PMIC
The PMIC TPS659113 mounted on Naon provides Thermal shutdown protection and hot-die detection functions. These functions refer to the PMIC junction temperature and not to the CPU Tj, but they can be used to implement some protection/detection strategies.
Using a contactless sensor on carrier board
Another option for measuring the CPU's case temperature is the use of a contactless sensor that can be mounted on carrier board. See for example: http://www.ti.com/product/tmp006.
- Determine the maximum reachable Tj, taking into account all your application requirements and the worst operating environment conditions
- Consider the opportunity of using the OPP modes (see NELK Power Management) for CPU power consumption optimization, making sure that enough power is available for computing requirements.
- Consider the opportunity of using the extended temperature range component (that has Tj_max=105°C) rather than the commercial temperature range model (Tj_max=90°C)
- According to the estimated consumption, the system requirements and the maximum reachable Tj, consider using a heat sinking/cooling system.