The low voltage range: If the voltage falls below a process-dependent threshold, the leakage currents dominate the energy consumption.

© Fujitsu

The functionality of IoT devices can be divided into two areas: on the one hand, the area of data acquisition and the temporary storage of data, which can be operated permanently with minimal power consumption, and on the other hand, the area of data processing and communication. These are energy-intensive tasks that only take place at certain intervals. Back-bias technology can therefore be used here, which dynamically adjusts the bias voltage of a circuit to the current requirements.

The low-voltage range poses a number of challenges: Since neither typical logic libraries nor transistor models are suitable for near/sub-threshold applications, special components are required. In addition, SRAMs (static volatile memories) no longer function reliably in this range. This makes the intermediate storage of recorded data more difficult. The transistor itself also causes problems. Optimized for operation at 0.9 V, it often delivers unpredictable or inadequate results. However, potential errors associated with this often remain undetected, as the transistor parameters in series production are usually only checked at nominal voltage.

Data is recorded and temporarily stored in near-treeshold mode. The more energy-intensive data processing and communication takes place in dynamic back-bias mode.

© Fujitsu

Fujitsu Electronics Europe (FEEU) acts as a distribution partner for Mie Fujitsu Semiconductor's (MIFS) Deeply Depleted Channel (DDC) technology. The company understands the challenges of the low voltage range and addresses them as follows: First, it provides a transistor and associated simulation models specifically designed for the low-voltage range. Second, in collaboration with the Swiss Research and Development Center Centre Suisse d'Electronique et de Microtechnique (CSEM), it has developed a logic library and memory compiler designed for near/sub-threshold applications. Thirdly, MIFS controls transistor parameters in production down to the sub-threshold range, allowing problems to be reliably identified.

DDC technology has two further advantages: Firstly, the variance of the transistor parameters is significantly lower compared to standard technologies, the SRAM memory content is therefore stabilized in the targeted voltage range. Secondly, the back-bias sensitivity should be mentioned: The characteristics of the logic can be influenced or modulated by applying a substrate voltage - with standard technologies, such an influence is only possible to a very limited extent. DDC technology allows such modulation to be carried out either statically or dynamically: A static implementation works like trimming discrete circuits. In the dynamic case, circuit parts can be modulated according to the current performance requirements of the application. This approach has a positive effect on leakage currents and therefore on the energy balance of the application.

Synergies between components

It is not only the energy efficiency of individual components that is important, but also their interaction on a PCB. The correct combination requires experience in the low-voltage range, and suitable components often have to be collected from all over the world, as the number of suppliers is limited. FEEU, for example, offers MCUs and RTCs from the American manufacturer AmbiqMicro and is therefore aimed in particular at customers for whom in-house chip development is not an option. Both products operate in the near-threshold range and are therefore designed for energy efficiency. They significantly undercut the power consumption of typical MCUs. The processor core of the MCUs is an ARM Cortex M4F, whose power consumption is even lower than that of the Cortex M0+. In addition to a 13-channel ADC, the standard interfaces of the Apollo MCUs include configurable GPIOs. These are suitable for implementing I2C and SPI interfaces, among other things. A maximum of 512 KByte Flash and 64 KByte RAM are available as memory. The Apollo MCUs are available in two different packages, either in a 41-pin CSP with 27 GPIOs or in a 64-pin BGA with a total of 50 GPIOs. In addition, the RTC integrated in the MCU is also available as a single component. The RTC variant with integrated power management unit is particularly suitable for users who do not want to or cannot change their existing MCU. Efficient RTCs are particularly indispensable when data is only recorded at relatively short intervals. During the long inactive phases, power consumption must be reduced as much as possible.

Other components are also important for IoT applications. It has already been mentioned that data storage in the low-voltage range is a challenge - and non-volatile memories in particular have a comparatively high power consumption during write accesses. FRAMs, which are highly energy-efficient, are an alternative. In addition, standard components, such as crystals and chip resistors, must be compact and low-power in order to meet the requirements of the IoT.

Author:
Dr. Klaus-Peter Dyck is Senior Manager Marketing & Application at Fujitsu Electronics Europe in Langen.