POWER
Energy Harvesting & Power Management ICs for IoT
Small stand-alone sensor devices, designed for use within the Internet of Things ecosystem, are often powered by photovoltaic cells to make them operate autonomously. The purpose is to protect the environment by reducing the use and waste of primary batteries, eliminating the impact of cabling cost and preventing maintenance from replacing batteries regularly.
Energy Harvesting IC
A few key components are required to design such a power supply. Firstly, a special DC/DC converter is used to convert energy from a solar cell and to store the collected energy in a battery or supercapacitor storage element. The R1801 energy harvesting buck DC/DC converter is especially made for this purpose and draws an ultra-low quiescent current of only 200 nA, allowing the use of the harvester circuit even in a low-illuminated environment when the generated level of energy is moderate. As soon as sufficient energy is available on the input side, the buck DC/DC converter will be enabled to transfer energy from input to output until the energy drops below a threshold. This process repeats and slowly increases the voltage on the output, where the energy storage device is connected, until the required output voltage level is reached. Once the output voltage reaches 90%, the Power Good output becomes active and enables additional circuits, such as a buck- boost DC/DC converter, to make a stable supply voltage for other peripheral devices. The Power Good output remains active until the output voltage drops below a certain level, regardless of whether an input voltage is present or not. This keeps circuits on the output active even when the solar cell does not generate power.
One significant advantage of the R1801 is that
the output and maximum power voltages can be selected according to the product version. This means that it is possible to match the settings of the R1801 with the specifications of the solar cell to maximise efficiency. A reverse current protection is built-in to keep the energy in the energy storage device when the input voltage is lower than the output voltage. The minimum required start-up power is only 1 µW, and its peak efficiency is around 80% at 10 µA output current.
Buck-Boost DC/DC Converter A secondary buck-boost DC/DC converter IC is required on the output of the energy harvesting IC to stabilise the fluctuating voltage of the energy storage device. For this purpose, we use the nano-power IC RP604 or RP605, just to ensure that the collected energy is not largely wasted in the internal circuits. These ICs are designed for applications that are primarily in sleep mode and only periodically wake up to take a measurement, send data, and then return to sleep mode. The RP604/RP605 have an impressive low quiescent current of only 300 nA. The operating voltage ranges from 1.8 to 5.5 V and delivers up to 300 mA in buck mode, while in boost mode, the output current is less and related to the input voltage level. Peak efficiency is around 92%, depending on input and output conditions. The RP605 has an additional battery voltage monitor circuit embedded to measure, for example, the voltage of the energy storage device. The BM- output provides an analogue voltage which can be connected to an A/D converter for further measurement.
LDO Regulator
For this kind of application, one could also select a nano-power LDO regulator RP118/RP124
30 DECEMBER/JANUARY 2024 | ELECTRONICS FOR ENGINEERS
to power peripheral circuits and require only 300 nA quiescent current at no load. In addition, the ICs offer three operation modes. 1.
Low power mode
2. 3.
Fast transient response mode Off mode
The devices switch automatically between the low power and the fast transient response mode based on the output current demand of the application. The performance of the LDO is enhanced in fast transient response mode. In particular, the ripple rejection, as well as the response speed to line and load transients, demonstrate much better results compared to a conventional LDO with low current consumption. The Chip Enable pin controls the Off mode and turns the LDO offline, reducing current consumption to a minimum. Also, when needed, the RP124 has an additional battery voltage monitor circuit embedded.
RIOT-02 Evaluation Board Finally, to experiment with these power management ICs from Nisshinbo Micro Devices, a special evaluation board RIOT-02 is available. It is a complete board with ICs for energy harvesting and power management, an energy storage device, MCU and sensor ICs.
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