TIDUEO0C July 2019 – March 2021
The TIDA-010053 reference design combines the TPS62840 (buck converter), the TPS63900 (buck-boost converter), and the TPS610995 (boost converter). The load can be either an NB-IoT module connected to the cellular network, or an electronic load which simulates the load profile. The buck-boost and boost devices support a single LiMnO2 primary cell configuration while the buck device enables use cases with two in-series primary cells.
There are multiple constraints in smart metering applications, which will influence the selection of the power architecture. Smart water, heat or cold meters as well as add-on RF-enabled modules for mechanical flow meters typically are size-constrained physically (see standards documents EN1434 for water and EN4064 for heat meters). Most of these meters operate from a single primary cell, for example electronic add-on modules with RF functionality with 1–2 Ah capacity achieve a lifetime of at least 5-6 years, if using the wM-Bus RF protocol (EN13757) T- or C-mode for data communication in the 868-MHz Industrial, Scientific, Medical (ISM) unlicensed frequency band.
The new NB-IoT communication modules do offer similar mechanical size and PCB footprint as existing Sub-1 GHz RF solutions but deliver longer range coverage at the cost of higher average power consumption while using higher transmit power in the licensed mobile network frequency bands.
As mobile networks are controlled by the mobile operators, including the power saving features which heavily influence the power consumption of the NB-IoT module, it is close to impossible to predict the battery lifetime of an NB-IoT module inside a smart meter or any other IoT product.
This reference design proposes a solution for this challenge by measuring the battery drain with a dedicated device such as the BQ35100 as implemented in the TIDA-01546 design. Integrating the BQ35100 battery-gauge device allows for in-system precise measurement of the State-of-Health (SOH) for the LiMnO2 cells and wirelessly reporting the SOH value over the NB-IoT network. Updating the SOH status by the BQ35100 device takes approximately one second and can be done very infrequently, for example starting with a fresh primary cell just once after several weeks or even months to save energy. The subsequent SOH update period can be continuously adjusted based on an algorithm which considers the expected battery drain and correlating that to the measured SOH data, see Section 18.104.22.168.
To obtain the highest SOH accuracy, TI recommends measuring the battery with the BQ35100 device only after a sufficient relaxation time after the high-current pulse for RF transmission occurred. For example, the BQ35100 gauge device is enabled 10 minutes after the last current pulse used for NB-IoT data transmission but the 10 minute delay period can be programmed to any suitable value. The ADS7142 system health monitor in the TIDA-01546 design is used to detect the high-current pulse and the application waits for the appropriate battery relaxation time, before running an SOH measurement.