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Ultra low power. Three words tossed around pretty casually within the semiconductor industry, but particularly so when referring to Bluetooth low energy-enabled radio System-on-Chips (SoCs). However, depending on the features of the radio SoC and the needs of the target application, the claim of “ultra-low-power” may not always be accurate or applicable.
So what features should a system-level designer look for in order to make their Bluetooth low energy technology-enabled device truly low power?
For IoT edge-node devices or “connected” health and wellness applications, designers must consider at least the following parameters when comparing system power consumption levels between Bluetooth low energy radio SoCs:
Power Conversion. Most Bluetooth low energy radio SoCs operate at wide input voltage levels to satisfy the needs of different batteries. However, the battery voltage is most often different from the operating voltage of the Bluetooth low energy RF path as well as the operating voltage of on-chip Flash, processing cores, and memory. Hence, effective on-chip power conversion becomes important to get to the lowest possible power consumption.
Duty-cycling. Receiving (Rx) and Transmitting (Tx) currents are obviously important for the power budget; however, the paradox of most Bluetooth low energy applications is that most of the time they aren’t actually doing anything. Sampling, transmission and reception of data typically happens at very low data rates, which is why the sleep current of the device often becomes the dominant factor in the application’s overall power budget.
A Truly Ultra-Low-Power Radio SoC
In response to the rapidly evolving needs of IoT and “connected” health and wellness applications, ON Semiconductor has recently announced the availability of RSL10, a multi-protocol Bluetooth®5 certified radio SoC offering the industry’s lowest power.
For RSL10, its defining “ultra-low-power” characteristic begins in applications that are typically battery powered (e.g., devices using 1.5 V AAA batteries, 3 V 2032 coin cells, 1.25 V 10A ZnAir batteries), and where the data throughput during normal operation is relatively low.
The Bluetooth low energy RF path of RSL10 operates “natively” at 1.1 V but, to ensure that battery voltages ranging from 1.1 V up to 3.6 V can be accommodated, RSL10 facilitates effective, on-chip DC/DC conversion as well as regulation to feed other parts of the system with appropriate voltages.
For instance, if the RSL10 is powered by a 3 V 2032 coin cell (which would be the case in many medical or remote sensing IoT applications), the following Sleep Mode currents can be achieved:
25 nA with wake-up from external pin.
40 nA with wake-up from external pin or internal timer.
100 nA with wake-up from external pin or internal timer and 8 kB RAM retention.
Most alternative Bluetooth low energy radio SoCs require two to three times the amount of current to maintain similar modes.
Additionally, RSL10 draws 3.4 mA in Rx Mode and 4.6mA in Tx Mode – in both cases these numbers are achievable using the 2 Mbps data rate established by the new Bluetooth 5 standard.
So how do these numbers apply in a real world scenario where an application developer needs to calculate battery life time based on knowledge about the duty cycling of the application?
Consider the example shown in the figure below, where a remote sensing application is in its active Bluetooth low energy transmit duty cycle once every 2 seconds of its operating life. The remaining time the application is in Sleep Mode (wake-up from external pin) or not transmitting or receiving data.
Bluetooth Low Energy Technology Duty Cycle Power Considerations
For a complete advertising event with a 24-byte payload (Transmit power 0dBm), RSL10 will consume approximately 700 uA for a duration of 7 ms. Using this information, it is easy to relate the Bluetooth low energy power consumption to the battery capacity.
To see the real-time power consumption benefits of RSL10, visit the ON Semiconductor booth (#931) at Internet of Things World in Santa Clara, California.
To learn more about RSL10, watch the RSL10 Overview Video!
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