How a Bluetooth Low Energy Radio System Can Achieve Run-Time of Ten Years On a Small Coin Cell Battery
By Bob Card
Americas Marketing Manager, ON Semiconductor
In many small, portable IoT applications, the ultimate challenge for the design engineer is to provide reliable wireless connectivity while operating on a single coin cell battery for ten years.
This is no easy task: most inexpensive coin cells only offer a capacity of around 240mAh. But this Design Note demonstrates that it is possible, by selecting a radio System-on-Chip (SoC) which has low Sleep mode current consumption, to achieve the ten-year goal over both short- and long-distance wireless connections.
If the application can only draw on energy capacity of 240mAh, the wireless device must spend most of the time asleep, only waking up occasionally to make a wireless transmission (see Figure 1). For example, a 7ms wake time, a 5s Transmit interval and 120 wireless transmissions per hour yield a duty cycle of 0.14% wake time and 99.86% sleep time. This shows why it is essential for the radio SoC to consume very low power in Sleep mode.
Fig. 1: Sleep and On duty cycles
For battery-powered applications operating over short- or medium-range links, typically up to 50m, the Bluetooth® Low Energy radio technology is the preferred choice. For longer-range transmissions over 1,000m, ON Semiconductor supports Sub-Gigahertz Software-Defined Radio (SDR) technology with its AXM0F243 wireless microcontroller.
A Bluetooth Low Energy radio operates in the 2.4GHz Industrial, Scientific and Medical (ISM) band using a 40-channel partition scheme, with 2MHz channel spacing. Three RF channels (37, 38 and 39) are reserved for advertising functions, which allow the discovery of devices in the vicinity. Channels 0-36 are reserved for data transmissions. The advertising channels occupy different parts of the spectrum to provide immunity against interference from 802.11 or Wi-Fi® radio transmissions (see Figure 2).
Fig. 2: Bluetooth Low Energy advertising channels (Source: Accton Marketing)
The data unit of an advertisement packet, called the Protocol Data Unit (PDU), has a two-byte header which specifies the type and length of data payload up to 37 bytes (6 bytes for the advertisement address and up to 31 bytes for data).
Connectable and non-connectable transmissions
Bluetooth Low Energy advertisement packets can be either ‘connectable’ or ‘non-connectable’. Figure 3 shows the operation of an ON Semiconducotor RSL10 System-in-Package (SIP) wireless SoC, measured by a power analyzer and showing connectable (left) and non-connectable (right) advertisement events, both at 0dbm Transmit power.
While both events use channels 37, 38 and 39, and last for 7ms, the connectable event includes a Receive pulse for each channel. This makes sense, since the connectable event is also intended to receive incoming transmissions. The resulting power analyzer measurement reveals the average current for each transmission type: 711.624µA for the connectable event, and 504.307µA for the non-connectable.
In addition, the RSL10 SIP’s Deep Sleep current is 160nA, with 16kbytes of data retained in RAM for the Bluetooth Low Energy protocol stack, and running an internal timer to wake itself up.
Fig. 3: power consumption for transmission of connectable and non-connectable packets
RSL10 SIP battery life
Figure 4 demonstrates that the actual battery life for the RSL10 SIP-based system on the conditions above will range from 10.97 years for connectable transmissions at a 2.5s advertisement interval, to 27.26 years for non-connectable transmissions at a 5s advertisement interval. These calculations assume the use of a 240mA CR2032 coin cell and a PDU of 5 bytes.
Fig. 4: actual battery life of the application using an RSL10 SIP
For longer-range wireless transmissions, ON Semiconductor’s AXM0F243 narrow-band SoC achieves a link budget of 153dB and can transmit a distance of 37km (23 miles) when operating at a frequency of 915MHz with a 30dB fade margin. For transmissions over a 1.1km range, the AXM0F243 exceeds the desired 10-year battery life (see Figure 5).
Fig. 5: actual battery life of a long-range application based on the AXM0F243 SoC
So with the right radio SoC, achieving a 10-year battery life over short or long range is entirely possible.