Accounting for BLE Peak Power Demands in Power-Limited Designs


Bluetooth Low Energy (BLE), also known as Bluetooth Smart, offers power-efficient connectivity and interoperability essential for wearables, IoT applications, and other smart devices. For designers, limited power budgets in these designs continue to challenge their need to meet consumer demand for both connectivity and extended operating time. By understanding BLE peak power demands, engineers can meet both tight power budgets and efficient connectivity requirements using available ICs and modules from manufacturers including Dialog Semiconductor, LSR, Murata Electronics, Panasonic, and Texas Instruments, among others.

The rapid acceptance of wearables and IoT devices has brought heightened interest in the ability to leverage low-power design methods, including energy harvesting, to extend the operating life of batteries or even eliminate them entirely. Consumer demand for easy wireless connectivity continues to challenge designers’ ability to balance available power sources with power required by the application in general and wireless communications in particular.

In wireless communications, power consumption is a function of a complex set of factors. In considering the power budget for a design, engineers must not only include more familiar device current requirements for receive, transmit, and quiescent states but also account for power requirements related to the communications protocol itself. A simple protocol able to initialize quickly, rapidly transmit a short burst of data, and return quickly to a sleep state will typically result in lower overall power requirements than a protocol that might offer more complex functionality but require more time – and power – to do so.

For some applications, the need to achieve particular objectives for transmission range and data rate within a specified power budget can dictate use of a custom communications protocol. Here, protocol designers can pare away overhead -- trading reduced flexibility for longer range, higher throughput, and lower power, for example. For the emerging class of wearable and IoT applications, however, standards-based communications is a base requirement for a market filled with an installed base of products, including smartphones, tablets, and other mobile devices. For communicating with these products, Bluetooth has become the common connectivity option and BLE has rapidly emerged as the preferred option for connecting wearables and other personal electronic devices to these hosts.

Small payloads

Optimized to deliver small payloads as quickly and efficiency as possible, BLE was designed to consume minimal power and communicate with the largest range of host devices with the lowest possible latency. In fact, BLE is capable of achieving 260 kbps data rates, but at the cost of incrementally higher power requirements that could push many wearables or IoT applications beyond their power budget. Applications requiring higher data rates are best served by other connectivity options such as Bluetooth BR/EDR.

Wearables, IoT devices, and most sensor applications spend most of their time in a sleep or quiescent operating state, wakened by external events or periodically by timers to process data. Consequently, quiescent power consumption is of critical importance. At the same time, the ability to wake rapidly is important for maximizing the efficiency of power utilization. Power-limited designs can ill afford power consumed by a lengthy initialization phase or protracted handshake protocol to set up a connection session.

Indeed, an important strategy in BLE’s low-power definition is to keep the wireless radio turned off as much as possible. When communications are required, BLE uses simple communication methods that reduce radio-operation time. In fact, BLE devices require only 3 msec to establish a connection, complete a communications transaction, and return to the quiescent state. In contrast, Bluetooth can require up to 100 msec to complete a link-level connection.

Understanding BLE peak power

For any low-power design, wireless communication can dictate peak power requirements, and in energy-harvesting designs drive additional requirements for energy conversion, power management, and energy storage. During operation, different phases of a communications transaction require different but largely predictable processing times and power for connection events. For example, a typical design using the Texas Instruments CC2541 will demonstrate distinct peak power phases associated with wake-up, RX, TX, and processing (Figure 1).

Figure 1: During a single connection sequence, a BLE device such as the Texas Instruments CC2541 exhibits multiple peak current demands as it receives and sends packets. (Courtesy of Texas Instruments)

For the example illustrated in Figure 1, the wake-up phase consumes about 6.0 mA and requires about 400 μsec. (The initial spike shown in the figure is due to the sudden power draw caused by the recharging of capacitors in the CC2541’s internal voltage regulator as it wakes up -- a power spike normally eliminated by external capacitors.) After wakeup, the device undergoes a short power plateau for processing (7.4 mA, 340 μsec) followed by a communications initialization phase where the device prepares its RX and TX circuitry. This phase results in a brief peak (11.0 mA, 80 μsec) just prior to the full RX phase (17.5 mA, 190 μsec) where the device listens for a packet from the master. After RX, the device returns briefly to the base power plateau (7.4 mA, 105 μsec) before once again returning to its peak power level for TX (17.5 mA, 115 μsec) when the device transmits a packet to the master. After TX, the device returns to its base power plateau to handle processing for a duration largely dependent on the application in general and data payload in particular. In this case, the processing phase lasts 1280 μsec and consumes 7.4 mA. Finally, the device executes a transition to sleep (4.1 mA, 160 μsec) before actually returning to the sleep state.

The specific details of the BLE power profile will of course depend on the specific application and BLE device itself. In fact, the total processing time for each connection event will not always be exactly the same from application to application, device to device, or even event to event. However, the time and power required to receive and transmit data will typically remain very stable. By understanding this characteristic BLE power profile, however, engineers can better optimize power budgets in power-limited designs in general and energy-harvesting designs in particular. Furthermore, by anticipating peak demands designers can determine the need for and required capacity of energy-storage devices such as supercapacitors or rechargeable batteries needed to provide reserve power in ambient-powered designs.

Reduced power

Engineers can find BLE solutions with even lower power requirements than those suggested in Figure 1. For example, the Texas Instruments CC2640 achieves RX at only 5.9 mA and TX at 6.1 mA (0 dBm). A member of TI’s SimpleLink wireless family, the CC2640 combines an ARM Cortex-M3 32-bit core main processor with an ARM Cortex-M0 dedicated to wireless operations. Along with an extensive set of digital peripherals, the device includes TI’s sensor controller comprising an ADC, comparators, and other analog peripherals – all capable of collecting analog and digital data autonomously while the rest of the system is in sleep mode (Figure 2). Other SimpleLink devices, such as the TI CC2650, combine BLE with additional connectivity options such as ZigBee, 6LoWPAN, and others.

Figure 2: For its CC2640 BLE device, Texas Instruments combines multiple ARM cores with an extensive set of digital and analog peripherals for autonomous data collection required by wearables and IoT applications. (Courtesy of Texas Instruments)

The Dialog Semiconductor DA14580 combines an ARM Cortex-M0 core, BLE core, and digital peripherals (Figure 3), achieving power consumption of only 4.9 mA for RX and for TX (0 dBm). The device requires only 600 nA in sleep mode and is able to operate with a supply voltage as low as 0.9 V. The DA14580’s integrated radio transceiver implements the RF portion of the BLE protocol and together with the Bluetooth 4.0 PHY layer provides a 93 dB RF link budget for wireless communication.

Figure 3: The Dialog Semiconductor DA14580 operates with a supply voltage as low as 0.9 V and requires only 600 nA in sleep mode. (Courtesy of Dialog Semiconductor)

Along with dedicated BLE devices such as the TI CC2640 and Dialog DA14580, designers can find BLE modules that combine filters, crystals, antennae, and other discrete components to provide a complete drop-in BLE connectivity solution. For example, the LSR 450-0119 SaBLE-x BLE module builds a drop-in solution around the TI CC2640 IC, while the Murata Electronics LBCA2HNZYZ module and Panasonic PAN1740 module combine the Dialog DA14580 with the other components required to deliver a drop-in BLE solution.

Conclusion

For wearables and IoT device designs, Bluetooth Low Energy offers an attractive wireless communications option that combines ubiquitous connectivity with the kind of low-power performance required by energy-harvesting and battery-powered designs. By understanding the detailed power requirements of BLE communications, engineers can account for peak-power demands in power-limited designs. Using available dedicated ICs and complete drop-in module solutions, engineers can quickly add BLE wireless communications to low-power applications.

For more information about the parts discussed in this article, use the links provided to access product pages on the Hotenda website.

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