ALLPCB
Power conversion for wearables
Ultra-low-power power conversion is critical for maximizing battery life in wearable devices. Below are recent low-power products and efficient DC-DC conversion approaches relevant to wearable designs.
LDOs and buck-boost modules
TI's TPS727xx series 250mA LDOs feature very low 7.9uA quiescent current, low dropout voltage (100mA typ 65mV, 200mA typ 130mV, 250mA typ 163mV), a wide output voltage range, and fast load transient response. The LDOs also provide high power-supply rejection ratio (PSRR); in RF applications they show about 70dB at 1kHz, and they can work with small, low-cost ceramic capacitors.
TI also offers the TPS82740B 200mA buck-boost converter module, which achieves up to 95% conversion efficiency and draws only 360nA Iq in active mode and as little as 70nA in sleep. The small module integrates a switching regulator, inductor, and input/output capacitors into a 9-bump MicroSiP component, achieving a footprint of about 6.7 mm2.
Boost converters
Boost converters are generally less efficient than buck converters, but boosting battery voltage is common in systems such as display drivers. Maxim's MAX8627 is a 1A boost converter that raises a single-cell lithium output from 3V to 5V with up to 95% efficiency while drawing only 20uA Iq. Silicon Labs offers TS33x boost converters with industry-leading low Iq down to 150nA; the TS33x accepts inputs from 0.9V to 3.6V and provides eight selectable outputs from 1.8V to 5V.
Bluetooth, microcontrollers, and low-power system design
When extending wearable battery life, every system element must be considered. A common power-saving approach is to turn off high-consumption functions such as processing and display when they are not required. Bluetooth Smart (BLE) is the dominant wireless interface in many smartwatches and other wearables and is commonly used to relay information from smartphones to wearable devices.
Texas Instruments provides a Meta Watch development system that includes an SDK/API to support integration with phone applications and web services. The development kit includes a smartwatch with display, a 3 ATM stainless steel case, strap, crystal, a vibration motor, a 3-axis accelerometer, and an ambient light sensor. The Meta Watch platform is optimized for low power and is based on TI's 16-bit MSP430 ultra-low-power microcontroller and a CC2564 Bluetooth controller interface.
Selecting an MCU is important for wearable power management. Efficient MCUs can transfer data quickly and return to sleep modes, reducing overall energy consumption. Designers now have broad MCU choices, and 32-bit devices have become more cost-competitive versus 16-bit parts. ARM's Cortex-M series, from ultra-low-power Cortex-M0/M0+ to higher-performance Cortex-M7, offers cores suited to different wearable applications. Many vendors, including Texas Instruments and STMicroelectronics, provide Cortex-M-based MCUs; ST's STM32 family includes low-power lines such as STM32L1 and STM32L0.
Sensors and signal conditioning
Power management for the many sensors in wearables must also be addressed. Sensor technology is a key driver for wearable applications, but designers should not overlook sensor front-end circuitry. STMicroelectronics offers low-power signal conditioning for wearable sensors; for example, its QA4NP low-power quad operational amplifier consumes about 580nA per channel at a 1.8V supply.
Summary
This article highlights selected low-power power management techniques and products. Designing system-level solutions that integrate efficient power converters, low-power MCUs, low-power radios, and sensor front-ends is essential for meeting the ultra-low-power requirements of wearable devices. These low-power techniques are also relevant to other battery-powered and energy-harvesting applications.
