With an estimated figure of 600 million devices to be sold in the coming years and a future certainly optimistic also linked to the advent of IoT and the upcoming industry 4.0, the market for wearable devices is growing strongly. Today, wearable devices are not just gadgets but also include health monitoring tools used in many fields of medicine.

Yet an obstacle for their full deployment in these markets is their energy autonomy: in order to reinforce the concept of “wearable,” small batteries are necessary and improvements should be made in energy efficiency and power management.

The amount of information managed by a normal wearable device, the visual LED interface and the BLE communication protocol require efficient power management solutions for long-life products, but also provide new opportunities for recharging by using energy harvesting solutions.

As a first consideration, designers should optimise energy savings, making the most of low-power solutions in sleep mode for a definite period of time, relying on physical interrupt options to awaken the wearable, for example, a vibration or a sudden movement. A fitness device may work in active situations for data collection, but it might save energy when no motion is detected, remaining in sleep or ultra-sleep mode indefinitely. Currently, compact lithium-ion battery technology is the predominant source of energy in wearables, however, capacity is closely linked to the size of the battery and capacity largely degrades within a few years of use.

Powering up the wearables

One way to efficiently increase the life of the battery is to decrease the power consumption of the many sensors placed inside a wearable device. All the sensors play an important role and operate at different voltages from that of the battery, hence they requires a DC/DC converter (see figure 1). Improving conversion efficiency directly impacts the lifetime of a battery. The choice of a switching regulator is also a key factor to maximise the efficiency and to determine the levels of power consumption for each working step. A low quiescent current can give a maximum yield of 80% at 1mA depending on the input voltage and output values.

multi-rail DC-DC configuration (cr) Figure 1: A multi-rail DC-DC configuration.  

Inductor-based DC-DC switching converters are the preferred choice for their superior efficiency compared to linear regulators, however, adding up multiple inductor-based switching regulators to address each voltage requirement may prove too costly. Multi-rail DC/DC or switched-capacitor converters should also be considered as a possible alternative to linear regulators to improve overall efficiency and extend battery life.

The TPS82740A module from Texas Instruments is specifically designed to deal with the power requirements of wearable devices such as smart watches. The module comes in a "MicroSIP" technology (System-in-Package) that includes the switching capacitors and the input/output inductor in a package only 6.7mm².

The classic configuration as displayed in figure 2 does not require external components. The operating principle used by TI is based on Direct Control with Seamless Transition into Power Save Mode (DCS-Control). The device runs on a rechargeable lithium-ion and Li-primary battery such as Li-SOCl2, Li-MnO2 and two or three alkaline batteries. Input voltage up to 5.5V also allows operation from a USB port or thin-film solar modules if one wants to rely on energy harvesting.

TI's TPS82740A module (cr) Figure 2: TI's TPS82740A module in a classic configuration.  

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