According to research firm MarketsandMarkets, the wearable technology market is expected to grow from USD $15.74 billion in 2015 to $51.60 billion by 2022, which translates into a CAGR of 15.51% between 2016 and 2022. Wearable devices contain, among other components, voltage regulators of the switching or low-dropout (LDO) variety. No doubt, then, that the voltage-regulator market is bracing for growth as well.
It’s quite common to find switching-regulator ICs in power-supply designs for wearable applications. Designers find them more flexible because they can achieve peak efficiency in the 90th percentile. Although switching regulators generate more noise than linear regulators, their efficiency is far superior as well.
Some existing power solutions combine LDOs and switching regulators, while others just use switching regulators. For example, Intersil’s ISL9120 (Fig. 1) operates in either buck or boost mode with a pulse-frequency-modulator (PFM) controller that automatically alternates between buck, boost, and automatic bypass modes to maintain a steady output voltage. The PFM controller helps attain up to 98% efficiency at higher load and greater than 86% at lower load conditions. In doing so, it achieves less power drain with less heat, which in turn extends battery life.
1. Only a single inductor and very few external components are needed for the ISL9120 buck-boost regulator. (Courtesy of Intersil)
During stay-alive operation, the ISL9120 goes into forced bypass mode, which reduces power consumption to a quiescent current of less than 0.5 μA. To illustrate a scenario where forced-bypass-mode operation would come into play, think of powering an LDO that may be in standby mode with near-zero output current. When the buck-boost is in bypass mode, it saves the regulator 41 μA of quiescent-current consumption.
Know Your IQ
The forced-bypass mode is just one design option that may be used when designing wearable electronics. Among other aspects that need to be considered is the current consumed by the device in different power modes, like standby mode and shutdown.
The quiescent current (IQ) is very important, because wearable devices spend a lot of time in standby mode. IQ is the current consumed when a circuit is not driving a load. Therefore, the lower the value of the IQ, the more you can extend the life of a battery.
Designers have been trying to achieve better energy efficiency in wearable devices by lowering IQ values. Several solutions are available in the marketplace, with IQ values continuing to drop as designs evolve. Nowadays, it’s more common to find a switcher regulator for low-power applications that reaches nanoampere IQ values using fewer external components. The result is a smaller solution with high efficiency that minimizes heat dissipation during active operation.
At Embedded World 2017 in Nuremberg, Germany, Maxim Integrated announced its latest boost regulator. The MAX17222 (Fig.2) features 300-nA IQ at its output and 95% peak efficiency to minimize heat dissipation. The switch regulator also provides a shutdown mode that does not allow any quiescent current from the load after shutoff. With this shutdown mode, it’s possible to use alternate sources to regulate the output. The regulator draws just 0.5 nA in shutdown mode.
2. The MAX17222 boost regulator requires a single configuration resistor and small output filter. (Courtesy of Maxim)
With switching regulators, Maxim’s designers are able to achieve lower IQs using fewer external components. The MAX17222 utilizes a fixed on-time, current-limited, pulse-frequency-modulation control scheme that allows for both continuous conduction mode (CCM) and discontinuous conduction mode (DCM).
Today’s designers are increasingly merging pulse-width modulation with a power-saving PFM mode when operating under low loads. Doing so helps to maximize the battery life of wearables. They also are developing smaller and more flexible designs that can further extend battery life in wearables.
Clearly, many aspects need to be considered when designing a low-power system. Many solutions currently circulate the market, and new options are quickly emerging thanks to evolving design approaches. Depending on the needs of the project, one solution might be better than another. No matter what solution is chosen, however, knowing and controlling quiescent currents will always help to extend a battery’s lifetime.