Let’s be honest—for most engineers, the characteristics of the various power-regulator topologies are of only moderate interest at most. That makes some sense, since the subtleties of SEPIC (single-ended primary inductance converter), CV/CC (constant voltage/constant current), discontinuous conduction, and continuous conduction (and many more) are not what generally concerns the regulator user.
Instead, the basic parameters are of interest, starting with the dc output-rail voltage, the available current, and the necessary efficiency. After those top-tier factors, one must consider issues such as footprint, cost, required passives or MOSFETs, noise, and regulation. “What regulator topology will get us there?” isn’t the issue for designers. Instead, it’s “are we there yet?,” so that the design team can focus on the rest of the circuitry and its software.
However, there are applications where the power-regulation subsystem is a major aspect of the design effort. For example, for fitness-related wearables and “hearables” (wireless earbuds), a large part of the user experience is mostly dictated by what the battery capacity and regulators can deliver. These devices are severely size-constrained, yet the need to operate for many hours from a single charge isn’t only desirable, but mandatory.
- SIMO Switching Regulators Extending Battery Life for Hearables and Wearables
- SIMO Technology Overview Using the MAX77650 PMIC
- Hearables Get Longer Life with SIMO
1. The power tree of a hearable (or wearable) device shows an array of dc rails, all regulated from a single battery of very limited capacity.
The power flow for a representative hearable device (Fig. 1) shows the rails as well as the efficiency and dissipation associated with each. The power-management function includes a battery charger, a buck converter, and an LDO to power the sensors, while a second IC (a dual LDO) powers the microcontroller, Bluetooth function, and audio.
There’s an interesting characteristic in the normal, day-to-day operation of wearables and hearables: They operate at a low-to-moderate active-duty cycles, but are also often in standby mode. The device may be active for 4, 8, 12, or more hours between recharging periods, in principle, but for much of this time their actual mode is non-active. Therefore, standby current is a critical parameter for lengthy battery life on a single charge.
A SIMO Regulator Benefits the Design
The wearable/hearable operating cycle has major consequences for the power-delivery subsystem. Obviously, a larger battery with more milliampere-hour (mAh) capacity isn’t an option for these devices. Therefore, the useful battery life rating is closely tied to operating efficiency and standby current.
The first approach of the design team might be to find the “best” regulator (switching or LDO) for each rail with respect to efficiency, IC size, and overall footprint, including required passives. While this approach may produce optimum results locally for each rail, it doesn’t produce a power subsystem, which is optimum from a system-wide, high-level perspective.
As a result, the regulator ICs and passives for each of the rails add up to a larger burden in size than the end-product can accept. Further, there’s no coordinated operating management among these rails, so this function must be implemented using another IC, or via the system microcontroller.
Instead, a better approach is to step back and look for a single-IC regulator solution that provides the multiple dc outputs needed, along with the active and standby efficiency. That’s where the single-inductor multiple-output (SIMO) regulator technology offers multiple advantages.
This architecture combines regulator functions to minimize die size, package size, and need for external passives, and still provide the needed performance. It’s not just packaging of multiple, yet unrelated, regulators in a single space-saving device. The multiple-output device includes functions such as an internal voltage reference, provision for measurement of key voltage/current parameters, and even a state-control function to manage the operation of individual rails and even sequencing among them.
By providing multiple outputs, the SIMO approach—along with the regulator’s low standby current—enables the regulator IC to extend battery life in space-constrained, battery-powered products. As a result, this topology is well-suited for the demands of wearable and hearable designs. The regulators deliver power with minimum losses and the architecture eliminates some otherwise duplicated components.
2. Using a clever design, only a single external inductor is needed to provide three independent dc rails, each having its own switching regulator.
The simplified block diagram of a SIMO (Fig. 2) shows how much of the circuitry for the multiple outputs is shared. This particular implementation uses a clever design that combines the roles of several inductors into a single one, significantly saving on the footprint needed to place these external passives, as well as BOM cost. Also, the control function is integrated across all outputs, providing further performance advantages.
An Example Makes the Benefits Obvious
A SIMO such as the MAX77650 from Maxim Integrated greatly improves the wearables/hearables regulator situation (Fig. 3). It provides three independently programmable power rails, yet needs only a single inductor to minimize total solution size. In addition, it incorporates a 150-mA LDO with high ripple rejection for audio and other noise-sensitive applications.
3. The tiny, single-chip MAX77650 SIMO from Maxim Integrated provides three switcher-regulated power rails, a 150-mA LDO, battery-temperature monitor, three LED drivers, and an I2C port for setup and monitoring.
This single IC, measuring a mere 2.75 × 2.15 × 0.7 mm, integrates the battery charger and regulation needed to power the sensor of a typical wearable (3.3 V), the microcontroller (1.2 V), the Bluetooth link, and audio (1.85 V). Meanwhile, the highly configurable linear charger (LDO) supports a wide range of Li+ battery capacities. Operating current is 5.6 μA for the three SIMO rails plus the LDO, while standby current is just 0.3 μA—a decrease of more than an order of magnitude.
It also includes battery temperature monitoring for additional safety. Its high-frequency operation (up to 1 MHz) allows for the use of a small inductor, further minimizing the required space, and it even has three current-sink drivers for indicator LEDs, for more savings. A bidirectional I2C interface allows for configuring and checking the status of the device and its functions. The comprehensive 82-page datasheet provides details on performance of the regulators, of course, across multiple parameters and conditions, as well as I2C I/O, setup, registers, sequencing, and state-diagram details.
4. The power tree of the previous hearable, now supplied by the MAX77650 SIMO, shows a dramatic improvement in efficiency and performance.
Using the MAX77650 in the hearables example cited above, three of the four loads connect to the Li+ battery via a high-efficiency SIMO switching regulator; the fourth load is powered by the LDO from the 2.05-V SIMO output, with 90.2% efficiency (1.85 V/2.05 V) (Fig. 4). Overall system efficiency is greatly improved, at 78.4%. The table is a high-level comparison of the power performance between both solutions.
As a further benefit, the PMIC’s internal on/off controller implements a startup sequence for the regulators (Fig. 5). It also provides power-up/power-down supervisory functionality.
5. The MAX77650 SIMO also includes a state machine to manage the startup sequence (shown) as well as power-up/power-down functions.
Although SIMO devices are highly integrated and require just a single inductor, they do need a few other external passive components. Realizing their full performance potential depends on choice of top- and second-tier specifications of these passives, such as selection of capacitors with low ESR at the operating frequency.
Is the SIMO approach the “best” that can be done along any single dimension of power-regulator performance for tightly constrained applications such as wearables and hearables? In most cases, it may not be. The reality is that like all design choices, it represents a compromise among the various tradeoffs inherent in every design and component. That’s the reality of engineering design.
6. The SIMO topology balances the tradeoff between footprint and dissipation, thus solving the challenge of providing multiple regulated dc rails for wearable and hearable devices.
However, using a SIMO regulator makes sense for products that are severely limited in their available space, have multiple-rail requirements, and need both operating efficiency along with low standby-current demands. In these situations, using a SIMO device can simplify the BOM, of course, and offer an excellent tradeoff between footprint and dissipation (Fig. 6).
Furthermore, the SIMO PMIC incorporates functions that would otherwise take additional effort to add and manage, such as rail sequencing and management, temperature monitoring, and a simple, one-port interface to the system microcontroller.