March 12, 2019
| By: Christine Young
Blogger, Maxim Integrated
Electronic gadgets that fit in the palm of your hand—with plenty of room to spare—perform an array of fairly complex functions. From wrist-worn fitness bands that continuously monitor vital signs to hearables that conduct on-the-spot language translation, these sophisticated devices come with some tough design considerations around:
Compared to traditionally larger power solutions, using a single inductor, multiple output (SIMO) buck-boost architecture can help address the prevailing challenges encountered by many of today’s smart, connected devices, says Gaurav Mital, principal member of the technical staff, product definition in the Mobile Solutions Business Unit at Maxim. Teaming up with Hackster.io, Mital recently hosted a 45-minute webinar, “Why Portable Electronics Perform Better with SIMO PMICs,” to discuss this technology.
“IoT devices are on an exponential rise and consumer devices are driving this growth. These are wearables, hearables, remotes, vital-sign monitoring…,” Mital said. “We want solutions that are small size. We have to manage the heat very well because now we have devices sitting in our ear canal or close to our eyes. We don’t like to charge our devices too frequently, so if we can increase the time between the charge cycles, that is what I would want in a product I’m using. Then comes the noise, so when I’m doing noise sensing, those are very small signals we are getting from our body, so we have to make sure the power solution isn’t inducing any additional noise.”
Figure 1. Earbuds and a host of other hearables, wearables, and IoT devices can benefit from the battery life boost that SIMO PMICs can bring to these small, portable electronics.
SIMO Architecture Brings Higher Efficiency, Smaller Size
Portable, connected devices commonly have these components: a Li+ charger, 5V near-field communications (NFC)/sensor, 1.85V Bluetooth® and audio (low noise), and a 1.2V microprocessor. These loads are typical for internet of things (IoT) devices. Output voltages for these rails are generally between 1.2V to 5V, and even broader for some devices. High system efficiency, small solution sizes, and low quiescent current are musts. Mital presented a power-management IC (PMIC) with a SIMO architecture as a way to address these requirements. In particular, a three-output SIMO buck-boost regulator with a linear charger, 3x current sink, power sequencing, a 150mA low-dropout regulator (LDO), and I2C serves as a full system solution that is scalable across multiple platforms. “When you have one solution powering the whole [system], you don’t have to worry about characterizing other parts,” he said.
A traditional switching-regulator topology calls for each switching regulator to have a separate inductor for each output. Inductors are large and costly, making them less-than-ideal for small designs. By contrast, in a buck-boost SIMO architecture, a single inductor regulates up to three output voltages over wide output voltage ranges. When an output goes below a certain threshold, this triggers the inductor to service that output. Since the outputs have different voltages and loads, this servicing process is not sequential; rather, it happens on demand, Mital explained. This architecture reduces solution sizes while maintaining efficiencies.
To highlight the efficiency advantage, Mital presented a traditional power tree with one inductor. In this case, a 90.2% efficiency reflects only the individual regulator efficiency, not that of the overall system. With a battery current of 49mA and VBATT MIN of 3.4V due to the 3.3V LDO, the overall system efficiency here is 69.5%. Mital then compared this scenario with a SIMO power tree with a single inductor. In this case, the battery current is 43.4mA (a 5.6mA savings, by comparison) and the VBATT MIN is 2.7V due to the PMIC operating range (which allows more discharge). The system efficiency of the SIMO solution is 78.4%, which is 8.9% more efficient than the traditional power tree.
Always On, Low Power, and Better Performance
During his session, Mital highlighted a few use cases to demonstrate the advantages of designing with always-on, low-power PMICs based on the SIMO architecture. For noise-sensitive headphone amplifiers, for instance, SIMO technology in the power supply supports some techniques to reduce output voltage ripple:
In test setups of the headphone amplifier application (shown in Figure 2), Mital compared a setup where VDD and DVDD are supplied by LDOs and one where VDD and DVDD are supplied by the MAX77650 ultra-low-power PMIC with SIMO and power path charger. Available in a 2.75mm x 2.15mm x 0.7mm wafer-level package (WLP), the MAX77650 features 5.6µA operating current, 0.3µA shutdown current, and an I2C-compatible interface. The test case revealed that using the SIMO PMIC to power the audio codec increases battery life without affecting audio quality. The inband spectrum input signals tested include: no signal, -60dBFS, and -3dBFS with nominal 32Ω load. Inband spectrum fast Fourier transforms (FFTs) show nearly identical noise and frequency content for VDD and DVDD supplied by discrete LDOs versus VDD and DVDD supplied by the SIMO PMIC. The noise floor and harmonic content remain unaffected by the SIMO driving VDD and DVDD supplies on the headphone amplifier.
Mital walked through several other use cases to show how highly integrated SIMO PMICs can lower power consumption and extend battery life for small, portable electronics. Smart earbuds present a true wireless solution with multiple rails, as shown in Figure 3. Since both the left and the right earbud require the same capabilities, the MAX77650 can be used in both. Mital notes that Maxim tries to maintain a consistent register map in its different SIMO PMIC families so users may easily port firmware from one platform to another.
There are also larger applications besides hearables and wearables that can utilize SIMO technology. Sleeping aids, in a headgear form factor, require a lot of processing power for the volumes of data gathered by their multiple sensors. The main system for this application can utilize a MAX77714 complete system PMIC with 13 regulators, 8 GPIOs, real-time clock, and flexible power sequencing for multicore applications. The design’s subsystem can utilize a SIMO PMIC to service the power rails for the accelerometer, for audio, and for the proximity sensor. Together, Mital said, both PMICs can drive more processing in this high-performance solution.