Fundamentals of Next-Gen Power Solutions for Effortless Location Tracking

Introduction

In a vast array of industries, ranging from consumer to industrial, there is an increasing necessity to track location in real-time with extremely high precision and accuracy. Be it locating a loved one, tracking packages, navigating outdoors, or managing assets, having accessible, compact, accurate, precise, and continuous location services gives peace of mind. A critical component of a compact location tracking system is the power solution, as its efficiency and noise characteristics directly influence the runtime and precision of the entire system. Consequently, it is important to innovate next-gen power solutions to further the capabilities of location trackers. This application note discusses the technical challenges of designing a power solution for compact location tracking systems and focuses on the advantages of using Analog Devices' single-inductor multiple output (SIMO) power management integrated-circuits (PMICs). Primary emphasis is given to noise reduction, power savings, and footprint minimization.

GNSS Overview

The global navigation satellite system (GNSS) was started in 1978 with the launch of the United States' global positioning system (GPS) satellite constellation. Since then, GNSS has grown to include satellites from Russia, the European Union, China, Japan, and India. To enable location tracking, each satellite continually transmits three key pieces of information:

• Time of transmission, within 3ns precision.
• Satellite location composed of altitude, latitude, and longitude (based on known orbits and exact time).
• A partial almanac that contains information about satellites the on-earth receiver can communicate with next.

With GNSS information from at least three satellites, a receiver can calculate its rough position on Earth based on the transmission time of satellite data (equivalently distance to the satellites) and the known location of the satellites. When more satellites are used, the location calculation of the receiver becomes more accurate and precise, but at the cost of using more power.

Portable GNSS Device Challenges

As GNSS signals travel at least 20,200 km to reach earth and are spread-out in a 28.6° cone, there is significant signal attenuation between the satellite and receiver. Location accuracy is directly related to the amount of noise in the signal relative to the signal strength. Atmosphere, interfering signals, and physical barriers such as dense woods or buildings add noise and reflected signals, which make accurate location tracking challenging. When initially rebooting a GNSS receiver (cold-start), the device must look for usable satellites. To quickly lock onto useful satellites, the receiver must make several data intensive computations, which in turn require more power. Powering mobile GNSS devices is difficult as consumer trends require next-gen technology to increase accuracy, precision, and processing speed, all while extending battery life and reducing overall form factor.

For wearable or portable location devices, there are three main design considerations:

• Minimize internal noise to ensure highest fidelity of GNSS signal.
• Maintain low power consumption to ensure long battery life despite continuous use.
• Ensure small solution size to increase wearability or portability of the end-product.

There are four power rails to consider for a GNSS receiver. Figure 2 depicts a common GNSS receiver and power-management configuration. The most power intensive input to the receiver is V_CORE, which manages the location calculations. While in use, V_CORE can draw up to 20mA continuously. Power is also supplied to the receiver's low-noise amplifier (LNA), which is crucial in receiving and demodulating the satellite signal. The LNA voltage rail must have as little inherent noise as possible to maintain the highest fidelity of GNSS messages. To manage digital communication, memory, and other system functions, there is also power supplied to V_DD and V_IO.

The power solution displayed in Figure 2 is an Analog Devices SIMO PMIC. This power solution implements a single-inductor multiple output (SIMO) topology, which has several benefits regarding the GNSS receiver. By using a SIMO PMIC, the four required power rails for the GNSS receiver are provided while maintaining low noise, high efficiency, and small number of external components. These benefits of using a SIMO power-management solution are expanded upon in the following sections.

Refer to the SIMO white paper for more details on the SIMO architecture.

Noise Management

For a GNSS receiver, internal noise mitigation is especially important as the received signal may already have significant inherent noise, which degrades the performance of location tracking. Modern GNSS receivers are becoming highly sensitive (pushing -160dBm), meaning they can receive very low power signals. While this means a decrease in size of end devices through smaller antennas, it also means noise considerations are even more important. To retain high quality signals, it is imperative for the internal systems that demodulate and decode the GNSS signal to have a stable power source. Thus, a low-dropout (LDO) regulator is used to power the receiver's LNA and ripple is minimized on the other power rails. Using a SIMO power solution, the peak inductor current is configurable, which consequently provides control of output voltage ripple amplitude. Figure 3 shows the ripple for each channel used to power the GNSS receiver. The blue line corresponds to V_CORE, which manages the location calculations. The red line corresponds to V_DD, which is supplied to the digital communication. The green line corresponds to V_IO, and the orange line corresponds to V_LNA, which powers the noise sensitive LNA. For accurate operation, ripple of < 20mV_PP is advised for V_CORE, V_DD, and V_IO. And for V_LNA, the rail powering the most noise sensitive subsystem, ripple of < 5mV_PP is advised.

Power Savings

GNSS receivers often do not need to receive and process data continuously. For example, in slow moving applications, readings may only be needed every 30s to provide accurate location. Consequently, it is beneficial for a GNSS receiver to enter a low-power mode when on standby. The power loss of the GNSS receiver is proportional to the operating voltage squared. To conserve power, the SIMO PMIC can enable a low-power mode by reducing the voltage supplied to the GNSS receiver. Voltage scaling can be controlled by one of the PMICs general-purpose input/output (GPIO) pins for dynamic control. When the GNSS receiver operational demands are lower (e.g., after cold-start or between location readings/calculations), the SIMO PMIC can scale down the supply voltage. Figure 4 provides an example of the SIMO PMICs voltage scaling capabilities regarding the GNSS receiver system requirements. The blue line depicts the SIMO power rail used for V_CORE. When a high-power state is desired, the GPIO switches low to set the V_CORE voltage to 0.7V. Otherwise, when a low-power state is desired, the GPIO switches high to change V_CORE to 0.5V.

Since battery life is vital for wearable and portable device usability, the GNSS receiver system efficiency is a principal design consideration. Compared to a traditional discrete power solution, the SIMO PMIC displays significantly improved system efficiency and reduced power consumption, which enables either longer battery life or reduced battery capacity (and size) in the end-device. As shown in Figure 5, when powering a wearable GNSS receiver with a SIMO PMIC, system efficiency is increased 15.5% (right) when compared to a traditional discrete power solution (left). As the SIMO PMIC has three easily programmable switching outputs, the headroom for the LNA linear regulator can be adjusted to achieve greater system efficiency. Consequently, the current drawn from the battery is reduced with the SIMO implementation, which in turn extends battery life by 25%.

Size and BOM Reduction

Space is a premium in wearable and portable devices with GNSS receivers. To maximize space available for components whose performance is largely dependent on size, such as the antenna and battery, it is critical to reduce the size of control circuitry and PCB volume. As the Analog Devices SIMO topology only uses one inductor to regulate three high-efficiency switching outputs, considerable space is saved, and fewer components are required than in a discrete approach. Figure 6 shows a footprint comparison between a traditional power management system (left) and a SIMO power management system (right). The traditional system includes two discrete bucks, two linear regulators, and twelve passive components. The SIMO power management system includes a single monolithic PMIC and seven passive components. Both solutions cover the voltage rails required for the GNSS receiver. However, the SIMO PMIC reduces solution size area by 8.7mm2 or by ~30%. As inductors are the largest passive components, most space saved stems from the SIMO PMIC only requiring a single inductor.

Summary

When considering the power needs and device constraints of a wearable GNSS receiver, Analog Devices' single-inductor multiple output (SIMO) power management integrated-circuits (PMICs) provide an excellent solution. As low noise, long battery life, and small solution size are paramount in the GNSS system, the SIMO power solution tackles these design criteria by implementing programable ripple control, on-the-fly voltage scaling, and regulation of three output rails using just one inductor. Consequently, when utilizing the SIMO power solution, the GNSS receiver can last longer or use a smaller battery, have increased accuracy and precision, and easily fit into wearable form factors.