How to Make Your IoT Gadget Smaller and Cooler



Turning away from typical approaches can stretch battery life and use space more successfully.

Charging and fuel gauging ICs—at the heart of every battery management system—are critical components of ever-shrinking mobile and IoT electronic gadgets. While size is shrinking, complexity is increasing. Faster charging with minimum heat generation calls for high-efficiency charging at a high input voltage and a high charge current. This article reviews a typical approach for meeting the challenges of reduced space requirements and minimizing heat generation. It then presents a new solution that delivers more efficient power in a smaller space, enabling longer battery life and smaller form factors.

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Figure 1: Smartphone at End-of-Charge

Typical Power Management Implementation

A typical charger and fuel gauge system is shown in Figure 2. In addition to the charger and the fuel gauge, it includes two safeout low dropout regulators (SFLDO) that drive the system’s USB interface. For simplicity, the external passives are not shown.

Figure 2: Typical Charger and Fuel Gauge System

Figure 2: Typical Charger and Fuel Gauge System

All active and passive components for the system diagram shown in Figure 2 are accounted for in the solution drawing of Figure 3.

Figure 3: Typical Charger and Fuel Gauge System Footprint (55mm2)

Figure 3: Typical Charger and Fuel Gauge System Footprint (55mm2)

The 1.5µH, 3.5A inductor (in a large 2520 package) and the two external LDOs present a huge challenge in terms of space. This solution occupies a board area of about 55mm2.

An Integrated Solution

The PMIC in Figure 4 (MAX77818) integrates the charger, fuel gauge, and LDOs in a single chip. This eliminates the wasted space associated with using multiple components. Another advantage of this solution is the ability to use a lower value 0.47µH inductor that carries 3.5A in a smaller 2016 package.

Figure 4: Highly Integrated Charger and Fuel Gauge System-on-Chip

Figure 4: Highly Integrated Charger and Fuel Gauge System-on-Chip

All active and passive components of the system diagram shown in Figure 4 are accounted for in the solution drawing of Figure 5.

Figure 5: Highly Integrated Charger and Fuel Gauge System Footprint (37mm2)

Figure 5: Highly Integrated Charger and Fuel Gauge System Footprint (37mm2)

The total board space occupied by the components is a mere 37mm2, saving 33 percent of the real estate. In addition, the MAX77818 draws only 20µA of total quiescent current. This saves valuable battery life, which enables either system size reduction by using the smallest battery possible or longer use time between each charge.

More Efficiency in Less Space

A comparison between the efficiency (from CHGIN to BATT) of the MAX77818 charger solution and a competitive solution is shown in Figure 6. To provide a true comparison, both solutions are powered with a 1µH inductor in a 2520 package. The integrated charger solution exhibits higher efficiency across most of the load current range even with a substantially higher switching frequency. At full load (3.5A), the efficiency of the integrated solution is well above 88 percent, while the competitive solution is just above 86 percent, a more than two percent efficiency difference.

Figure 6: MAX77818 vs. Competitor Efficiency

Figure 6: MAX77818 vs. Competitor Efficiency

Even with the large 2520 inductor, the integrated solution is 30 percent smaller than the competitive solution.

MAX77818 Integrated Charger and Fuel Gauge

The MAX77818 switching charger is designed with a special CC, CV, and die temperature regulation algorithm. The ModelGauge™ (m5) algorithm delivers battery fuel gauging with the highest accuracy available while operating with extremely low battery current.

The fuel gauge must be loaded with a factory characterization file (INI) to match the battery in use. Maxim has developed a vast battery database of cell characteristics and behaviors over a wide range of test conditions based on customer use cases. This allows Maxim to provide an initial configuration file that works well with the battery until a more specific battery characterization file is created to achieve the highest accuracy operation. To learn more about the terms of support for battery characterization, please contact technical support.

Additional Advantages of Integration

A fuel gauge IC has temperature-sensing capabilities to conform to the correct charging profile based on the battery at hand. That’s not normally the case for the charger IC. However, JEITA, a Japanese electronics protocol that has become the standard for lithium-ion (Li+) batteries specifies charger behavior vs. temperature. For example, to control charge current and voltage based on temperature, which reduces the current at cold temperatures, the charger would need to integrate a temperature sensor or a microprocessor host to provide temperature information. But the MAX77818’s integration permits the fuel gauge and charger to easily access the temperature, an important function when the JEITA mode is enabled. Integration reduces silicon overhead and potentially lowers the thermistor countFinally, with the fuel gauge and charger in the same package, there is less chance of having PCB layout issues.

Conclusion

We have discussed the shortcomings of a typical charger-fuel gauge implementation and introduced a new, highly integrated solution. The new solution, compared to an alternative implementation, delivers a full load efficiency advantage of more than two percent and a 30 percent reduction in PCB area, enabling longer battery life and smaller form factors.


Bakul-DamleBakul Damle is the Mobile Power Business Management Director at Maxim Integrated. His current interests include battery and power management specifically in fuel gauges, battery charging, energy harvesting, wireless charging, and battery authentication. He has several patents in test and measurement. Bakul holds a Master of Science degree in Electrical Engineering from the California Institute of Technology and a Bachelor of Technology in Engineering Physics from the Indian Institute of Technology.

JiaHuJia Hu is an Executive Business Manager for Mobile Power at Maxim Integrated. He has more than 15 years of experience in the semiconductor industry. Jia holds a master’s degree in Electrical Engineering from the University of Washington and bachelor’s degrees in Electrical Engineering and Economics from the University of California, Berkeley.

Nazzareno-Rossetti_2017Nazzareno (Reno) Rossetti, Principal Marketing Writer at Maxim Integrated, is a seasoned Analog and Power Management professional, a published author and holds several patents in this field. He holds a doctorate in Electrical Engineering from Politecnico di Torino, Italy.

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