Power Tip Power Supply: The Keys to Maximum Energy Efficiency

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There are various measures you can take to extend the service life of a battery-powered device or to minimize the size of the battery used. Essentially, it’s all about making the system as energy-efficient as possible. This power tip shows you which adjustments you need to make.

Frederik Dostal is a Field Application Engineer for Power Management at Analog Devices in Munich.

Electrical circuits should operate as energy-efficiently as possible. In addition to optimizing the overall circuit, the power supply must also provide the highest possible voltage conversion efficiency. The factors that can be adjusted to increase efficiency will be discussed below.

Figure 1 shows a wearable sensor system. It is a compact, battery-powered platform that provides various vital sign measurements, such as blood oxygen levels, heart rate, respiratory rate, impedance cardiography, bioelectrical impedance analysis, and skin temperature.

To make the battery as small as possible for this application, the device must operate very efficiently. There are various ways to optimize the power supply to achieve this.

High Integration: High integration is beneficial not only for space savings but also for power supply efficiency. Figure 2 shows the block diagram of the MAX20356, a PMIC that includes a battery charge controller, three switched step-down converters, four linear regulators, a switched buck-boost converter, a fuel gauge for determining the current battery charge, as well as extensive monitoring functions and voltage control circuits.

These functions could be provided by different integrated circuits. However, in a discrete design, the internal power consumption would be much higher. The reason for this is that many necessary support functions—such as a voltage reference for regulating the output voltage in a PMIC—need to be present only once but can be used by all integrated circuit blocks.

Efficient Voltage Conversion: Of course, voltage conversion must be as efficient as possible. This is optimized using the standard control parameters for voltage converters: switching frequency, passive components such as inductors and capacitors, modes such as pulse skipping or burst mode in the low-load range, and optimized dead time of the internal converter MOSFETs.

Low Quiescent Current / Self-Consumption: The PMIC’s self-consumption—or required quiescent current—must also be very low. Modern semiconductor processes and optimized circuit design help achieve this. The current flowing through various resistor dividers in the circuit is also relevant. Increasing the resistance values results in a lower bias current. However, this increases the impedance, which can make it easier for interference to couple into the circuit. An increase in impedance can only be implemented after thorough evaluation.

Rapid Scaling of the Processor Supply Voltage: Dynamic Voltage Scaling (DVS) offers one way to reduce power consumption in a system. A microcontroller requires a higher supply voltage during periods of higher computational load and a lower supply voltage during periods of lower computational load or when in standby mode. If the DC/DC converter constantly adjusts the voltage to meet these demands, a significant amount of power loss can be saved. Thanks to its very fast response, system efficiency is significantly increased depending on the specific application.

Minor disturbances in the power supply: Finally, the relationship between energy efficiency and disturbances must be highlighted. When a sensor is powered by a voltage converter, disturbances such as voltage ripple are also transmitted from the power supply to the sensor. Some sensors can tolerate higher voltage ripple and interference at various frequencies if they have a higher current available.

When the current is low, the power supply line must have very little interference to ensure the quality of the sensor data. In some applications, therefore, there is a direct correlation between the cleanliness of the supply voltage and the system's power consumption. 

* Frederik Dostal is a Field Applications Engineer for power management at Analog Devices in Munich

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