How to Design Efficient Power for Portable Devices

Among the many challenges of designing accurate, high-performance, essential analog circuits is designing their power supplies to provide efficient low-power to precision signal-chain amplifiers, sensors, data converters, and more. In this spotlight article, we discuss not only the attributes of efficient power supplies, but also the types, design considerations, and some examples of how efficient power extends battery life for IoT and other portable applications.

The Importance of Efficient Power and Battery Life

For any portable design, battery life is important. To extend battery life, designers specify components such as low-power microcontrollers, sensors, radios, and efficient power supplies. The power supply provides energy to drive all of the device's functional blocks. Power supplies typically consist of regulators, such as switching regulators that boost or buck the voltage, or low-dropout (LDO) linear regulators. Some also have power management ICs and perhaps even a battery charger.

While active current consumption is an important factor in extending battery life, standby current of the power supply is also important as the device spends more time at sleep and hibernation to save energy. Moreover, the power supply’s quiescent current is often the biggest contributor to a system’s standby power consumption, so utilizing efficient power ICs with  very low quiescent is a key strategy in extending battery life. Ultimately, extending battery life requires minimizing power dissipation. The higher the efficiency, the less power is wasted. Linear regulators provide significant advantages over switching regulators in simplicity, cost, and output noise, but not efficiency. Efficient power supplies utilizing switching regulators are good for portable designs because capacitors and inductors are used to store energy and convert the voltage.

Revisiting Linear and Switching Regulators

In this section, we’ll do a brief refresher on two of the most common power-supply ICs: linear regulators and switching regulators.

Linear Regulators

A linear regulator simply inserts an electronically variable resistor (in the form of a transistor) in series with the input DC to drop the voltage to the desired value of output voltage. If the input or load current changes, the resistance is varied by a feedback loop to keep the output voltage constant. A linear regulator can’t step up the voltage, but it can step down and regulate the voltage supplied to it with a minimal number of external components. Because these devices contain no switching elements, they generate little noise. The big disadvantage of linear regulators is power loss. When power is low, this effect is not necessarily an issue. However, let’s say you have a 5V load at 10A from a DC source of 10V. In this scenario, the power loss through the resistor is 50W, with a conversion efficiency of only 50%.

Figure 1: Linear Regulator

A linear regulator is usually, although not always, less efficient than a switching regulator. Low dropout linear regulators (LDOs) operate where the voltage of the source powering the linear regulator is near the regulator's output voltage, so efficiency in this situation is high, In that case, an LDO might be a better choice than a switching regulator because the LDO has less noise. But in general, a high-efficiency regulator provides a distinct advantage in portable designs, as less power is wasted, resulting in longer battery life.

Switching Regulators

Switching regulators are so named because they switch a power transistor, which, when used in conjunction with an inductor, efficiently converts one voltage to another. When these power transistors switch, they do so very quickly, as fast transitions improve the regulator's efficiency. To understand why, first consider the power transistor's power dissipation when it is not transitioning. When the transistor is off, voltage appears across it, but no current flows through it. So, no power is lost. When the transistor is on, a small voltage appears across it while appreciable current may flow through it. Thus, typically, a small amount of power is lost. When the power transistor transitions from an OFF state to an ON state, or vice versa, voltage appears across the transistor while current flows through it. Therefore, appreciable power can be lost. Speeding up the switching process reduces these transition losses.To minimize the power loss associated with the rectifier diode in a switching power supply, a synchronous configuration can be used. In a synchronous configuration, the rectifier diode is replaced with a MOSFET switch. This approach increases the efficiency of the switching converters even further.

Figure 2: Switching Regulators: (a) Step Down, (b) Step-Up, and (c) Inverting

Switching regulators are popular because they possess excellent efficiency when subjected to different combinations of input voltage and load current. The levels can be as high as 96% for both step-up and step-down switchers, although a step-down is typically more efficient, and up to 90% for an inverter. Also, if you need to step up, step down, or invert a voltage, switching regulators are the only devices capable of these operations for load currents above approximately 125mA.

Table 1: Linear vs. Switching Regulators

Portable Devices and Applications

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The following examples show how efficient power ICs are employed in battery-powered analog applications:

High Efficiency Buck-Boost Converter for Battery-Powered IoT Applications

The growing lithium-ion battery market is largely driven by an increasing inclination towards smart electronic devices with expanding functionalities. Primary cell batteries based on Lithium Thionyl Chloride (Li-SOCl2) chemistry or dual-cell Alkaline (AA or AAA) with operations as low as 1.8V is also gaining popularity in portable devices. No matter what type of battery is employed, these devices will discharge through a wide range of voltages while sustaining a power rail between 2.8V to 3.8V, specifically for powering MCU, Wi-Fi, BLE and GPS features.

The traditional buck or bypass boost plus low dropout (LDO) topologies are not ideal solutions for these smart devices because they are not the most efficient at extending battery life. This A better solution is a buck-boost convert such as the MAX77827 that can maximize battery life and address the power requirements of many internet of things (IoT) applications.

The MAX77827 has a quiescent current of 6µA and a peak efficiency of 96%, allowing it to support low-power requirements because, regardless of the battery voltage variations, it can automatically transition between buck and boost modes to provide a consistent output power supply. The IC buck-boost regulator utilizes a four-switch H-bridge configuration to realize buck and boost operating modes. This topology maintains output voltage regulation when the input voltage is greater than, equal to, or less than the output voltage. The buck-boost is ideal in one-cell Li-ion battery powered applications and two-cell Alkaline battery powered applications, providing 2.3V to 5.3V of output voltage range. High-switching frequency and a unique control algorithm allow for a very small small solution size, low output noise, and a very high efficiency across a wide input voltage and output current range.

Figure 3: MAX77827: Low-quiescent-current, buck-boost converter with a peak efficiency of 96%

Synchronous Boost Converter for a Wearable Heart Monitoring Patch

Internet of Things solutions often require small devices operating autonomously for long periods of time while consuming little power. A good example of this type of IoT device would be a wearable heart monitoring patch. Such a device, powered by a 100mAh alkaline button cell and consuming 100µA in operation, can last 3 weeks. In shutdown mode, the device may need to last up to 3 years, which requires a leakage current of 4µA or less. A typical voltage regulator, with a leakage current of 0.2µA and a total quiescent current of 10µA will subtract 1.8 months from the device’s shelf life and two days of operation. To extend the battery life for this situation, a designer should consider the MAX17225 nanoPower synchronous boost converter. Here's why.

The MAX17225 has a 400mV to 5.5V input range, 1A peak inductor current limit, 95% Peak Efficiency, 300nA Quiescent Supply Current, and an output voltage that is selectable using a single standard 1% resistor. It has a True Shutdown mode that yields leakage currents in the nanoampere range, making this a truly nanoPower device. The True Shutdown feature disconnects the output from the input with no forward or reverse current, resulting in very low leakage current. The MAX17225 also features low RDSON, on-board powertrain MOSFET transistors, to yield excellent efficiency even when operating at frequencies high enough to warrant a small overall PCB size. The MAX17225, ultra-low quiescent current, high-efficiency synchronous buck converter significantly increases the shelf and operation life of IoT devices.

Figure 4: MAX17225: Typical Operating Circuit

Storage Capacitor Bank Backup-Regulator for Mission-Critical Applications

Mission-critical conditions require regulated power even with the main power source is deenergized.  Thus, high-performance regulators charge a backup power source, such as a super-capacitor or capacitor bank, in just seconds to deliver continuous power to critical system components when the main power source is turned off. This solution is personified by Maxim’s Continua family of backup power regulators. Let's look at how they work, examiningg the MAX38888.

The MAX38888 is a storage capacitor or capacitor bank backup regulator designed to efficiently transfer power between a storage element and a system supply rail in reversible buck and boost operations using the same inductor. When the main supply is present and above the minimum system supply voltage, the regulator operates in buck mode and charges the storage element at up to 500mA peak inductor current. Once the storage element is charged, the circuit draws only 2.5μA of current while it maintains the super capacitor or other storage element in its ready state. When the main supply is removed, the regulator operates in boost mode and prevents the system from dropping below the minimum operating voltage, discharging the storage element at up to 2.5A peak inductor current. The MAX38888 is externally programmable for minimum and maximum voltage of the storage element, such as super capacitor, minimum system voltage, and maximum charge and discharge currents. The internal DC/DC converter requires only a 1μH inductor.

The MAX38888 reversible buck/boost regulator delivers has a 95 percent peak efficiency. Operating over 2.5 to 5V input in buck mode, the MAX38888 charges an energy storage device at up to 500mA peak inductor current. On supply failure, MAX38888 operates in a boost mode, providing 2.5 to 5V output at up to 2.5A peak inductor current from an energy storage device, discharging all the way to 0.8V. In portable electronics where the main power source is a battery, the MAX38888 doubles battery life by lowering quiescent current up to 15 times lower than competitive solutions during idle mode.

Figure 5: MAX38888: Typical Operating Circuit

Design Considerations

Higher performance and extended battery life pose a growing challenge for the designers of portable devices. However, the introduction of power supplies with different architectures and better performance are lessening this challenge. In this section, we will discuss the benefits and tradeoffs of different combinations of power architectures for efficient power solutions.

Low-Dropout (LDO) Linear Regulators: The LDO's lowest cost, lowest noise, and lowest quiescent current make it a solid choice for many applications. Its external components are minimal: usually a bypass capacitor or two. The newest LDOs offer dramatically improved performance.  Efficiency, though poor when VIN is much larger than VOUT, becomes very high when VIN approaches VOUT. In that case, the LDO benefits are very good.

DC-DC Converters: Available in buck, boost, buck/boost, and inverting topologies, DC-DC converters offer high efficiency, high output current, and medium-low quiescent current. On the other hand, they produce output ripple and switching noise. They also are more expensive, due to a more complicated control scheme and the need for an external inductor. But, better control schemes have added valuable features such as soft-start capability, current limiting, and selectable PWM or PFM operation.

DC-DC Buck Converter: In nearly all applications for which VIN is greater than VOUT, the DC-DC buck converter is more efficient than an LDO. This is especially true when VIN is much greater than VOUT (e.g., converting the output of a single Li-ion cell to 1.8V). The DC-DC buck converter exhibits some output ripple and switching noise, but these are not as severe as in other DC-DC topologies. One notable advance in control schemes is the implementation of duty cycles up to 100%, enabling the circuit to achieve low-dropout performance.

DC-DC Buck Converter with LDO: Combining the DC-DC buck converter with an LDO is useful in applications for which high efficiency and low noise are important. This arrangement, however, applies only when VIN is substantially larger than VOUT. If the minimum VIN approaches VOUT, the LDO alone should provide similar efficiency and lower dropout, usually resulting in the same or better battery life for much lower cost.

DC-DC Boost Converter: The most important feature of a DC-DC boost converter is that an LDO cannot step up the voltage (boost). On the other hand, boost converters have notoriously high output ripple and switching noise. They also require better control schemes to eliminate oscillation in the output and to reduce efficiency loss due to parasitic resistance in the MOSFET switch and external components.

DC-DC Boost Converter plus LDO: Combining a DC-DC boost converter with an LDO has two advantages: It implements a low-noise boost function (at a slight penalty in efficiency versus the noisy booster without an LDO), and it performs the buck/boost function with surprisingly high efficiency. A typical buck/boost application converts the output of one Li-ion cell to 3.3V. Efficiency is very high, because the battery spends most of its life near 3.6V, allowing the booster to idle and providing the LDO with a near-ideal input voltage. This system also delivers higher efficiency with smaller external components than the traditional single-ended primary-inductor converter (SEPIC).

Examples of Efficient Power ICs

Glossary

• Analog - A system in which an electrical value (usually voltage or current, but sometimes frequency, phase, etc.) represents something in the physical world. The electrical signal can then be processed, transmitted, amplified, and finally, transformed back into a physical quality.
• Boost Converter - A power supply that steps an input voltage up (boosts it) to a higher, regulated voltage.
• Charge Pump - A power supply which uses capacitors to store and transfer energy to the output, often stepping the voltage up or down. Charge is transferred from one capacitor to another under control of regulator and switching circuitry.
• Current-Sense Amplifier - An amplifier that measures current by measuring the voltage drop across a resistor placed in the current path. The current sense amp outputs either a voltage or a current that is proportional to the current through the measured path.
• Data Converter - A/D or D/A converter: An electronic circuit that converts analog signals to digital, or vice-versa. An analog signal is a continuously varying voltage or current. Its digital counterpart is a stream of digital numbers, each representing the amplitude of the analog signal at a moment in time.
• D/A Converter - Digital-to-analog converter (DAC): A data converter, or DAC, that receives digital data (a stream of numbers) and outputs a voltage or current proportional to the value of the digital data.
• DC-DC - Any of the family of switch-mode voltage regulators, these devices use an inductor to store and transfer energy to the output in discrete packets, resulting in highly efficient power conversion.
• ESD - Electrostatic Discharge: Release of stored static electricity. Most commonly: The potentially damaging discharge of many thousands of volts that occurs when an electronic device is touched by a charged body.

• Hot-Swap - A power supply line controller which allows circuit boards or other devices to be removed and replaced while the system remains powered up. Hotswap devices typically protect against overvoltage, undervoltage, and inrush current that can cause faults, errors, and hardware damage.
• Idle Mode - A method for improving the efficiency of switching regulators by skipping pulses when the circuit is lightly loaded. This variation in PWM (pulse-width modulation) combines the efficiency at low loads afforded by PFM (Pulse-Frequency Modulation) with PWM's efficiency and low-noise characteristics at higher loads. At light loads the circuit skips pulses as necessary (acting like a PFM circuit). At higher loads it acts like PWM. The net result is the maximum efficiency over the widest possible load range.
• Inrush Current - A momentary input current surge, measured during the initial turn-on of the power supply. This current reduces to a lower steady-state current once the input capacitors charge. Hotswap controllers or other forms of protection are often used to limit inrush current, because uncontrolled inrush can damage components, lower the available supply voltage to other circuits, and cause system errors.
• Inverting Switching Regulator - A switch-mode voltage regulator in which output voltage is negative with respect to its input voltage.
• LDO - Low Drop Out: A linear voltage regulator that will operate even when the input voltage barely exceeds the desired output voltage.
• Line Regulation - The ability of a power-supply voltage regulator to maintain its output voltage despite variations in its input voltage.
• Load Regulation - Load regulation refers to circuitry that compensates for changes in load. Most commonly: Circuits that keep voltage constant as load varies.
• Overvoltage Protection - Overvoltage Protector (OVP) refers to a circuit that protects downstream circuitry from damage due to excessive voltage. An OVP monitors the DC voltage coming from an external power source, such as an off-line power supply or a battery, and protects the rest of the connected circuitry using one of two methods: a crowbar clamp circuit or a series-connected switch. The crowbar short-circuits or clamps the supply line to limit the voltage, possibly triggering other forms of protection such as a fuse. See Crowbar.

• Point-of-Load - Point-of-load (POL) power supplies solve the challenge of high peak current demands and low noise margins, required by high-performance semiconductors such as microcontrollers or ASICs, by placing individual power supply regulators (linear or DC-DC) close to their point of use.
• PFM - Pulse-Frequency Modulation: A pulse modulation technique in which the frequency is varied with the input signal amplitude. The duty cycle of the modulated signal does not change. Because it is always a square wave with changing frequency, PFM is also referred to as square-wave FM.
• PWM - A method for using pulse width to encode or modulate a signal. The width of each pulse is a function of the amplitude of the signal. A technique used to modulate the power delivered to a load. In DC-DC switching regulators, the pulse width driving the main power switch (and hence, the duty cycle) is varied to maintain the desired output voltage. In DC motor-control applications, pulse width is used to vary motor speed.
• Quiescent - For an electronic circuit, a quiet state in which the circuit is driving no load and its inputs are not cycling. Most commonly used for the specification "quiescent current," the current consumed by a circuit when it in a quiescent state.
• Switching Regulator - A voltage regulator that uses a switching element to transform the supply into an alternating current, which is then converted to a different voltage using capacitors, inductors, and other elements, then converted back to DC. The circuit includes regulation and filtering components to insure a steady output. Advantages include the ability to generate voltages beyond the input supply range and efficiency; disadvantages include complexity.
• Synchronous Rectification - In switch-mode power supplies, the "steering" diode is replaced or paralleled with a FET switch to reduce losses and thereby increase efficiency. The FET is off during the inductor charge cycle, and then turned on as the inductor discharges into the load.
• Voltage Doubler - A capacitor charge pump circuit which produces an output voltage which is twice the input voltage.
• Voltage Regulator - A circuit which is connected between the power source and a load, which provides a constant voltage despite variations in input voltage or output load.