Power Conversion Efficiency for Portable Designs

Who needs to plug in? From smartphones to music players to health and fitness wearables, portable, battery-operated devices are everywhere. Cameras, tablet computers, earphones, hearing aids, smart watches, fitness monitors, hand tools and meters, and GPS trackers all demand an ingenious plan for power. More devices are joining this category every day.

When an engineer is designing for a battery-powered mobile or wearable application, the power conversion method is always a prime concern. Find a review someplace for any of these devices and you will see battery life and charge time predominantly discussed. The consumer demands long operating time between charges.

For any portable device you are designing, the size, weight, and efficiency of its power converter are essential things to optimize. Luckily there are some great power conversion technologies ready to help you out.

Figure 1. For portable devices, power conversion ICs must meet stringent size, weight, and efficiency criteria.

Battery Drives Power Conversion Choices

The first decision when starting your design would be the battery. A replaceable primary battery or a rechargeable battery? Or, a non-replaceable battery that lasts so long the user disposes of the entire product and buys a new one?

The battery type and voltage will determine many of your power conversion decisions. The more common types of batteries are alkaline, nickel metal hydride (Ni-MH), lithium-ion (Li-ion), and lithium-ion polymer (LiPo, LIP, Li-poly). Various lithium types have begun to take over lately as their cost has come down. The nominal cell voltage can be from 1.25V to 3.7V, depending on your selection. This choice will most likely be about energy density, cost, and cycle life (how many charge-discharge cycles).

We should add that energy harvesting via solar cell, thermoelectric, or vibration might be an option for a low-power portable design. For almost all applications, you will still require a small rechargeable battery—however, a large capacitor might work in some cases.

Small and light is certainly the name of this game. And a converter with extremely low quiescent current is usually required to efficiently handle inactive periods. Big transformers and inductors are not going to work here. We need to use high-frequency conversion for small-sized magnetics. And we need efficiency, efficiency, efficiency.

Supporting a Single Supply

If your application can work from a single supply that is higher than the battery voltage, a boost converter is the easy way to go. Topologies like resonant-mode regulators are available, but their control circuitry consumes too much power for small systems. Save those for designs over 8A. A half-bridge flyback boost converter with synchronous rectification is a great, but slightly complex, choice. There are a number of boost switching regulator ICs that take all that complexity inside. Look for a chip that has quiescent supply currents down around 500nA, better than 90% efficiency over an output range of 100µA to 100mA, and a startup voltage of less than 1V. Some will operate down to 0.40V, once they start up at >1V.

This type of converter chip can come in tiny 2mm x 2mm 6-pin μDFN or 0.88mm x 1.4mm WLP packages. Switching frequencies of 250KHz or more mean the few external components required will be small. Look for a fixed frequency, or better yet a fixed dithered frequency, for reduced electromagnetic interference (EMI). Maximum output current will vary greatly with input voltage because of small chip power limitations, so be sure to check the safe operating range along with inductor current and ratings.

Is a Buck-Boost Topology Your Answer?

A buck-boost controller solves the battery voltage concern. Standard buck-boost switching circuits usually produce a negative (inverted) output voltage. The single-end primary inductor converter (SEPIC) topology gives a non-inverting output, but requires two inductors (see Figure 2). These inductors can be made as a coupled inductor, which reduces their footprint and cost. If we are running at 1MHz or 2MHz, they are not very big. However, the series capacitor must be non-polarized and, therefore, will be large to handle the required current.

Figure 2. Basic block diagram of a SEPIC converter.

Because the series capacitor blocks direct current, the average current through it is zero, making the second inductor the only source of DC load current. The average current through that inductor is the same as the average load current and is independent of the input voltage.

There are similar topology buck/boost converters that use a four-switch H-bridge configuration and only a single inductor. Since the four switches are all inside a tiny WLP package, that’s OK. These devices must have N- and P-channel FETs and an internal boost supply to run them. They usually operate around 2.5MHz. You can look for an input voltage range of 2.3V to 5.5V and efficiencies of over 90% across output load currents from 1mA to 1A. The single output voltage is usually limited to around 2.5V to 4.2V in devices I have seen. High efficiency at low output current is enabled by skip-mode switching.

Figure 3. A four switch H-bridge configuration buck-boost converter requires few external components and uses a serial I²C setup and monitor interface. These ICs, such as the MAX77801, come in small packages.

Multiple Supply Rails Needed

Most portables are smart. They’re equipped with a microcontroller of some kind and have a few sensors—including temperature, acceleration, direction, biopotential, and gyroscopes. They may use Bluetooth wireless. There very well may be a lighted LCD touchscreen.

This means you will certainly need multiple power supply voltages. You may dream of having everything run on 1.8V, but you’ll likely end up with at least three supply rails. Luckily there are a number of power ICs with multiple outputs that fit this application very well in the form of multi-channel integrated power management ICs.

Figure 4. A 30-bump, 0.4mm-pitch WLP, 6x5 array.

Single-inductor, multiple output (SIMO) buck-boost regulator ICs typically provide three or four independently programmable power rail outputs, plus a 150mA low drop-out (LDO) linear regulator with high ripple rejection for audio and other noise-sensitive applications. This type of PMIC probably has an I²C interface for configuration and status checking. They usually require very few external components and likely include a battery charger plus a battery temperature monitor for safety. Some also feature current sink driver outputs for LEDs. Some are specifically targeted at running LCD displays.

Most of these ICs take only μAs of operating current. Their three or four buck-boost outputs provide output currents of 25mA to 100mA each, depending on the input voltage. And they often occupy a small 6x5 WLP package (Figure 4).

Figure 5. The SIMO converter scheme with three outputs.

The SIMO architecture basically shares one inductor. There are a few trade-offs for SIMO. Because a single inductor is essentially providing buckets of energy to alternate outputs, the output voltage ripple will be somewhat high. A little more filtering may be needed. Voltage accuracy will not be what one might get from a single regulator. These chips often have an output voltage of between 0.8V and 5.25V with accuracy of ±2.5%.

Other examples of SIMO power management ICs have three buck regulators, three or four LDO linear regulators, and a buck-boost regulator, providing up to seven regulated voltages, all in one chip. These ICs provide everything one needs in a single chip and include a 5mA to 500mA battery charge circuit.

Designing for portable and wearable devices is not easy. Keeping efficiency high and footprint and price low is tough. But, the newest conversion ICs can help simplify the design process while enabling you to meet your performance targets.

So now you, the design engineer, can be a hero—for a change.