Tiny Voltage Regulators Meet Wearable Electronics Space Constraints

Wireless sensors built into glasses, watches, motion trackers, and even clothes promise to revolutionize connectivity and form a key part of the Internet of Things (IoT). Some analysts forecast the sector could be worth a staggering $131 billion by 2020. Although wearable electronics represent a leading-edge technology, it is just the latest example of a space-constrained design to challenge the engineer.

Compact products put the power supply designer under particular pressure as he or she is expected to come up with a robust circuit, relying on a tiny battery and taking up just a few tens of square millimeters to power a processor, memory, peripheral components, and possibly even an energy-hungry display.

Fortunately, the engineer can take advantage of one of a new generation of tiny DC/DC switching voltage converters (“regulators”). These highly integrated power modules fit tiny footprints yet offer efficiency and features on par with much larger devices. This article takes a closer look at switching voltage regulators designed for space-constrained applications.

Working with small batteries

Wearable electronics are an exercise in Lilliputian engineering. A key design criterion is that the product should be unobtrusive. That places constraints on size and weight and in turn on the capacity of the power source. Some products, such as smart watches with powerful computation capabilities and GPS, are designed to act as “hubs” for a wirelessly connected “body area network”, and will likely use Li-ion batteries that consumers accept need topping-up every few days. However, buyers will be much less likely to tolerate recharging or frequent battery changes for the small sensors (for example, heart-rate monitors and speed and distance pods) that make up the peripheral elements of a personal network.

Engineers need to come up with sensors that can run for months from tiny batteries such as Energizer’s CR2032 coin cell (a lithium/manganese dioxide [Li/MnO2] battery that is a popular choice for powering products such as electronic watches). Energizer’s coin cell supplies a nominal 3 V and has a nominal capacity of 240 mAh before the battery voltage drops below 2 V.

Assuming a sensor such as a heart-rate monitor is used for an hour a day, a simple calculation (240 mAh/90 x 1 hr) reveals that the average current draw on the battery must be lower than 2.7 mA for the cell to last more than three months. However, actual battery capacity is heavily influenced by demand pattern and discharge rate.¹ For example, almost all wearable sensors incorporate a low-power wireless technology such as Bluetooth low energy or ANT+ that apply a pulse drain to the cell as the radio periodically transmits data, in addition to the constant drain from powering the unit’s other electronics (see Figure 1).


Figure 1: Dayton heart rate monitor incorporates Bluetooth low-energy wireless technology and runs for over a year on CR2032 battery (Courtesy of Nordic Semiconductor).

Figure 2 illustrates how this pulse load affects the battery’s “accessible” capacity. In the example, the duty cycle is 1 ms ON/14 ms OFF, demand is 23 mA at 2.7 V discharged through a 120 ohm load. From the graph it can be seen that, for this scenario, the accessible capacity of the battery before the voltage falls below 2 V is closer to 170 than 240 mAh. Therefore, the average current draw for the heart rate monitor must be less than 1.9 mA (170 mAh/90 x 1 hr) if the battery is to last for three months.



Figure 2: Discharge curve for CR2032 coin cell under pulse drain (Courtesy of Energizer).

Eking out the energy

Being restricted to an average current of less that 2 mA and a peak current draw of less than 30 mA (about the maximum that can be extracted from a CR2032 without seriously compromising capacity²) while working with little space makes the design of the power supply for a wearable sensor difficult.

Managing the power budget is best done in incremental design steps that consider the physical silicon, operating voltages and modes, voltage regulators, techniques such as adapting the clock rate, circuit layout, and software. Each step yields small results, but the overall effect is significant.

However, the most critical component in the power supply circuit is the power module itself. The more functionality that can be integrated onto the chip, within the constraints of the bill-of-materials (BOM) budget, the greater the likely power savings and circuit board real estate reductions. (See the TechZone article “Design Techniques for Extending Li-Ion Battery Life.”)

Power modules that integrate a switching voltage regulator with its controller are a good place to start when designing a portable product with long battery life. There are some drawbacks compared to linear regulators, for example, increased electromagnetic interference (EMI), greater cost, and increased design complexity, but when power saving is a major consideration the switching regulator’s impressive efficiency outweighs the other disadvantages.

Major semiconductor vendors offer a wide range of extremely compact and power frugal chips targeted specifically at portable designs. Texas Instruments (TI) supplies the TPS62240, a high- efficiency synchronous step-down (“buck”) switching regulator optimized for battery-powered portable applications.

The TPS62240 operates at 2.25 MHz fixed switching frequency and enters a power-save mode at light load currents to maintain high efficiency over the entire load current range (see the TechZone article “The Advantages of Pulse Frequency Modulation for DC/DC Switching Voltage Converters”). In addition, by entering a “sleep” mode, the current consumption of the regulator can be reduced to less than 1 µA.

The high operating frequency of the TPS62240 allows the use of smaller inductors and capacitors which help to decrease power supply size. The chip itself is available in a five-pin TSOT23 and six-pin 2 x 2 mm SON package. The chip maintains over 90 percent efficiency for input voltages from 2 to 3 V and output currents over 2 mA (see Figure 3).


Figure 3: Texas Instruments’ TPS62240 maintains high efficiency over a wide output-current range.

Linear Technology offers a range of switching voltage regulators for small portable devices. A proven solution is the company’s LTC3549 buck switching regulator that is supplied in a 2 x 3 x 0.75 mm DFN package. Similar to the TI chip, the Linear Technology product operates at a fixed frequency of 2.25 MHz in order to reduce the size of the external inductor and input and output capacitors.

The LTC3549 operates across a 1.6 to 5.5 V input range and can supply up to 250 mA output current (V_IN = 1.8 V, V_OUT = 1.2 V). The device uses “Burst Mode” technology to increase efficiency at light loads and extend battery life. The supply current drops to 50 µA during Burst Mode operation and less than 1 µA in “shutdown” mode. Efficiency peaks at 93 percent (V_IN = 1.8 V, V_OUT = 1.5 V, load current = 20 mA).

Targeted at space-constrained portable applications, Maxim Integrated offers the MAX1672. The product is a buck-switching-voltage regulator combined with a linear regulator that the company claims helps reduce the size of the external inductor and filters ripples in the output voltage. The device is packaged in a 6 x 5 mm QSOP and accepts an input voltage of 1.8 to 11 V while providing an output of 1.25 to 5.5 V using two external resistors. Typical efficiency is 85 percent. The chip can supply up to 300 mA (V_IN = 2.5 V, V_OUT = 5 V).

The expanding wearable electronics market represents a lucrative opportunity for electronics designers. By basing a product’s power supply around one of a wide range of proven switching regulators, engineers can make the most of a small battery’s capacity while ensuring that their circuit design squeezes into the compact form-factors consumers expect.

For more information on the parts discussed here, use the links provided to access product pages on the DigiKey website.

References:

1. “High pulse drain impact on CR²032 coin cell battery capacity,” Kjartan Furset, Nordic Semiconductor, Peter Hoffman, Energizer, 2011.
2. “Coin cells and peak current draw,” Mathias Jensen, Texas Instruments, 2010.