From glasses and watches to fitness and sleep monitors, the latest wave of consumer products — featuring miniaturized wearable technology and the Internet of Things (IoT) — continues to demand smaller and more efficient power supply technologies than ever before.
System designers will get as much of the system functionality as possible into an ASIC, but there will always be a call to add more subsystems, from wired and wireless connectivity chipsets to novel product differentiation features. Adding features via additional chipsets after ASIC definition means supporting additional power supply rails. Furthermore, the tiny battery size used in wearable devices limits the total available energy, which means total system power efficiency is very critical to meeting consumer expectations for a long operating life.
Manufacturers of switching DC-DC buck converters have risen to the challenge by shrinking their parts to less than 1 mm through advances in chip scale packaging (CSP). The smallest such buck converter today measures 0.8 x 0.8 mm, thanks to a CSP package with bump pitches as small as 0.28 mm. In many products, the regulator device also includes internal MOSFETs, further minimizing the board space needed for power conversion.
But package size is only one factor wearable technology designers must consider to minimize the board space needed for power regulation. Here are three other key considerations.
- Switching frequency: The complete power solution (assuming integrated MOSFETs) includes an inductor and two capacitors. By picking a regulator with a switching frequency of between 2 MHz and 6 MHz, designers can reduce the inductor capacity to less than 1 micro-Henry, shrinking the passive circuit elements and the active silicon.
- Real-world power efficiency: Switching converters with operating frequencies of more than 1 MHz DC-DC present a tradeoff between efficiency and circuit size, since higher frequencies give up 1-2% efficiency. Today's switching converters offer up to 95% efficiency, so most often wearable device designers are willing to balance the efficiency/size tradeoff to get the smallest possible form factor. Another important factor is wearable devices stay on for extended periods (days or weeks). That requires processors to be put into low-power standby states. To extend battery life during standby states, many power converters use power save modes to reduce switching frequency, which saves additional power.
- Programmable output: Changing voltages dynamically is a popular technique for conserving power by moving to a lower voltage during standby mode. With a buck converter that features digitally programmable Vout, there is no need for additional resistors that would ordinarily set the output voltage. This reduces the board space needed, along with the component count.
Wearables present board designers their biggest challenge ever in delivering the functionality people expect is such a limited space. But by following the tips above, designers can implement power solutions they need for key functionality while maintaining their form factors.