When discussing wearables, it’s almost impossible to avoid a discussion about battery power. Consumers are demanding batteries that last for days, but they’re also looking for batteries that support more and more power-hungry applications. While those issues both have a simple solution, no one wants to lug around a massive battery to power their portable device and any batteries must fit into increasingly slimmer and smaller designs.
While much of the discussion about battery power focuses on how to reduce the battery power needed by certain applications, designers can’t ignore that wearables spend much of their time in “deep-sleep” state. Essentially, they spend a lot of time not running applications, but during the sleep time the batteries are still leaking.
It may seem like the battery power lost during sleep mode would be minimal, but with wearables evolving in the direction of more and more usage, sleep state leakages are starting to become a more serious problem.
“A 1 meg pull-down resistor can draw 4uA in a 4V system = 16uW. If you are powering a laptop at 70W, this is trivial. However, if you are pushing the limits and use a super-small battery the power loss becomes a big deal,” says Adrian Mikolajczak, COO & VP Marketing at GLF Integrated Power. “Recently, Panasonic released its smallest rechargeable 0.013A-hour battery for wearables. A 20uA average current consumption (equivalent of five times 1 meg pull-down resistors sprinkled on the board) will discharge the battery in under one month, without any features turning on. For these small systems, every uA counts.”
The sleep leakage is becoming a bigger problem as consumers demand wearables that last longer in between chargers, according to Mikolajczak. Future wearables–and some current options–may not ever be able to be charged at all as in the case of smart running shoes. These shoes are the evolved version of the traditional LED light shoes for kids. With current technology, designers are adding wireless sensors to track metrics and even transistors. Unfortunately, it’s nearly impossible to offer a charging option for this technology. (Who wants to charge shoes, anyway?) With this in mind, it’s imperative that the shoes have a solution for circuit leakage since they will be sitting in warehouses on sleep mode for some time before they even begin to be used by the customer.
Weirdly, the solution for this new technology problem is actually not a new technology. A load switch can be the key to reducing the amount of leakage since it shuts off the systems that are not in use. GLF has a 5nA typical off-state leakage at 3.3V, and 9nA leakage typical at 4.5V for a switch that is only 30 to 35mOhm typical in that voltage rage when on.
“To use the example above, five of these load switches sprinkled on a board in the off state would let a wearable powered by that 0.013Ah Panasonic battery survive over 500 months, or over 40 years, if that were the only source of power loss,” says Mikolajczak “Of course, the battery would self-discharge and likely corrode long before this, but I think it makes the point.”
It seems as though the load switches are coming into their own as more generations of wearables emerge. Much of the focus for first generation wearables was just to get a prototype out the door, but second and third generation must be more attuned to customer lifestyles, which means longer times between charges.
As the batteries need to last longer and longer periods of time, the load switches could be a age-old solution to a new problem. Many of the people at GLF started at semiconductor companies, so they saw the adoption of load switches into computers, laptops, and eventually cell phones. Wearables are really the next stage of battery challenges, but they’re certainly not the end of the line. The applications for this type of technology makes sense for the next wave of electronics, including smart credit cards, smart labels, and mobile medical monitors.