Looking to develop a wearable device, such as a watch, oximeter, or blood pressure monitor? The small size and high functionality required in a smart watch pose two basic challenges to the system designer: how are you going to fit everything you need within the required form factor, and how do you power the device?
Here are three solutions to enable ultra-low-power operation in wearable electronics:
The key to long battery life is operating with the lowest current consumption by reducing system activity when it is not needed. In addition to turning certain functions off, this means operating in the sleep or standby mode of the microcontroller and in the power save mode of the power supply. For example, when the user is not looking at his or her watch, you turn its display off. Or when the SimpleLink™ Bluetooth® low energy CC2541 wireless microcontroller (MCU) is able to operate with just the sleep timer and not Timer 1, the supply current is reduced from around 90 µA to just 0.6 µA—a savings of over 99%! Additionally, any background task is interrupt driven in the code. Thus, the microcontroller operates always in sleep mode, waking up only when an interrupt commands it to.
2. Have lowest current consumption in standby mode
A key technology driver is reducing the current drawn in these standby modes. For instance, the MSP430F59xx (FRAM) with EnergyTrace++™ microcontroller consumes just 450 nA in standby mode due to its extremely low leakage FRAM memory. And if you power this microcontroller with a TPS82740A, the current drawn from your single-cell Li-Ion battery is only about 750 nA when you are powering its leakage. In this case, the 360-nA quiescent current (IQ) combined with the DCS-Control topology delivers such a low standby power consumption. And if your current consumption is that low, you certainly need load switches to disconnect sub-systems that are off from their supply voltage to eliminate their leakage currents from the system.
3. Integrate to save precious PCB space
Now that you have the power consumption down to a reasonable level so that the battery won’t need recharging every day, the second hurdle is space—where are you going to put everything (microcontroller, sensors, power supply, battery, etc.)? For this challenge, integration is your friend. The TPS82740A is a MicroSIP device, which integrates all required passives AND a load switch! This is more than 75% smaller than a discrete TPS62740-type implementation with the same optimized ultra-low power performance. The MSP430F59xx also integrates numerous functions such as a temperature sensor, differential input analog to digital converter, 8-mux LCD display driver, and 256-bit encryption. Just add a battery and a few other system-specific sensors and you’re basically done!
Operating in standby mode, having lowest current consumption in standby mode, and integrating to save board space are three ways to make your next wearable design a success. What are some other ways to save power in a wearable design?
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