The Lowdown on Sleep Current: Does Active Current Still Matter?

Many of the most common capacitive sensing-enabled products, such as remote controls, set-top box front panels and wall-mounted control interfaces, tend to stay inactive much more often than they’re active. A television remote control might sit on a coffee table 16 hours of a day before the user finally gets the chance to sit down and watch some TV. Since these devices stay in an ultra-low power state the vast majority of the time, the amount of current the MCU controlling the device draws when the device is active doesn’t matter, right?

Wrong. The truth is that a design that’s been optimized for minimal current draw remains power conscientious during all operational states, from a design’s lowest power idle state to its most computationally intense active state. In capacitive sensing applications, a low power state means that scanning happens at a lower frequency, and a ‘candidate touch’ will switch the MCU to an active mode where scanning happens at a higher frequency to create a system that’s responsive to user input.

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Let’s say a designer is creating a system powered by two AA batteries. The system uses a capacitive sensing MCU that runs two states of operation: the lowest power state uses 500 nA and the highest power state uses 10 mA of current. The design team estimates that the system will be active 30 minutes of every day, where the capacitive sensor needs to be in its highest power state to scan buttons and respond to user input. Thirty minutes a day is about 2 percent of the device’s time, so the average current draw when factoring in both active and low power modes works out to be around 200 uA. Excluding other components in the system that might also draw current, the two 2700 mAh AA batteries will be drained in about three years of use.

Now let’s run those same numbers with a system that has an active mode current draw of 5 mA instead of 10 mA. Even though the system only runs in active mode for 30 minutes a day, the average current draw for this 5 mA system works out to be 100 uA – half of the previous design. This means that we’ve effectively doubled our battery life by reducing our active mode current by 50 percent. An extension of battery life to this degree can mean all the difference in customer experience.

So how can a developer reduce active mode current draw? Two strategies can be used: use as little current as possible while in active mode, and stay in active mode for as little time as possible. Silicon Labs’ capacitive sensing MCUs help developers pursue both strategies through numerous design optimizations:

  • Our fast and accurate capacitance to digital converter means that the system can do more scans per unit time than many competing MCUs.
  • Capacitive channel binding means that the capacitive sensing block can scan multiple inputs simultaneously, when the system only needs to determine if a touch is happening somewhere on the sensing surface. When firmware detects a ‘bound touch’, the system can break the binding and scan inputs individually.
  • A low power oscillator can be used when system timing is not critical. This oscillator provides around a 20MHz system clock at a fraction of the current draw of the precision internal oscillator.
  • A 150 uA suspend mode gives developers an easy-to-enter/quick-to-exit intermediate low power state when the system is waiting for an event to occur but cannot yet enter its lowest power state.
  • Memory retention across low power mode means that the system doesn’t have to reinitialize capacitive sensing registers or RAM when it enters active mode. This means a system can get back to sampling and processing quickly and revert back to a low power state as quickly as possible.

All capacitive sensing applications trade current draw for responsiveness. Generally, the more current allocated to capacitive sensing, the more robust and responsive the system becomes. In designs with slower capacitive sensing MCUs or MCUs with less design attention paid to low power operation, developers often have to sacrifice battery life to preserve the user experience. With a combination of a fast capacitive sensing block and low power design features, Silicon Labs makes it easy to design a responsive system that is power conscientious in all modes of operation.

Learn more about our full line of capacitive sensors here.

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