Guest blog written by David Isbister, director of hardware at Whistle
When we set out to design the Whistle Activity Monitor, we knew we would face a number of challenges, due to the extremely small form-factor and the functionality we wanted to include. This form factor required using a much smaller battery than a typical mobile device, such as a cellphone or tablet, but still needed to have wireless connectivity to the Internet or to a smartphone. Before we even began, we created an extremely-detailed power usage model for our device, to help us pick the right components, power supply structure, and software architecture to maximize our battery life without sacrificing the user experience.
We knew we wanted to include wireless connectivity with our device, since the customer experience of syncing data using a cable is unacceptable in 2013. We also wanted to use wireless connectivity to detect which family member is with their dog. This, in particular, led to choosing to include Bluetooth (using the TI CC2564) in our device. We chose to use a dual-mode BT/BLE device to ensure connectivity with the maximum number of devices currently in the market.
One of the things that needed to be decided upon was which type of power supply to select. In most small form-factor devices, linear regulators have been used, due to their small size, low-cost, and low quiescent currents. However, linear regulators are inefficient, as they dissipate power linearly with the difference between their input and output voltages. This can mean efficiencies at 1.8V of 40-50% and 78-90% for 3.3V (based on a Lithium-Ion battery voltage range of 3.6-4.2V).
In the past few years, switching regulators have made up most of the gaps with LDOs:
Of course, the best reason to use switching regulators over LDOs is efficiency. The switching regulators we chose (TI TPS62400-series) have excellent efficiency with >90% efficiency at currents >1mA.
Our actual usage model is a gigantic spreadsheet, but it can be mostly distilled down into these two graphs (note that the Y-axis in the first graph is logarithmic):
What do these two pictures mean?
Since the battery is about 2/3 of the entire volume of our device, power supply design was critical to determining the physical size of our device. By using switching power supplies, we managed to reduce our power usage, especially in our higher-power modes, which allowed us to reduce the size of our device.
Another way we plan to maximize battery life is to intelligently utilize our wireless interfaces (Bluetooth, and Wi-Fi) and our mobile device software (on iPhones, and soon-to-be on Android) to find the most efficient communication method for each connection. For example, if our device is within Bluetooth range of an iPhone when it is attempting to sync, it will use the Bluetooth connection (rather than the higher-power Wi-Fi connection).
In the end, we hope to create a tiny device with wireless connectivity that lasts over a week on a single charge. This means the Whistle can spend less time in its charger, and more time on its dog’s collar.
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