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How to Extend Battery Life in Wearable Applications

Editor’s comment: We will see here how a low-leakage semiconductor switch can save the day and your design. All switches are not created equal!

There is little doubt that wearable and IoT devices have changed our lives. Wearable applications have helped many of us to improve our lifestyles with suggestions of when to move, what tie goes with which shirt, when and how often to exercise, and how much to sleep. In order to provide these functions, wearable devices embed a variety of sensors such as accelerometers, altimeters, light sensors, and heart rate monitors.

However, the physical metrics these sensors are meant to capture vary slowly in time, which is common in the human being or in the natural environment. Consequently, the sensors are in OFF mode for most of the time and only turn on for a very short time to capture data. In addition, when in normal operating conditions, the power consumption of sensors specifically designed for wearable applications is already quite low. So, if designers want to improve the operating lifetime of a wearable device, they need to decrease the current consumption of the sensors when in OFF mode and disconnect all loads from a switching signal such as an I2 C clock and data lines.

Leakage current

Commonly labeled as leakage current or stand-by current, its impact on the overall power consumption of the device is often underestimated. For example, the standby current of a display (display OFF) can be as high as 10μA. By considering a battery capacity of around 30-40mAh, this consumption represents something between 0.02 and 0.05% of a battery’s capacity. It doesn’t seem like that much! But since there are many sensors that contribute to this unwanted current consumption, its impact can cost something like 5% or even 10% of the battery lifetime if not properly controlled.

Of course, the block of major weight and volume in a wearable device is, in fact, the battery. Lower power consumption allows designers either to increase the operating lifetime or to reduce the size of the battery, and therefore of the whole equipment.

So, there is a need to turn these sensors really OFF, or better, to disconnect them from the battery. In order to do this, one of the most effective solutions is to use ultra-low leakage switches between the battery and the supply pin or leaky inputs of the sensors.

When devising the STBC02/03 battery management ICs, ST designers took into account this application need, and together with a Linear Li-Ion Charger, embedded two Single Pole Double Throw (SPDT) switches. These switches have a leakage current of less than 1nA thus dramatically reducing the leakage current consumption of sensors, displays, and other peripherals in wearable and IoT applications. In the STBC02, the switches are driven through a single wire interface while in the STBC03, they are driven by two dedicated logic inputs. In the following sections, some practical examples of use of these switches are shown.

Examples of ultra-low leakage SPDT Switches Uses

1 Eliminate leakage current in Sensors and Heart Rate Module (HRM)

In this scheme, the supply voltage to sensors, BLE network processor and Optical Heart Rate Module (OHRM) is cut. In this way, several microamps of leakage current are saved. In particular, the Optical Heart Rate Module, which embeds green LEDs, can draw high leakage current.

2 Eliminate leakage current of a Boost Converter

A boost converter is typically used to supply the LCD or OLED display and the Optical Heart Rate Module. In battery powered application, the boost converter should have the true shutdown feature, which avoids current path from the battery to the load through the rectifying diode. Even when using such a device, the leakage current through the blocking diode can be as high as several hundreds of nanoamps. By inserting the STBC02/03 switches in the current path, this leakage current is reduced to few nanoamps. Since the current flowing through the boost converter when in the ON state can reach some tens of milliamps, it is a good practice to put more switches in parallel (two in the above case) in order to reduce the path resistance.

3 Eliminate leakage current of the Display

The display is one the leakiest peripherals in a wearable application. An OLED display has a typical leakage current of 1 µA but can reach 10μA. An LCD display is less leaky with a max leakage current of few microamps. In both cases, the leakage current reduction provided by the STBC02/03 switch insertion is very high, bringing this value down to few nanoamps. Also, the interface pins can be disconnected if they have poor leakage performances.

4 Eliminate leakage current on LED driving

It is common to use the MCU I/Os to turn on and off some user interface LEDs. Blue or green LEDs, due to their high forward voltage, are connected directly to the battery or to the 5V rail provided by the boost converter. Some of the best low power MCUs have low leakage I/O pins that exhibit around 10nA of leakage current. This is quite a good value, but still 10 times higher than the one provided by the STBC02/03 switches. In addition, the four switches of the STBC02 are driven through a single wire digital interface—saving three MCU I/Os.

5 Reduce power consumption on I2C Bus

Even when not addressed, the clock and data pins of the sensor are subjected to go high and low at the bus communication frequency. Each pin capacitance on the bus needs to be charged and discharged at each cycle. By considering a bus voltage of 1.8V and an I2 C bus frequency of 400 kHz, each pin contributes with 3.6μA. By multiplying this value to the number of pins connected to the bus, the amount of wasted current can be considerable. The STBC02/03 switches help reduce this wasted current by disconnecting the bus lines to avoid switching the pin capacitances at the bus clock frequency.

Conclusion

It has been shown that, even if leakage or stand-by current consumption coming from different components in the system is quite low, the sum of each contribution determines a significant reduction in the battery lifetime. By implementing specific power distribution architectures able to make sure that ‘off’ is ‘off’, these current consumptions are drastically reduced or even eliminated. The STBC02 and STBC03 battery management ICs from STMicroelectronics, with embedded ultra-low-leakage switches, allow the easy implementation of these architectures.

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