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 I2C clock and data lines.
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
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.