TI Technical Article: Do all voltage rails need low quiescent current?

All designers of ultra-low-power systems are concerned about battery life. How often does a fitness tracker battery need to be recharged? And for a disposable battery system, how often does a technician need to service a smart meter or replace the battery? Obviously, the goal is to maximize battery life. For a fitness tracker, a week of battery life is ideal, while a smart meter could last 20 years or more. What factors need to be considered when designing each subsystem to achieve this life?

Many systems have one or two voltage rails enabled, which power the system microcontroller (MCU), key sensors, or communication buses. These always-on voltage rails must be highly efficient to extend battery life. Optimally designed subsystems minimize the current consumed by each always-on subsystem—often less than 10µA or even 1µA in total. As I've discussed in my technical articles , achieving ultra-low power requires optimizing each of these subsystems. Low current consumption by the voltage rails means ultra-low power supply quiescent current (IQ), such as the 60 nA IQ TPS62840 .

You might think that minimizing the current consumption of each power supply during operation is the most important thing. Reducing IQ improves efficiency, which in turn reduces battery power consumption and extends battery life. But can efficiency always be improved significantly? For systems that operate at relatively high load currents, such as displays and some sensors, the answer is clearly no; the output power far exceeds the current consumption IQ. For example, if a fitness tracker's display draws 12 V at 5 mA (60 mW total power), then the 100 µA IQ draw from a 3.6 V battery (0.36 mW total power) is insignificant.

More important for these types of subsystems is the power consumption when disabled. To save power, ultra-low power systems shut down power-hungry subsystems most of the time. Therefore, shutdown current is also very important to the life of the system battery. This current, often called leakage, can be very large, so a load switch must be added to disconnect the power supply to the subsystem and further reduce the shutdown current. The TPS62748 high-efficiency step-down converter provides the load switch and ultra-low quiescent current IQ of 360 nA for these systems.

When a load switch is not used, if there is a connection path between the device and the load, both the leakage current into the device itself and the load conditions must be considered. This is a common situation with boost converters, so special circuits are sometimes added to disconnect these paths, such as the isolation switch in the TPS61046 boost converter. In other cases, this path is specially optimized to allow bypass operation - powering the load with a shutdown current consumption of less than 50nA in a disabled device.

Choosing the right type of device for a particular subsystem—ultra-low quiescent current IQ or ultra-low shutdown current—is very important. These nuances are common in every ultra-low power system from wearables to smart meters to medical devices. So it’s important to carefully consider the application requirements before choosing the best solution.