voltage rails and low quiescent current

What is a voltage rail? Does it require low quiescent current? All designers of ultra-low power systems are very concerned about battery life. How often do fitness tracker batteries need to be charged? And for disposable battery systems, how often do technicians need to maintain smart meters or replace batteries? Obviously, the design goal is to extend battery life as long as possible. For a fitness tracker, a battery that lasts for a week is ideal, while a smart meter can last 20 years or more. To achieve this endurance, what factors need to be considered when designing each subsystem?

Many systems often enable one or two voltage rails that power the system microcontroller (MCU), critical sensors, or the communications bus. These normally-on voltage rails must maintain high efficiency to extend battery life. Optimally designed subsystems minimize the current consumed by each normally-open subsystem—the total current is typically less than 10µA or even 1µA. As I said in the technical article, to achieve ultra-low power consumption, these subsystems need to be optimized. The low current consumption of the voltage rail means the supply quiescent current (IQ) is ultra-low, such as 60 nA for the IQTPS62840.

You may feel that it is most important to minimize the current consumption of each power supply during operation. Lowering IQ can improve efficiency, so reducing battery power consumption can also extend battery life. But can efficiency ever be significantly improved? 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 referred to as leakage, can be so large that a load switch must be added to disconnect power from the subsystem, further reducing shutdown current. The TPS62748 high-efficiency buck converter provides load switching 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 leakage current into the device itself and the load must be considered. This is a common situation when using 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 specifically optimized to allow bypass operation—powering the load with less than 50nA shutdown current consumption in the disabled device.

It is very important to select the appropriate device type for a specific subsystem - ultra-low quiescent current IQ or ultra-low shutdown current. These nuances are common in every ultra-low-power system from wearables to smart meters to medical devices. So the application requirements need to be carefully considered before choosing the best solution. The above is the relationship between voltage rail and low quiescent current requirements. I hope it can help you.