Low power has its limits, here’s why

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All new electronic products share one design goal: low power operation. Many of these products are battery operated, so it makes sense to get the lowest possible current draw. This in turn will provide longer battery life, fewer charges, and an extended period of operation. For large factory IoT networks, this can save money due to reduced maintenance.

Wearable devices like headphones, smart watches, and medical monitors also require long battery life, as do all kinds of electric meters and field sensors. While the device spends most of its life in sleep mode, it requires special design considerations.

It turns out that the main limiting factor is what is called the quiescent current (iQ). I understandQ and how to avoid its problems is explained here.

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Define IQ

IQ has several definitions. A good place to start is the current required to keep the IC or circuit alive with minimal functionality until it is called upon to be turned on. Permanent devices such as remote keyless entry systems on cars are representative of this need. You don’t want it to drain your battery while the car is idling, but you want it to be responsive when you’re ready to drive. Other definitions that help clarify IQ include:

  • The overload current used by the integrated circuit to stay alive without any load or switching operation.
  • Current derived from the input and output pins. A buck converter draws current from the input, while boost and buck-boost converters draw both input and output.
  • IQ Usually has three main parts: the shutdown current of the device, any non-switchable IQ and by changing IQ.
  • Note: IQ is not a zero input current.
  • IQ therefore has different meanings depending on your CI and how it is used. You will need to define it for your use case.
  • Efficiency varies greatly with charging current; the best efficiency is usually with heavy loads.
  • In addition to current to keep critical circuits alive, additions to IQ come from noise, both switching and analog and leaking capacitors, and other components with even a tiny amount through a dirty PCB.
  • Obviously the more IQ, longer battery life. But it is not that simple.

If you need to update on IQ, contact the supplier of the devices you plan to use. For example, Texas Instruments offers a variety of resources, both print and video training.

A solution

the the overall result of the battery life is the function of all the power management part of the product. It usually starts with an MCU which typically sets multiple shutdown modes, such as sleep, hibernation, etc., and how long the MCU stays in each mode. The power system also includes one or more DC-DC regulators or converters. These devices require careful selection.

Texas Instruments offers several integrated circuits that can help achieve the lowest IQ and the best performance. For example, the The TPS61094 is an example with a boost converter with a buck mode (see figure). Low voltage drop (LDO) regulators are also used. With several modes available, it is easier to optimize the circuit for the application. Seek the lowest IQ for each one you use, no more than a few hundred nanoamps. The pins of the IC to focus on are IN, OUT, GND and EN. Check from the datasheet if there are others.

Finally, the total iQ is really the sum of two or more iQs of several chips. Some integrated circuit suppliers have a formula for achieving this amount.

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