Software engineers can be quite impressive from a power management perspective. With each passing year and each improved version of product (such as a wearable or smart meter), the operating code is further refined to make the best use of every coulomb in the battery. Algorithms are optimized for power, dynamic voltage scaling (DVS) is used to power the processor with just enough voltage, and ultra-low power modes are invented to reduce standby power consumption to sub-µA levels. What an amazing job you do!
But what if all that effort and all those late nights you put in to optimize your code or algorithm didn’t make a significant improvement in battery run time? What if your new MSP430F59xx FRAM with EnergyTrace++™ microcontrollers don’t improve your power consumption at all? (After all, it does draw a mere 450 nA in standby.) You might say something like, “But I know this portion of code uses less power! I measured it myself! Mr. Power Supply Engineer, why is your battery not lasting any longer?”
The problem may be that you have simply reached the limits of the current power supply architecture, perhaps a TPS62231. You are operating at such a low power level, that you have reached the point of diminishing returns. No longer does efficiency improvement in your code result in a corresponding improvement in battery run time. At this power level, reductions in output power move the power supply’s operating point to a much less efficient level. So, the input power consumed is only slightly reduced. Thus, the battery is drained at almost the same rate.
Before you throw out your momentous code improvements and resign your product to being just as good as the previous version (but not any better), you might give this tip to your power supply engineer: go find an ultra-low power optimized power supply, such as the TPS62740, and redesign your power supply. Recent advances in power management have produced power save mode techniques and quiescent current (IQ) reductions that combine to produce some very non-traditional efficiency numbers, such as: 80% efficiency at 10 µA of load current and 50% efficiency at 1 µA of load current. These are truly quite high values for such low output powers. In fact, when your MSP430F59xx MCUs are in standby, drawing just 450 nA, the current drawn from your lithium ion cell will be only slightly higher—perhaps less than 750 nA, depending on the state of charge of that lithium ion cell.
Now, the power supply engineer and software developer can rejoice together as they combine to minimize the power draw of portable devices. Perhaps unfortunately for you, the software engineer, these new and improved ultra-low power optimized power supplies and microcontrollers mean you have more work to do on the next product’s code revision!