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Power/Alternative Energy??

Maximizing a portable's battery

Posted: 01 Sep 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Juan Alvarez? Texas Instruments? TI? battery? factory-installed battery?

Factory-installed batteries in products such as smoke detectors, security devices and thermostats are required to last for more than 10 years. Hence, longer battery life has become a critical component of a portable application's design.

To maximize battery life, the designer needs to minimize the average current consumption of the system. Using multiple low-power operating modes and selecting the right components are critical to keep the average current consumption as low as possible.

Multiple low-power operating modes include shutdown and real-time-clock standby operation. Main components include the mcu, power supply and signal chain. Selecting the best MCU for the application has become more challenging as variations among MCUs have grown in terms of cost, peripheral design and mix, cpu architecture and level of on-board integration.

For a portable industrial-measurement application, placing priority on the critical application requirements of long battery life, high-performance analog peripherals and adequate user interface will yield the best MCU choice for the application.

Here are some tips for designers to achieve the longest battery life possible.

Do

  • Achieve the lowest current consumption in standby mode. Many portable applications spend more than 99 percent of their time with the CPU idle. Current consumption will gravitate toward idle mode, which can either be turned off, wait for an external interrupt to wake up or run a real-time clock operation using a timer.

  • Use MCUs with a fast wake-up time. Active mode has the highest current consumption. The CPU is wasting active-mode current consumption as an MCU is waking upgoing from an idle to an active state. The designer needs to consider MCUs that wake up as fast as possible and can execute code as soon as possible. Waking time is that period from an interrupt to a stable and fast clock. An MSP430F20x1 wakes from an interrupt in less than 200ns.

  • Use low-power brownout-reset (BOR) protection. Any portable application requires BOR or low-voltage detection to ensure that the system will reset itself if the supply voltage drops below the specification. Many MCU vendors provide a brownout-protection feature, adding between 20A and 70A of current consumption. A designer will want to use a low-power BOR, since this function must be "on" all the time. For example, the MSP430 16bit MCU has a "zero" power brownout reset protection.

  • Integrate as much as possible. Some MCUs incorporate functionality that you can eliminate from the board, including high-resolution ADCs, op amps and 12bit DACs. Integrating these functions enables register communication instead of slow serial communication that will add current consumption. Every component added to the board will increase leakage current.

Don't

  • Use the cover sheet of a spec to select an MCU. A designer must read data sheets carefully, looking at worst-case operating temperature, maximum specified values and operating supply voltages.

  • Assume that operating modes are comparable across different MCU vendors. Each vendor will specify current-consumption values differently. For example, an MCU vendor may not include all the functions that are active on a given operating condition.

  • Use multiple power supplies. Some designs require multiple or complex power supplies. A low-dropout regulator or a boost converter can be expensive. It adds cost and increases the average current consumption, because the power supply will most typically be on all the time. Consider a 3v supply.

  • Use polling. Some MCUs recommend an infinite loop so you can cycle through all the peripherals you are monitoring. This technique is inefficient, since it takes CPU overhead and significant current consumption. Consider MCUs that have good interrupt- capability support. For example, some low-power MCUs have an interrupt vector for up to two 8bit ports, while others support only a few I/Os.

  • Use the CPU. Read the user's guides carefully to be sure you benefit from all the features an MCU has to offer. Remember that every time you execute a useless line of code, you are draining your battery. A low-power MCU will include features in hardware to keep the CPU off while peripherals are executing specific tasks that don't require any processing. Auto-scan is an example of this. An ADC can automatically scan various channels and store the values in a temporary buffer, flash or RAM without CPU intervention.

- Juan Alvarez
Manager
MSP430 Microcontroller
Texas Instruments Inc.




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