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Low Voltages & Fast Transients Can Cause Problems

Don't postpone the system power engineering in your embedded design. In the old days you could wait until the hardware was working and the code written. Then you slapped a 5 volt supply on the product and shipped. These days power engineering is a critical part of your design task. You need to consider the power system before you even pick an MCU (microcontroller unit).

Low-power systems are a specialty. The 2013 Design West conference has an entire track dedicated to low-power design. The low voltage supply rails of such systems are a challenge. You have to insure that the power supply chips you use can create and maintain these low voltages. The lower voltages also mean less noise immunity for all your digital circuits. So your power system has to make both low voltages and maintain low noise. You may need to design in filters or add a low dropout linear regulator after the main switching regulator. Once you have the solid power hardware you can look at writing code that minimizes power consumption and takes advantage of the shutdown features of the various chips in your system.

Those shutdown features make a whole new problem for your power system. It must be able to deliver power over a wide range of loads since you may be shutting down most or all of your system from time to time. Then when your code fires up the hardware, you have to make sure the power system does not droop due to this transient load application. So now you need a low-voltage, low-noise, wide-load-range power system with good transient response.

Some of your low-power embedded designs might take advantage of energy harvesting. You will want to check out Todd Baker's talk about “Energy Harvesting for Microcontrollers” at Design West. More and more hardware is available to do the power creation from “free” energy sources such as heat, vibration, or light. You will want to keep up on the developments since you may well be called on to design such a system, or it might be the basis for your next patented design.

If you have operated exclusively in the digital or software domain, be sure to check out the tech fundamentals track at Design West. There are sessions for “Analog for Digital Engineers” as well as “Fundamentals of LEDs.” Driving LEDs has different power considerations than other loads, and you will want to get up on the basics in this course. The analog course might give you understanding as to why some chips need a negative voltage to give you best results.

And speaking of extra power rails to get best performance, you need to understand why FPGAs (field programmable gate arrays) and other digital systems may need a multiplicity of power rails that your system has to turn on and off in a specific order with tight timing. UBM's Max Maxfield has a Design West course on FPGAs and programmable logic that will give you the basis for designing systems with these powerful chips. You may also want to check out the “FPGA Design for Embedded Systems” course.

Other embedded power considerations are literally life saving. Check out the “Designing Embedded Systems That Do Not Damage Humans” course. It only takes a few milliamperes to stop a human heart. Be sure to learn how good power and system design keeps these safety considerations paramount.

Another key requirement for embedded systems is USB connectivity. Don't forget the USB standard requires you supply 5 volt power. Sometimes that requirement is a key factor in your power system design. Embedded systems with multiple voltages are always a complex challenge. You might create an intermediate bus from the battery and then buck and boost to get all the system voltages that you need. It is easier on the downstream power chips to have a stable input voltage of the intermediate bus. But this also causes an efficiency penalty. Maybe you are better making all the voltages needed by feeding the battery voltage directly to the power chips. Often, a combination of techniques is required, especially if your analog circuits need a negative voltage, your USB needs +5V, and your FPGA needs a handful of power rails to get the best performance.

Be sure to learn as much as you can at Design West, since power systems for embedded designs are only getting more and more complex. When you register for the conference, you will have access to the proceedings afterwards, so you can read up on some of the sessions you could not attend in person. Keep analog and power considerations in mind, and I will see you at the 2013 Design West conference, where we can trade stories of embedded systems gone wrong due to last-minute power design.

To register for Design West, go here.

4 comments on “Low Voltages & Fast Transients Can Cause Problems

  1. TheMeasurementBlues
    April 3, 2013

    It's one thing to supply 5V for USB, but keep in mind the fifth pin on a mini-USB and micro-USB connectors. In some systems, you need to ties that pin to ground or pull it up to 5V to get your device to charge.

  2. Brad Albing
    April 3, 2013

    Paul – thanks for some good info on power supply considerations with respect to the whole project. And for some good resources to learn more (i.e., Design West). I looked at their various tracks and spotted another one that looks pretty good -Analog Interfaces for Low Power Design:

    http://www.ubmdesign.com/sanjose/schedule-builder/session-id/39

  3. Brad Albing
    April 3, 2013

    Oh – just one more of a more general nature, but still good for anyone doing analog design, Hands-On Analog Basics – Beginner Knowledge and Veteran Refresher:

    http://www.ubmdesign.com/sanjose/schedule-builder/session-id/188

  4. Brad Albing
    April 4, 2013

    Scott – you're right. In that situation, your job (as the engineer) is to get the problem solved and get the job done – not to be popular. But if we wanted to be popular, we'd be on TV.

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