My last post dealt with designing your product to get long battery life. The title implied it was about primary batteries but in reality most of what I wrote would apply to get the best efficiency from any power source. This week I’m going to continue in a similar vein but with considerations for using rechargeable batteries.
Myths and legends are usually the subject of incredible beauty, unsurpassed strength or attempting what mere mortals assume is an unobtainable goal. It is often difficult to separate myth from reality or even know where one ends and the other begins. The same can be said for much of what you read these days about products with a 10 year battery life. When you see advertisements for wireless devices claiming 10 year battery life from a coin-cell battery it is easy to think that is the stuff of myths and legends. Yet, given the right hardware/firmware design and the appropriate battery technology and capacity for a certain application, 10 year battery life is certainly a doable thing.
You may not expect to read about multiple amp loads in the context of “Low Power Design”. It could be an indication of how wide-spread the push for energy efficient products has become. In many embedded applications it is quite common for a micro drawing a few milliamps to control motors or solenoids that require several amps of current or high-brightness LEDs that draw several hundred milliamps. Even if these current ranges are way above what your design has to deal with, the information presented here may be applicable to your design.
Several of my recent posts have mentioned the very negative impact of heat on power consumption. This is the first of a two part series of posts on thermal management for low power devices. This information is mostly taken from my "Low Power Design" PDF e-book.
As semiconductor geometries have shrunk, in recent years leakage current has become a significant component of the overall power consumed by ICs. As parts heat up, their leakage current typically increases. It is not uncommon for parts to consume twice as much current at their highest rate temperature than at their lowest. For example, the AD8226 op-amp is rated for -40°C to 125°C. The quiescent current ranges from 325uA at -40°C to 425uA at 25°C to 600uA at 125°C. This is nearly a 100% increase across the temperature span and nearly a 50% increase from “room temperature” to the maximum temperature. You should conduct your current measurements at the temperature your product will normally operate at if not at the temperature extremes too.
This post will focus on “current leaks”, the points of power consumption that are often designed into a device without much thought. In some cases they are things you have some amount of control over, in other cases they are things you have little control over but have to account for in your power budget in order to get an accurate power consumption or battery life estimate. While technically not leaks as in leakage current, I call them leaks because they are where your battery life drains away.
Last week was incredibly busy so I wasn’t able to put the time in to complete the third part of the “Leakage currents & current leaks” post. This will be a short post with a link to a white paper on our website for more details.
Most engineers consider the oscilloscope their first tool of choice for hardware development work. Yet very few engineers ever consider how accurate their scope is. Most of the major oscilloscope manufacturers place great importance on the timing aspects of their products. Multi-gigahertz sample rates are fairly common today in mid-range digital scopes today yet most of those scopes only have 8-bit A/D converters. While timing accuracy is often spec’d in double-digit ppm, voltage measurement error on the same scope can be as much as the signal level you need to measure using a scope’s lowest volts/division setting.
Virtually all semiconductor devices have some amount of leakage current. It is interesting to note as operating voltages and device power consumption keep dropping, leakage current is becoming a larger percentage of a device’s power consumption. In most cases there isn’t much you can do about leakage currents other than be aware of them and account for them in your power analysis. In some cases there may be a significant difference in leakage current levels from manufacturer to manufacturer for devices that perform the same function so it pays to take the time to include leakage current comparison in your component selection. For a CMOS device that isn’t actively being clocked, leakage current can make up a significant part of its power consumption that may be called out as “standby current” or “quiescent state current” in the datasheet.
Accounting for all of the obvious points of power consumption and their current levels for active and sleep states can be a difficult task. In every design there are also points of power consumption that are often overlooked. Every type of semiconductor device has some amount of “leakage current” that may or may not be called out in its datasheet. Leakage current can make up the majority of the deep-sleep current draw for a modern micro when it shuts off power to most of its internal circuits. There may also be aspects of your design that can “leak” microamps to milliamps of current that you don’t take into account.
The CMicrotek µCP100/µCP120 are ideal for engineers developing wearable/portable devices, wireless "IoT" devices and energy harvesting powered devices. The µCP100 features a current range from 5nA to 100mA, the µCP120 is targeted at higher power applications such as WiFi with its 50nA to 800mA range. With a voltage range from millivolts up to 20VDC they can be used with all of the common logic power supply voltages and small battery technologies. The µCP100/µCP120 work with any scope with a standard BNC probe connector or can be used as a pre-amplifier for a data acquisition system for longer term tests such as voltage/temperature range testing.
It's been a while since I posted last but I plan to start doing that regularly again. We are now offering the “Low Power Design” PDF e-book for free on our website. I'll continue to cover low power design here along with relevant industry news and some occasional CMicrotek news.
So, about the e-book. Most of the information on the web about low power design focuses on hardware design (and mostly the same basic information repeated over and over) yet the firmware will often determine whether a product meets its power targets or not. Our e-book offers tips and techniques for both low power hardware and firmware design. Much of the information contained within the e-book hasn’t been published and is based on over 20 years of experience in developing battery powered devices and efficient firmware for high performance products. The “Low Power Design” e-book provides a good introduction to the many issues encountered in designing low power devices for engineers new to low power design while including enough in-depth discussion that more experienced engineers can find it useful too.
It’s been a few crazy busy months at work but I should be getting back on track with regular postings next week. This week I just have a few things that might be of interest ...
First, my company started a crowdfunding campaign on Indiegogo last week. We have working prototypes of our µPower Analyzer and µCurrent Probe and need help to ramp up production of the µCurrent Probe. We’re offering pre-orders on both products at a 20% discount. We also have a low-cost version of the µCurrent Probe for the Maker community planned for later this year but if we can pre-sell the first production run of 75 units (with a 30% discount!) we’ll move that up for production in the June. The link is below, please help us spread the word about this.
I was introduced to a company a few months ago that has some super low current real time clocks. Ambiq Micro in Austin, TX has a family of RTCs (with and without power management features) with 7X lower current than the industry standard RTCs. They use a technology developed at the University of Michigan called “Subthreshold Power Optimized Technology” that allows their RTCs to operate as low as 14nA. Ambiq has announced they are working on an ARM Cortex M0 micro utilizing this technology to be sampled later this year (ARM is one of their investors).
I came across these related articles this past week. They have some good numbers showing just how much of an impact firmware/software can have on power consumption. The first article shows that encrypted storage uses about 6X more power than unencrypted storage and in one test the encryption software accounted for 42% of the CPU utilization. The second article indicated that the Flash memory used for data storage only used 1% of the power required for storage operations - the OS, file system and encryption code accounted for the other 99%.
This week I covered several approaches to make large switch statements execute faster and save power. Switch statements are a great way to help keep your code readable but it is easy to forget they can turn into a long sequence of if-then-else statements.
This week I'm continuing with some of low power firmware concepts. It may seem counterintuitive but some of the concepts I've used for low power firmware are the same things I did for the firmware on high performance disk controllers. To a large degree, whatever you can do to make your firmware run quicker will also make it more power efficient.