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Design Articles

The Benefits of Single-Cell MCU Operation

By combining efficient and optimized power components with an MCU device, it is now possible to have a capable, ultra-low power ‘system on chip’ that can run from a single battery cell all the way down to 0.9 Volts.

Steve Diaper – Field Applications Engineer, Silicon Labs

Until recently, even the lowest voltage, lowest power MCUs on the market required a minimum supply of 1.8 Volts to operate – requiring at least two alkaline batteries in series for battery operation. However, a new MCU family now offers a minimum operating voltage of just 0.9 Volts – the end of life voltage of a single alkaline battery.

By running from a single battery, you could, for example, within a similar form-factor, replace two smaller cells with a larger single cell battery – while increasing product battery life. Alternatively, you could take the two existing batteries and connect them in parallel rather than series, again increasing product battery life by a worthwhile amount. Such parallel battery connections do require a mechanical means to prevent reverse connection of the two cells, but are otherwise a good way to maximize battery lifetime.

Another possibility would be to remove one battery, thereby making your product both smaller and cheaper. You might imagine that removing a battery would reduce the battery life of your product by half. However, as we shall see shortly, that need not be the case

Operating from a Single-Cell

It is, of course, possible to create a system using a standalone DC-DC boost convertor, which can run a standard MCU from 0.9 Volts, by boosting the input voltage to provide a 1.8 Volt or greater supply. However, this standalone approach has a number of limitations within a battery-powered embedded system. In order to minimize current consumption, it is desirable to disable the DC-DC convertor when it is not needed. However, if the DC-DC convertor is turned off, then the MCU will be without a supply voltage, and will be unable to maintain a real time clock, or restart the system without an external input of some kind. Worse, the MCU will lose its entire RAM contents whenever the DC-DC is disabled. However, without disabling the DC-DC convertor, then the standby current consumption of the system, even with the MCU in sleep mode, can remain high, often greater than 20uA.

Aside from this, there is the active efficiency of the DC-DC convertor and MCU to consider. Most standalone DC-DC solutions are designed to be most efficient when delivering at least 150mW (and in most cases considerably more) to the load – and are a lot less efficient at lighter loads. By contrast, a typical MCU device will draw less than 30mW from the supply – which can result in a surprisingly low DC-DC efficiency in the range of 50-70%.

So, is there another, more efficient solution? What if you were to integrate an optimized, low power DC-DC convertor on to the same silicon as the MCU? This could readily reduce system cost, and board space. If you also included the ability to retain the RAM contents and run a real-time clock using the low input voltage, down to 0.9 Volts, then the MCU could have control of its own power system. If you were also to optimize the standard MCU peripherals and functions, including standby mode, wake up from sleep and fast code execution, for the lowest possible leakage current and power consumption, then the device could support single-cell operation, while still having a battery lifetime comparable with a dual-cell implementation.

figure 1

The Advantage of an Integrated Solution

This describes the approach that has been adopted by Silicon Labs for their recently introduced C8051F9xx MCU family. In detail, the device integrates a highly optimized step-up DC-DC convertor into the MCU which can boost the incoming battery voltage, between 0.9 and 1.5V, up to a programmable output voltage of between 1.8 and 3.3V. This boosted voltage is then used for the I/O pins and analog peripherals of the MCU. As shown in Figure 1, by using an optimized 65mW DC-DC convertor, the convertor efficiency can remain as high as 80 to 90%.

Not only that, but since this DC-DC convertor is capable of supplying a total output of 65mW, the boosted output voltage can also be used to provide a voltage supply for external components. In this way, some of the potential problems with interfacing to other higher voltage ICs or sensors, driving blue LED’s at 3 Volts, or even providing sufficient voltage to drive an LCD or OLED display, can be avoided.

To further improve system efficiency, the MCU core and digital peripherals of the new family run from an internally regulated 1.7V supply, consuming only 170uA/MHz at speeds of up to 25MIPS. Figure 2 gives a simple overview of the power architecture within this new MCU family.

figure 2

Functional Efficiency

Providing an efficient, integrated power-supply system is not, of course, the whole story. The different operating modes and switching times, and the analog, digital and communications peripherals can all have an effect on the overall power consumption of the system.

The most obvious datasheet parameters of a low-power MCU include the standby and active-mode power consumption figures. Manufacturers often, as above, quote a figure of milliamps per megahertz (mA/MHz) to account for different clock speeds used with the device.

In relation to this, when looking at active power consumption, it is counter-intuitive, but often true, that a higher active clock speed is more efficient in terms of average power consumption than running an MCU at a much lower speed. A CMOS processor is typically much more efficient at the faster end of its operating capability, and can then spend more time in a low-power standby or shutdown mode.

For the same reason, a well-designed, fast ADC can also provide efficient system measurements. The speed of the ADC in a given system may, however, be restricted by higher input impedances that require longer acquisition times. In addition, for consistent ADC results in a battery-powered system, it is common to use a separate reference voltage, and this is sometimes integrated into the MCU. However, the efficiency of the system can be compromised if a fast ADC, capable of giving a result in a few microseconds, has to wait several milliseconds for the voltage reference to stabilize.

The ADC and Voltage Reference modules used on the new devices from Silicon Labs offers some of the shortest wake-up and processing times on the market. The high-speed internal voltage reference is stable within 1.7usec – and is therefore ready as soon as the MCU wakes-up - allowing the 300ksps 10-bit ADC to begin conversion immediately.

It is common within mixed-signal MCUs for the relatively simple analogue comparators to be interrupt-driven, capable of waking the device and operating somewhat independent of the processor core. However, further power efficiencies can be realized by adding some degree of ‘autonomous’ operation to the ADC module as well.

The latest Silicon Labs ADC module supports both a burst-mode – performing a series of 16 conversions and automatically accumulating the result without MCU intervention – and a window-comparator mode – only interrupting the MCU when the results are within a particular ‘window’ of values of interest - as well as offering the ability to synchronize to the ‘quietest’ part of the DC-DC convertor’s operating cycle.

Alkaline is not the only Battery

figure 3
figure 4

Several single-cell battery chemistries are suitable to provide an input voltage between 1.5 and 0.9 Volts for the integrated DC-DC convertor of these MCUs. This includes almost all AA and AAA style batteries – Alkaline, NiMH, NiCd and Lithium primary among them – as well as Zinc-Air and Silver Oxide ‘button cells’.

However, for other battery types, the nominal battery output is higher - such as Lithium ‘coin cells’ with a voltage of between 3.0 and 2.0 Volts. In addition, there may be other reasons for using a higher supply voltage. Such applications can still take advantage of  the ultra-low power consumption and efficiency, by configuring the device in a ‘two-cell’ mode. Referring again to Figure 2, you can see that the DC-DC convertor can be disabled completely, allowing the MCU to support an input voltage of between 1.8 and 3.6 Volts.

Estimating System Battery Life

To allow designers to readily estimate the battery life of a new design– a task commonly undertaken using a complex spreadsheet - Silicon Labs offers a simple, freely downloadable PC utility – the ‘Battery Life Estimator’.

For any system or application, when given the designer’s choice of battery type, and the ‘Discharge Profile’ - a number of basic power consumption parameters as shown in Figure 3, the software compares single, dual series, and dual parallel battery configurations in terms of total battery life – taking into account self-discharge and shelf-life effects. The output of the software is a graph showing battery voltage against time, and a numerical estimate of battery life as shown in Figure 4.

By using and modifying the saved Discharge Profiles with measured or estimated values, designers can evaluate the long term effect of different system features with their choice of battery configuration, and even compare competitive MCU solutions.

Summary

By combining efficient and optimized power components with an MCU device, it is now possible to have a capable, ultra-low power ‘system on chip’ that can run from a single battery cell all the way down to 0.9 Volts.

Silicon Labs’ new C8051F9xx family is unique in the general purpose MCU market in being able to operate in this way. It does this while supporting full-speed 25MHz processing, unrestricted operation of the 300ksps ADC, and even re-writing of the Flash program memory of the device. Despite offering all these features, as well as up to 64KB of on-board Flash program memory, and 4KB of RAM, these devices come in packages as small as 4 x 4mm.

For more information about single-cell MCU operation, see www.silabs.com/point9.

Silicon Laboratories
Austin, TX
(512) 416-8500
www.silabs.com


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