ELECTRONICA 2016 SUPPLEMENT COVER STORY You can keep power supply capacitors
small if power supplies are shut down in sleep mode. It takes energy to charge them and if the power supply is shut down when in a sleep mode then the energy in the capacitors is normally wasted. For example, a 1μF capacitor on the power supply of circuitry, which is shut down 100 times per second will consume 165μA at 3.3V (same calculation as before). Many ICs will take less than that in shutdown or sleep mode, so it is often better to keep circuitry powered but in a sleep state than to actually do power switching to save power. The exception to this advice would be if
the device used didn't have a sleep mode or if its sleep mode was not low current. If you can use 100nF instead of 1μF you could save a lot of energy.
LOW SUPPLY VOLTAGE Devices will consume less power at lower voltage even if they don't consume less current. Therefore, if a microcontroller is powered by 1.8V instead of 3.3V, power consumption will be around half for the same current. Usually digital devices will also consume less current at lower voltages, so the power is further reduced. Look out for the maximum clock speed also being reduced - it is not uncommon for the maximum clock speed to be lower at lower voltages. So, while the current will be lower, it will also take twice as long to run the microcontroller's code. For example, a Microchip nanoWatt XLP
PIC24F16KA102 microcontroller running at 2MHz consumes 695μA at 3.3V, but it only requires 363μA at 1.8V, which is 70% less power - a massive saving. However, at 3.3V the microcontroller can run at a maximum speed of 32MHz, while at 1.8V the top speed is 8MHz. Choose the clock speed to suit the
application, not just the fastest possible. This probably won't be known until the code is finished. Most microcontrollers have an adjustable clock multiplier, allowing the clock speed to be changed by the application code. If the microcontroller code is the limiting factor in terms of executing the code and going back to sleep, then the fastest processor may also be the most efficient. However, if something else will actually be slowing things down and the microcontroller will be "marking time", the microcontroller could be woken by a periodic interrupt instead. An alternative approach would be to
dramatically slow down the processor when it is just "waiting" and then speed it back up when there is something to do. Ideally you would sleep instead, but clock startup times and the power wasted while waiting for the clock to stabilise can mean sleep is not the best option in some circumstances.
32-BIT MICROCONTROLLERS Does every design really need a 32-bit processor? They seem to be everywhere but the work actually being done by them can be minimal. A 16-bit or 8-bit microcontroller may be more efficient in some applications. It does depend on what code is running and the efficiency of the compiler. If there is a Bluetooth or Internet interface, then more memory may be required and a 16-bit or 32-bit processor should be used. A TCP/IP stack with a web server also requires a 32-bit processor. Creative software writing can minimise the resources needed and therefore minimise power consumption. Wider data width processors consume
more power in a number of ways. Accessing 32-bit RAM and Flash memory is more power hungry than accessing 16- bit memory. Also, leakage current increases with wider memory. That
/ ELECTRONICS
Figure 4: PIC24F16KA102 family general block diagram
ELECTRONICS | OCTOBER 2016 S11
CLOCK STARTUP Power is wasted while waiting for the system clock to stabilise. If code can be run while the clock is still settling then that can help, but if there is any other part of the circuitry relying on a stable, accurate clock the processor has to wait, resulting in wasted energy. Some microcontroller manufacturers' clocks are quicker to start than others.
Figure 2: Wearables for health & fitness
Figure 3:
ADP5301 functional block diagram
points towards keeping memory size to a minimum both by efficient code structuring and writing, and also not choosing a processor with massive amounts of excess memory.
RF POWER If there is a RF wireless interface, e.g. Bluetooth Smart, consider the transmitting distance. Not only are there power savings by transmitting with lower power, the receiver sections can have adjustable sensitivity and will take less power when sensitivity is set lower. For example, the Nordic Semiconductor nRF52832 has a high receiver sensitivity mode using 10.9mA but normal sensitivity only takes 6.1mA
CUSTOM ICS The ultimate lower power solution might be a full custom IC design with only the required circuitry. Circuitry that never drives off-chip consumes a lot less power. However, it is the slowest and most expensive way of developing a product. It is also probably why a Frederique Constant Smartwatch battery lasts 2 to 3 years and a Fitbit Flex lasts 3 to 5 days. There are a lot of aspects to consider
when designing a low power system. It can be a good idea to create a spreadsheet with all the parts of the system listed with current consumption, required duty cycles, voltages and total power consumed. Then for each design scenario calculate the projected power consumption. The only way to find out some of the information needed is to actually build a prototype and test it.
Mouser Electronics
www.mouser.com T: 01494 467490
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