When trying to reduce power consumption at the board level, the first thing to do is to reduce the current consumption (both active and leakage) of the board's components. Many designers try to achieve this goal by reducing the number of components outside the microcontroller. External analog parts are very common in embedded devices, and developers should know their analog parts well and treat them with particular care during design, especially in defining early-on how they will be powered in standby mode.
To keep external components to those that are strictly essential, designers should typically choose microcontrollers that integrate as many needed peripherals as possible. This is true of analog components, as well. If ADC resolution constraints are not too stringent, for instance, it is generally preferable from the current consumption point of view to choose a microcontroller with an internal ADC. Integration in the MCU enables better device management in active and standby mode.
Integrated microcontroller analog peripherals such as comparators, ADCs, and DACs should be analyzed carefully, though, and their external support components chosen with an eye toward maximally quiescent current values and measurement conditions. It is not uncommon that the current consumption of an analog comparator integrated in a microcontroller falls in the 20 to 30 μA range. This is not a problem when the device is operating in active mode. But compared with the 20 to 500 nA needed by a microcontroller in standby mode, this is really too much.
So, you'll want integrated analog peripherals that can be turned off when not being used. If the MCU you want to use can't have peripherals turned off, it may be better to avoid integrated analog and use an external, very low-power device. The Linear Technology LTC1540 comparator, for instance, needs an ultra-low quiescent current of only 300 nA. Care should be taken, though, to be sure that the way you shut the device down won't affect your measurements.
When even external device current demands are not low enough, partitioning the board's power lines into different rails should be a viable solution. In fact, the best way to cancel the power consumption of an electronic component is to turn it off. Many integrated circuits provide a shutdown pin that a microcontroller's I/O pin can control to help reduce the board's current consumption to a bare minimum through software management of the device's operating mode.
Other components haven't a shutdown pin, or they have a standby current too high for the design's energy budget. In these cases, the only possibility for implementing software power control is to design a sectioned power distribution system with the MCU controlling low power and low dropout switches for each section. MOSFETs are good candidates as power rail switches, as is the recently introduced ON Semiconductor NCP380 fixed/adjustable, current-limited, high-side power distribution switch.
In addition to active devices, another potential source of current leakage in standby mode is capacitors -- especially those over 0.47 μF. But even small capacitors can be a problem if not properly selected. Both in power supplies and analog circuits there are many capacitors that, put all together, can form a big source of leakage current.
A capacitor's leakage current is not constant, though, because it is influenced by the charging current, temperature, and the applied voltage. Leakage current increases linearly with temperature and voltage increase. The reason for these leakage currents is that dielectrics do not have infinite resistivity. The charging leakage current decreases logarithmically with time, arising from dielectric charge absorption (which seems to be affected by geometric factors) at work.
The choice of capacitor can do much to minimize this problem. For instance a 1000 μF, 35V aluminum electrolytic capacitor with non-solid electrolyte has a leakage current of 350 μA after two minutes that decreases to 70 μA after five minutes. The values for a tantalum electrolytic capacitor are 200 μA after two minutes and 45 μA after five minutes. These values will continue to decrease over longer periods, but it is clear that they should be considered an unwanted and hidden problem in low-power designs.
For MLCC ceramic capacitors the leakage current is in the order of one nA or less. So, for low-power applications ceramic capacitors should be the natural choice as long as they are available in the capacitance and voltages your design needs. Low-leakage tantalum capacitors should be used when higher capacitance values are needed.
In the next part I will start to analyze optimizations at microcontroller level.
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