AN-1059 Datasheet, PDF (1/5 Pages) International Rectifier – Application Note

AN-1059
Application Note
Extend Battery Life by Reducing System Power using the EnerChip RTC
Introduction
This Application Note discusses techniques to use the EnerChip
RTC CBC34803 or CBC34813 self-powered Real-Time Clocks (RTC)
to dramatically reduce system power in microcontroller based
applications. This in turn extends battery life significantly. The
internal battery in the EnerChip RTC also serves to backup the time
in case of a main battery swap or main battery complete discharge.
Now batteries can last the life of the product over many years.
Power Reduction Techniques
Typical microcontroller based devices can benefit from the power saving features in many microcontrollers.
These features include ways to shut down portions of the chip such as serial ports and other peripherals when
they are not being used, changing clock frequencies on the fly, or going into a variety of “sleep” modes that can
drastically reduce power by powering down subsystems not currently in use. The most common and generally
most effective method is to put the microcontroller and peripherals into sleep with only a wake-up timer
running. This mode can often reduce the current of the microcontroller to a few microamps or less of current.
Even so, the sleep current integrated over a long time can be a significant power drain. Some microcontrollers
can stop all timers and operations but keep a few registers alive to cut the current to only several tens of
nanoamps but they don’t have a built-in way to wake up since the internal timers are also suspended. The
microcontroller power reduction through sleep is a big benefit but often the sensors or user controls must stay
awake which increases the power usage.
This Application Note suggests methods of using an CBC348xx to utilize the lowest power suspend modes of
microcontrollers and/or to completely shut power down to peripheral chips for periods of time to drastically
reduce power in systems that need to respond to human-speed time delays. These techniques can also be
used to minimize power in slower, environmental sensor applications. The general techniques and results will
be discussed first followed by considerations for systems with higher active power. Lastly, specific registers that
are used to set up the CBC348xx to accomplish such power savings are listed. System power savings of factors
of ten or even one hundred are possible.
Sleep Power versus Active Power
Typical microcontroller-based devices where energy conservation is an issue spend a large part of their time in
a low-power “sleep” state. This is a state where the microcontroller is not running but is waiting for an interrupt
from either a sensor or a timer. When one of these interrupts is issued the microcontroller goes to a higher
power active mode to process the event and then goes back to sleep. The active mode operation may include
processing the sensor data and then may operate an actuator or send a message. Many times a message
may be sent via a low-power radio protocol which requires significant processing cycles to correctly operate the
protocol stack. The amount of processing depends greatly on the complexity of the protocol.
The average power consumption of the system is the sleep power times the percentage of time the system is
asleep plus the active power times the percentage of time the system is active divided by 100.
Pavg = (Psleep * % time asleep + Pactive * % time active)/100
Minimizing the largest of these terms provides the greatest power savings. In some cases the active power
term is much larger than the sleep power term either because the power per event is large or the active power
events happen very often.
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Doc AN-72-1059 Rev A
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