Specifications

BMA180
Preliminary data sheet
Bosch Sensortec
Rev. 1.0 Page 17 / - proprietary information - 06 March 2009
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Specifications within this document are preliminary and subject to change without notice. Document is not intended for publication.
b) In small bandwidth applications, sleep mode could be used to save a significant
amount of power by frequently changing between sleep and standard mode (see
next section).
4.2.2 Current consumption using duty cycling
For most of the applications a sensor must not stay permanently in normal mode. This allows a
significant reduction of current consumption by switching periodically between sleep and normal
mode. Of course, settling times, etc. have to be considered to keep the application running.
Following 2 examples are giving rough indications of current consumption by duty cycling
(depending on power mode and selected bandwidth). Examples are with respect to low
frequency applications.
Formulas:
- average current = average (sleep mode current; normal mode current)
- measurement time = time from sleep to normal mode + settling time of filter +
read-out time +time from normal to sleep mode
Example 1:
- sleep mode current = 1 µA, sleep time is 200 ms, filter is 150Hz
- current mode is low power mode (-> bw = bw_selected/2)
- start-up time from sleep is 2.5 ms
- read-out time is roughly 0.5 ms (ADC conversion time is 0.417ms)
Î output filter is 75 Hz -> settling time is 6 * 1/75 sec = 6/75sec = 80ms
Î overall measurement time = 2.5ms + 80ms + 2.5 ms + 0.5 ms = 85.5 ms
Î current = (200ms * 1µA + 85.5ms * 575µA )/285.5ms = 173µA.
Î result: approx. 70% decrease in supply current and power consumption
Example 2:
- sleep mode current = 1µA, sleep time is 500ms, filter is 1200Hz
- current mode is low-noise mode
- start-up time from sleep is 2.5ms
- read-out time is roughly 0.5 ms (ADC conversion time is 0.417ms)
Î output filter is 1200Hz -> settling time is 6 * 1/1200sec = 6/1200sec = 5ms
Î overall measurement time = 2.5ms + 5ms + 2.5ms + 0.5ms = 10.5ms
Î current = (500ms * 1µA + 10.5ms * 850µA )/510.5 ms = 18.5µA.
Î result: approx. 98% decrease in supply current and power consumption
Remark:
Sometimes it is necessary to perform a soft-reset after changing certain parameters (e. g.
mode-selection by changing mode_config bits). In case of a soft-reset, it is recommended to do
this reset after having switched from sleep to operational mode. In this case the total typical
wake-up and reset time at maximum bandwidth is much smaller than in case the soft-reset is
activated during sleep mode.