Recommended Sleep Current Measurements
Proper measurement of sleep current is often a needed step to accurately estimate battery life requirements. Special steps need to happen in order to ensure proper measurements are taken without upsetting the normal operation of the unit under test.
Measuring sleep currents can often cause problems with units because of the equipment used to measure very low currents accurately requires a large resistor in series with the power supply. This large resistor starves current from the unit during a momentary wake cycle forcing the voltage to droop to brownout levels very quickly often times causing the unit to achieve a bad state sometimes requiring a reset to resolve the problem. In order to achieve the lowest sleep current, Brown out detectors must be disabled during sleep modes. Even if the measurement equipment automatically changes current ranges, it is often too slow and cannot keep up with the sudden short bursts needed. During long cyclic sleep periods the units will often wake every 10 to 30 seconds momentarily to reset timers and perform other needed steps. These wake times are small and often not noticed when trying to measure sleep currents.
In order to compensate for the switching time of the equipment an additional large cap needs to be temporarily added when measurements are needed to allow for short pulses of current draw (See schematic below for details). 100uF is enough to handle 1.5 milliseconds with 20mA of current. The capacitor can be increased or decreased based on the switching time of the measurement circuitry and the momentary on time of the unit. The leakage current of the additional cap should be measured so to verify that it does not skew the low current reading. Note that the spike in current draw will be averaged by the capacitor. The actual magnitude of the current spike will no longer be visible, but the total energy consumed is still accounted for by integrating the current over time and multiplying by the voltage.
Internal pin pull-ups can also pull excess current and cause the sleep current readings to be higher than desired when driven or floating. Ensure that all pull-ups are disabled for input lines that have a low driven state during sleep. Ensure that pull-ups are enabled for floating lines/inputs that are not connected to other circuitry. It may not be possible to stop all leakage current on pins that contains an analog voltage where an ADC is used to read it unless the voltage can be disconnected during sleep. Each floating input not at a valid high or low level may cause some leakage dependent on temperature and charge buildup that may not necessarily be observed at room temperature.
Aug 23, 2017