Figure 2: Sensor sampling with sleep mode
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Selecting a low report rate and a longer sleep time are the keys to reducing the average power consumption. Report rate and sleep mode directly affect the capacitive sensing controller’s average current consumption, as defined in Equation 1
Equation 1: Average current calculation
IActive = Current consumed by the controller when it is sampling a sensor
TActive = Time it takes the controller to sample all of the sensors
ISleep = Current consumed by the controller when it is in sleep mode
TSleep = Amount of time the controller remains in sleep mode
As a general rule of thumb, a human finger cannot touch a button faster than 150 ms. Typical A-to-D conversion times range from 200—6000 µs. This means that several A-to-D conversions can be performed during the 150 ms a sensor is being touched. Optimize the report rate for your specific A-to-D conversion time.
Reducing CP helps to reduce TActive because the controller can use a lower A-to-D converter resolution. Selecting a capacitive sensing controller with lower active current reduces IActive. Keep in mind that sleep mode current has a minimal impact on the average current.