Use sleep mode and optimize the sensor report rate
Report rate and sleep mode work together to define how the capacitive touch sensors are sampled, as shown in Figure 2
. Report rate defines how often the sensors are sampled by the A-to-D converter. When a sensor is sampled, the capacitive sensing controller is in active mode. When a sensor is not being sampled, the controller can be put into sleep mode. In sleep mode, the controller powers down all internal blocks and peripherals. This mode is supported by most of the capacitive sensing controllers on the market.
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.