Considerations for Capacitive Sensing Technology in Mobile Phone Touchscreens 3

Mobile means low power

For mobile phones, the power consumption of capacitive sensing solutions must be very low. For battery powered mobile devices, the low-power target requires the controller to report to the main control device at a speed that is not too fast, and the scanning time for sensors should not be too long. If there are no other events to be processed, the device should enter sleep mode.

The key to extending battery life is to minimize the average current flowing through sensors when effectively scanning and processing data. The average current can be calculated by a simple time weighted average of active current and sleep current, so the longer the sleep time of the controller between scans, the longer the battery life.  diode

One practical limitation of long sleep intervals is system latency, which refers to the delay between touch events and the system's response to touch. In the eyes of non-technical users, significant latency is manifested as slow response to key presses. In extreme cases, extremely long sleep intervals can cause buttons to fail for a period of time.

In mobile phone design, solving the above problems lies in finding a good balance between fast sensor response and low power consumption. For mobile phone design, a latency of 30-50ms is a good goal. To lower power consumption, developers often put sensors into longer latency mode if there is no user input for a long time. This slower scanning mode, known as standby mode, has a delay of 100ms or longer. Once user input occurs, the system immediately enters a valid scanning mode with faster button response speed.

The following example calculation shows how to achieve an average current as low as 33mA in standby mode in a mobile phone design with 12 sensors. The scanning time is set to 0.5ms per sensor (t1=0.5 × 12=6ms). The reporting rate in standby mode is 100ms, so the sleep interval is set to 94ms (t2=100 ± C6ms). The sleep current and active current can be found in the controller IC data manual (ISleep=3 µ A, IActive=1500 µ A). By substituting these parameters into the formula for calculating the average current IAVE, the average current can be obtained as 93? A。 diode