Managing power in capacitive touch sensing applications, Part 2
Capacitive sensing enabled user interfaces is a widely accepted technology that often replaces existing mechanical switches and push buttons. Capacitive user interfaces are also used in battery-powered, handheld, and portable electronic devices. This penetration into portable and handheld electronics which mandate a long battery life and the constant focus on "green" technology has increased the importance of low-power applications.
This series of articles will feature various tips and techniques which will help you build a low-power capacitive user interface panel. The first part of this article discussed the following four techniques:
- Following layout best practices to optimize the sensor parasitic capacitance (CP)
- Using sleep mode and optimizing the report rate of the capacitive sensing controller
- Optimizing the report rate based on finger touch events
- Using the priority rule to wake up the capacitive controller from sleep mode
This part focuses on the next three techniques that can be used to optimize power consumption:
- Ganged capacitive sensor model to wake up the capacitive controller from standby mode
- Use of a proximity sensor to wake up the capacitive controller from standby mode
- Use of an external regulator to turn off the power to the user interface unit
Ganged capacitive sensor model to wake up capacitive controller from standby mode
The priority rule (discussed in Part 1) is useful only when one or a few specific sensors are used to wake the system up from standby mode. In most other cases, the system would be required to wake up when any of the sensors are activated. Typically, the greater the number of buttons that can wake up the system, the greater the average power consumed.
Ganged sensor sampling can be used to address this problem without increasing the average power. In this method, during the standby mode of the system, all of the physical sensors which can wake up the system are connected together to form a single virtual ganged sensor in the design. Scanning only the ganged sensor consumes lesser time than scanning all of the sensors; therefore, the capacitive controller can be in sleep mode for a longer time thereby reducing the average power consumed. (Refer to using sleep mode in Part 1 for more details).
If any of the physical sensors are touched, the sensor capacitance of the ganged sensor increases and a touch is detected. However, the specific button that was touched during the standby mode cannot be determined while sensing a touch on the ganged sensor.
To detect the button that was touched during standby mode, the capacitive controller should wake up and move into its active mode. The physical sensors need to be disconnected from the ganged sensor and scanned individually to identify the touched sensor.
In this method, the ganged sensor helps to optimize the average power by combining multiple physical sensors into a single virtual ganged sensor which ensures the capacitive controller reverts to active more only when a touch is detected. If after the capacitive controller moves to active mode, no finger touch is detected on the user interface panel for a certain period of time, then the capacitive controller should revert back to the ganged sensor mode.
The average system current consumption using this method is similar to that of the priority rule wake up method but provides the system the capability to wake up from a touch to any of the buttons.
Programmable capacitive controllers such as Cypress’ CapSense and CapSensePLUS controllers help to dynamically connect multiple sensors together to form a ganged sensor that optimizes the average power.
Proximity sensor
Use of a proximity sensor to wake up the capacitive controller from standby mode. This method is similar to the ganged sensor mode but involves using a capacitive proximity sensor instead of the virtual ganged sensor.
A capacitive proximity sensor is a loop of copper trace on a PCB or a loop of wire connected to the capacitive sensing controller. This capacitive proximity sensor can detect the presence of a hand when the hand is near the sensor without it actually touching the sensor.
Unlike all the other methods discussed before, which actually require a sensor to be touched, this method doesn’t require any physical touch to wake the system up. A capacitive proximity sensor is integrated into the user interface panel such that it enables the system to wake up when a user’s hand approaches the user interface panel. A simple construction is show in Figure 1 below.
In the standby mode, only the proximity sensor is scanned. Scanning only the proximity sensor reduces the total scan time thereby reducing the average power consumption. When the user’s hand approaches the user interface panel, the proximity sensor detects the presence of the hand and wakes up the capacitive controller. Once woken up from its standby mode, the capacitive controller moves to active mode and scans all the button sensors to detect the touches.
A value addition would be to control the backlight on user interface panels using the proximity sensor. Whenever a capacitive controller is in standby mode, the backlight can be turned off to indicate the inactive mode of the equipment. Once a user’s hand approaches the panel and the proximity sensor detects the same, the backlight can be turned on aiding the user in touching the correct buttons. This also helps reduce the overall power consumed by the end system. This is known as wake on approach.
External regulator turns off power
Use of an external regulator to turn off the power to the user interface unit. This method is significantly different from the methods discussed above. In this method, the host controller manages the average power consumption of the system.
Battery powered applications (e.g. mobiles phones) need extremely low power in standby mode. The system may contain multiple other units like an IR receiver and ambient light sensors which may also need to be in low-power mode.
In such applications, the host controller controls an external regulator or a power management integrated circuit (PMIC) that is used to control and regulate the power to the capacitive controller and other devices. Figure-2 below shows the block diagram of a typical implementation.
Figure 2: Block Diagram: External regulator with capacitive controller
The host controller turns off the power to the capacitive controller and other devices during the standby mode. This provides the lowest power in standby mode and allows for longer battery life in battery-powered applications.
This completes the second part of this article series. Other methods of optimizing the average power in capacitive sensing designs, especially methods using advanced capacitive controllers, will be covered in the next part.
Vibheesh B is a senior applications engineer working in Cypress Semiconductor’s Consumer and Computation Division and has specialized on Capacitive Touch Sensing applications since 2007. His responsibilities include defining technical requirements for new capacitive sensing controllers, developing new capacitive sensing controllers, conducting system analysis, debugging technical issues for customers, technical writing, and failure-analysis debugging.
If you enjoyed this article, you will like the following ones: don't miss them by subscribing to :
