Optimizing Power Consumption in Capacitive Sensing Systems Using Sensor Co-wiring

Capacitive touch buttons are rapidly replacing traditional mechanical buttons in many electronic products such as consumer, home appliances, automotive, and industrial. Although capacitive buttons have many advantages over mechanical buttons, system designers need to weigh certain parameters when creating capacitive sensing systems. These include signal-to-noise ratio (SNR), response time, and power consumption.

Since capacitive sensors are susceptible to noise both internal to the controller and external to it, SNR is critical to ensuring reliable performance of capacitive sensing systems. This article will focus on two other parameters.

Response time is a measure of how quickly a capacitive sensor responds to a touch. There is often a trade-off between power consumption and response time. In this article, we will explore the response time aspects that designers need to consider when optimizing power consumption.

Capacitive sensors need to be scanned within a specific time (called scan time) based on sensor characteristics such as parasitic capacitance and touch sensitivity. Scan time is the main contributor to the power consumption of capacitive sensing controllers. Power optimization is particularly important for battery-powered devices such as mobile phones and wearable devices including heart rate monitors. Power optimization can be achieved in many ways, including optimizing scan time and sensor scanning frequency. In this article, we will introduce and explain one of the more prominent power optimization methods for capacitive sensing systems, which is called sensor ganging.

>>> > Power consumption optimization

The key factors that determine power consumption are the sensor scan time and the sensor scan frequency. The sleep current value is generally much lower than the active current value. Therefore, when the capacitive sensing system is not in use, the capacitive sensing controller can be put into sleep mode to reduce the average current consumption. The scan-sleep-scan-sleep method is often used to optimize the power supply of the capacitive sensing system (see Figure 1). This method scans all sensors and then puts the controller into a low-power sleep mode. This is a cycle, and then this cycle is repeated continuously. A scan-sleep cycle is called a "refresh interval". Each refresh interval contains active time and sleep time. The "active time" includes the scan time, sensor data processing, and post-sensor scan activities such as: control feedback mechanisms such as LEDs and buzzers. The sensor scan time takes up most of the active time.

Figure 1: Current curve when using the scan-sleep-scan-sleep method

You can reduce power consumption by:

• Shorter working time, i.e. shorter scanning time or processing time of post-sensor scans

• Reduce the operating current for a given operating time

• Extended sleep time

>>> > Sensor co-connection

Sensor co-wiring is a method of reducing the power consumption of the capacitive sensing controller by reducing the amount of time the controller is operating. As the number of capacitive sensors increases, the power consumption for a given refresh interval increases; if the refresh interval decreases, the power consumption increases. For a given number of sensors, reducing power requires increasing the refresh interval. However, this affects the response time of the sensors. To achieve a good balance between response time and power consumption, all sensors can be combined and scanned as a single sensor. This is called sensor co-wiring. The group of sensors can be treated as a single sensor, and the capacitive sensing algorithm scans the co-wiring individual sensors as a single sensor. When a touch is detected and confirmed, the sensors are disconnected and scanned individually.

Sensor sharing is possible in devices such as the Cypress PSoC, where individual sensors can be connected to a global analog multiplexer bus. In mixed-signal devices such as the PSoC 4, an internal analog multiplexer bus can be used to connect multiple sensors to the CapSense block inside the firmware. A reference design guide that includes an analog multiplexer bus is provided at the end of this article, and describes how to connect capacitive sensors to the analog multiplexer bus.

‍ Figure 2: Scanning a sensor individuallyScanning a sensor after it is connected

>>> > Sensor Co-Connection Use Cases

Button/slider connection

In an application that contains only buttons or sliders, we can connect all buttons or sliders together and scan them as a single sensor before the user touches any button or slider. In order to obtain good system response time, the connected sensors can be tuned as proximity sensors by setting the sensitivity to a very high value. The sensitivity of a sensor indicates the minimum capacitance change caused by a touch that the sensor can detect. With proximity sensing, as long as the user approaches the device, the system can respond before the user touches the actual function button, thereby shortening the system response time.

For example, the backlight LEDs that enhance button visibility can be turned off when there is no activity. When the user approaches the device, the proximity sensor can detect the approaching hand and turn on the backlight LEDs to help the user operate the corresponding button. However, because the proximity sensor is extremely sensitive, it takes a longer time to scan, which increases power consumption. To further reduce power, the co-connected sensors can be tuned to a lower sensitivity so that they can operate as buttons. This means that the co-connected sensors only detect user touches, and all sensors are scanned individually after the user touches. This method has a longer system response time than the method where the co-connected sensors are tuned as proximity sensors.

Figure 3: PCB with buttons and slider

Close to joint

When multiple proximity sensors are included in an application - such as gesture recognition, all proximity sensors can be connected together and scanned as a single proximity sensor to detect the approach of a human hand in the Z direction (from the board's point of view). After the hand is detected, all proximity sensors are scanned individually to detect gestures in the X and Y directions. Another advantage of this approach is that the system can respond quickly to the approach of a human hand because the proximity detection distance can be increased after the proximity sensors are connected together, so it can detect a human hand at a longer distance than scanning the proximity sensors individually.

Figure 4: PCB with multiple proximity sensors

Row/column co-wiring in matrix or touch pad designs

In applications that include matrix buttons or touch pads, all rows or columns can be connected together and scanned as a single sensor before the user touches the matrix button or touch pad. It is not necessary to connect rows and columns at the same time because:

• This will increase the parasitic capacitance of the co-connected sensors. The parasitic capacitance must not exceed the upper limit supported by the capacitive sensing controller.

• The layout of the touch pad or matrix is ​​such that only connected rows or columns can detect a touch across the sensor area.

Hybrid sensor co-connection

Let's look at an application example where there is a proximity loop around four buttons. In this case the proximity sensor and buttons can be wired together and scanned as a single sensor. This allows the proximity detection range to exceed that of scanning the proximity sensors individually. This approach can be used when there are board size limitations and the proximity sensor size cannot be increased.

Figure 5: PCB with buttons and proximity sensor