Capacitive Proximity Sensing Technology in Smartphones

Infrared (IR) proximity sensors are currently widely used in smartphones to prevent accidental touches on the touch screen caused by the user's face during a call while reducing power consumption. The IR sensor has the advantages of long detection distance and fast response speed, but its expensive cost and complex and demanding assembly requirements have prompted mobile phone manufacturers to seek solutions with lower costs and simple structures. The popularity of capacitive proximity sensing in white goods, smart homes and other fields provides an effective way of thinking for mobile phone proximity sensing solutions.

1. System Structure

The structure of the capacitive proximity sensing system is shown in Figure 1. The controller detects the change in capacitance caused by the object approaching the mobile phone through electrodes. Once the change in capacitance exceeds the threshold set in the controller program, the controller will send an interrupt signal to the mobile phone processor. If the mobile phone is in call mode at this time, the host will turn off the LCD, touch screen and other components to achieve functions such as reducing power consumption and avoiding accidental touches.

The electrode is responsible for detecting changes in capacitance, and its design quality largely determines the overall performance of the system. The electrode is essentially a flat conductor, which can be a piece of copper on the FPC or an ITO film on the capacitive touch screen.

Figure 2 shows an example of the design of an ITO thin-film electrode. The size of the electrode directly affects the detection distance of the proximity sensor. When other designs remain unchanged, the detection distance increases with the increase of the electrode size. The shape of the electrode should be as smooth as possible to avoid right or sharp angles, and the electrode should be as complete as possible.

In mobile phone applications, the electrodes are usually rectangular to maximize the sensing area. At this time, attention needs to be paid to the corners of the arced electrodes. The electrode should be placed on the side of the FPC or ITO film close to the touch screen, and the other side of the back usually needs to be vacated. The area of ​​the front cover of the mobile phone corresponding to the back of the electrode should avoid large areas of metal, otherwise, it will affect the detection range.

The ground wire needs to be laid around the electrode to enhance the capacitance reference, shield the noise, and improve the linearity of the sensing direction. The distance between the electrode and the ground wire is recommended to be 0.5mm to 1mm. The width of the ground wire depends on the specific situation, and it is recommended that it be no less than 1mm. The lead from the electrode to the chip should be as short and thin as possible to reduce parasitic capacitance and coupling noise.

Another major factor affecting system performance is the controller. We chose the programmable CapSense controller CY8C20055 from Cypress with the new Quitezone technology. Quitezone technology provides unparalleled resistance to radiation and conduction noise and has ultra-low power consumption, which is very suitable for use in mobile terminals such as mobile phones.

This technology also achieves the industry’s best signal-to-noise ratio (SNR). In high-noise environments, Cypress’s patented CapSense Sigma-Delta (CSD) Plus algorithm can be used to detect capacitance changes as low as 0.1pF suitable for proximity sensing. In addition, CY8C20055 uses SmartSense auto-tuning technology, which can dynamically compensate for changes in the runtime environment in real-time, thereby ensuring the stability of performance and the consistency between channels.

2. Hardware Circuit

The circuit diagram of capacitive proximity sensing originally designed is shown in Figure 3. The peripheral circuit of CY8C20055 is very simple, the minimum configuration only needs 2 capacitors-modulation capacitor C1 and decoupling capacitor C2.

In this circuit, the PIN 3 of the chip is connected to the electrode to collect the capacitance signal. It is recommended to connect a resistor with a typical value of 560 ohms in series near the chip to suppress RF noise. Mobile products such as mobile phones need to pass strict ESD tests.

Because the electrodes are located on the upper part of the mobile phone, very close to the edge of the mobile phone, earpieces, earphone jacks, etc. (Figure 2), and the electrode area is relatively large, ESD arcs can easily pass through these openings or gaps are coupled to the electrodes after entering the mobile phone, which imposes a relatively large electrical shock on the chip pins, and there is a risk of damaging the pins.

In this design, an ESD protection component such as TVS is added on the side close to the chip to protect the chip. It should be noted that the self-capacitance of the selected TVS device cannot be too large. CY8C20055 provides I2C or SPI interface to communicate with the host. In this design, the host can configure sensing parameters, obtain data, turn off or wake up the chip, etc. through the I2C bus, and can also perform online upgrades of the chip program (Firmware).

In PCB or FPC layout, modulation capacitors and decoupling capacitors need to be as close as possible to the chip pins. When routing, it is important to avoid parallel electrode leads with I2C signal wires and power wires. If it is unavoidable, a ground wire should be added in the middle of the routing as isolation.