How to do the switch detection of TWS headset design?
TWS needs to detect the opening and closing of the charging compartment cover and whether the headset is in place. In this detection function, the Hall device is favored by more and more customers because of its sensitive response, small size, and low power consumption. In this article, we will introduce common switch monitoring solutions in the market, and the application of TI Hall sensor technology in TWS headsets.
Conventional switch detection scheme
Mechanical bullet detection
The mechanical spring needle has a simple structure and high accuracy requirements, but it has a short service life and is easily affected by dust, water vapor, vibration and other factors. The contacts are easy to rust, and metal fatigue damage is extremely easy to occur.
Reed switch detection
Magnetic reed detection is to seal the contacts containing precious metal materials in the glass tube by magnet induction. Therefore, the switch is not affected by moisture or other environmental factors, and the contacts will not be oxidized. The disadvantages are large size, difficult installation, and easy damage.
Infrared photoelectric switch detection
The infrared photoelectric switch installs the light transmitter and receiver face to face on both sides of a slot. The illuminator emits infrared light, and the receiver can receive the light when the cover is opened. When the cover is closed, the light in the slot is blocked, and the photoelectric switch is activated. The advantage of this scheme is that the detection speed is fast, but the power consumption is large, and it is also afraid of factors such as dust, water and vapor.
Hall switch detection
The Hall effect is used to detect changes in the magnetic field. Compared with reeds and mechanical switches, its digital output is clean and stable, without jitter, no impact, etc., long life and vibration resistance; compared to infrared light Detection, this detection method is not afraid of oil pollution, water vapor and salt spray pollution or corrosion, not afraid of dust, low power consumption, small size, light weight, easy installation. More and more TWS earphones have begun to choose switching Hall devices, and earphone insertion detection has gradually changed from a mechanical spring needle solution to Hall detection.
Hall effect and the principle of Hall sensor technology
The Hall effect is a process in which the carriers inside the energized solid conductor are shifted by the Lorentz force in the magnetic field, and then the induced voltage difference is generated, and finally the Lorentz force and the electric field force are balanced. Hall effect is often used in magnetic field measurement, electrical signal measurement and other directions.
Commonly used Hall devices are classified into linear Hall devices, switching Hall devices and latching Hall devices according to the type of output signal. The output of the linear Hall device is an analog signal, and the output voltage is usually in a linear relationship with the strength of the external magnetic field. The digital Hall device outputs a level of 01, among which digital Hall devices can be divided into three types: unipolar, bipolar and latching according to the characteristics of the induced magnetic pole.
The working process is shown in Figures 1 and 2. By detecting the strength of the external magnetic field, the output is controlled to be turned on or off, similar to a switch. When the magnetic flux density increases to a certain value, the Hall switch starts to act and outputs a low level. The magnetic flux density corresponding to this point is called the operating point BOp. Due to the effect of hysteresis, if you turn off the middle and Hall switch, you need to use When the magnetic flux density is lower than a certain point, the Hall switch is turned off. At this time, the value of the magnetic flux density is called the release point BRp.
The general structure of a digital Hall device consists of a Hall effect film, a voltage regulator, a sleep wake-up control circuit, a signal amplification filter circuit, an offset compensation circuit, a Schmitt trigger, and an open drain output.
Application of TI Hall sensor technology in TWS headset
In the TWS Bluetooth headset charging compartment, multiple magnets are often placed to attract the headset and the cover. The polarity of each magnet will affect the magnetic field induction intensity of the Hall IC induction point. If the omnipolar Hall IC is selected, the internal magnetic field of the charging compartment will maintain a high value even after the compartment cover is opened and cannot reach the release point of the Hall IC. Therefore, the magnetic pole opposite to the original magnetic field in the warehouse cover should be used, and a unipolar Hall IC that only acts on this polarity should be used to avoid interference from other magnetic poles.
In addition, the switch-type Hall IC is a logic control circuit. As shown in Figure 4, placing a filter capacitor at the input can provide a clean power supply environment to ensure that the internal logic can work normally. The closer the capacitor is to the chip, the better the filtering effect. The capacity is 0.1uF-10uF. Many engineers often encounter difficulties when calculating the effective total air gap between the switch Hall IC and the magnetic poles. TI provides an online calculation tool Hall Effect Switch Magnetic Field Calculator, and gives the TI Hall switch products under different specifications of magnets. The sensing distance provides reference and convenience for everyone’s design.
TI’s ultra-low power DRV5032 Hall-effect switch is available in seven versions in Table 6 in the TWS headset according to the different needs of customers. The IC has a wide supply voltage of 1.65-5.5 V and can be powered directly by a battery. Among them, the 5 Hz version has a current consumption as low as 0.54 µA under 1.8V power supply conditions, and the 20 Hz version has a current consumption as low as 1.6 µA under 3V power supply conditions.
Two different package specifications, SOT-23 and X2SON, are suitable for different warehouse structures. In addition, the unipolar version of the IC push-pull output saves space for pull-up resistors, and there are two levels of magnetic induction sensitivity to provide accurate and stable magnetic switching points. The operating temperature range supports -40 to +85°C.