[Introduction]Compared with mechanical switches, latches and switch chips play a more critical role in many applications. They improve flexibility, reliability, functional safety, repeatability, accuracy and size optimization of end assemblies. This article focuses ON how latches and switches work, the different types, and the Melexis latch and switch chip portfolio.
Intelligence and connectivity start with latches and switch chips
Latches and switch chips play a more critical role in many applications than mechanical switches. They improve flexibility, reliability, functional safety, repeatability, accuracy and size optimization of end assemblies. This article focuses on how latches and switches work, the different types, and the Melexis latch and switch chip portfolio.
What are latch and switch chips?
Magnetic latches and switch chips are based on the principle of the Hall effect and convert the magnetic field strength information of a magnet into a digital signal (1 or 0). Therefore, its output is On or Off, depending on the applied magnetic field. From the position of the magnet, the latch and switch chip can determine the actual position of the object.
The output changes when a defined threshold is reached.
This is Kagan acting based on two opposing thresholds. The output changes first when it reaches one of the defined thresholds (for example, the north magnetic field value), and then changes again when it reaches the opposite threshold (for example, the south magnetic field value). Between these two thresholds, the output is locked (remains unchanged).
What are the main types of latches and switch chips?
Latches and switch chips fall into three broad categories. For all three classes, the key parameters are two switching thresholds: BOP, the magnetic field operating point; BRP, the magnetic field release point.
The first type is the unipolar switch chip.
Such chips are activated only in one magnetic field range, which can be a north or south magnetic field. If the applied magnetic field strength is above the BOP threshold, the switching chip will enable the output. After the magnet is removed, the magnetic field strength will be below the BOP threshold and the switch chip will disable the output. This switch chip is mainly used for position detection.
The second category is bipolar switch chips, also known as latches.
The latch can be activated within one magnetic field range and deactivated when the opposite magnetic field is applied. In this case, the BOP defines when the output driver is activated (On), and the BRP defines when the output driver is deactivated (Off). Latches are typically used for PLC-controlled brushless DC motor commutation or DC motor index counting.
The third type is the omnipolar switch chip.
The function is the same as the bipolar switch chip, but can operate in two magnetic field ranges simultaneously. In other words, this type of switch chip can be activated in the magnetic field of the north and south poles, and is used to detect changes in the two magnetic field ranges.
Where are latches and switch chips used?
Latches and switch chips are ubiquitous and can be used in a variety of devices to simplify our lives. These “on-off” switching devices are widely used in automotive braking systems, transmissions, door locks and seat belts and other components. In electric bicycles, these devices are used for pedal assist systems, speedometers, and motor commutation, and in electric motorcycles, they are also used to trigger controllers.
In the home environment, latches and switching devices are widely used in various smart home appliances. These devices are used to ensure that the washing machine door is closed; Display the water level and flow of the coffee machine; detect the printer paper load and whether the lid is open or closed; and can be used for various power tools (motor reversing, position detectors, etc.).
Other applications include smart buildings (HVAC motor commutation), robotics (on/off detection, brushless DC motor commutation, end position detection, etc.), energy management (door open/close detection, etc.), and digital health (health management devices) Motor commutation, thermometer waterproof switch, etc.) and so on.
How to choose the correct output power?
In order to select the latch and switch chip suitable for a specific application, it is first necessary to select the correct output function. It depends on the specific module and the number of wires used to connect to another system. Standard configuration is 2-wire, 3-wire or 4-wire. This means you can choose from a 2-wire sensor chip that outputs a power supply current; a 3-wire sensor chip with an open-drain output; and a 4-wire sensor chip that outputs both speed (or pulse) and direction signals.
In some consumer applications, a push-pull output type can also be selected instead of an open-drain output. In this case, no additional external components (pull-up resistors) are required, further simplifying module assembly.
Such chips are typically used in remote sensor applications. The main advantage is that a 2-wire mechanical switch can be replaced while still using the same wiring harness. The current of the chip is used to identify whether the sensor chip has reached the operating point or the release point.
According to industry standards:
The current generated when the magnetic field strength is equal to or lower than BRP. The current drawn by the part is typically 3.3 mA or 6 mA, depending on the sensor chip selected.
The current drawn by the component when the magnetic field strength reaches the BOP threshold. This value is typically 14 mA.
The current level remains constant over the entire VDD range, ensuring stable readings even with some fluctuations. The loop in which the sense Resistor RSENSE is located converts the current output into a logic input for the Electronic control unit.
These chips feature open-drain outputs. To form a 3-wire system, a resistive load must be connected to the VDD line. Typical loads are between 10 kΩ and 100 kΩ. The load can be connected on the module side or the electronic control unit side. When the BOP threshold is reached, the output will approach 0 V. In this case, the output will be disabled and connected to VDD through a pull-up Resistor.
These chips output two signals: speed (or pulse) and direction. Speed is the normal output of the latch sensor, while the direction output indicates whether the rotation direction is clockwise or counter-clockwise. These chips are equipped with two Hall elements and ensure a constant width between boards by using a CMOS process.
Depending on the latching magnetic characteristics, the speed (SP) output will go low or high, respectively, when the south or north magnetic field is strong enough toward the top of the package. After the magnetic field is removed, the device remains in its previous state.
When a specific sequence of magnetic pulses is present on both Hall plates, the direction (DIR) output is latched low or high depending on the direction of motion of the applied magnetic field.
What are the features of Melexis latch and switch chips?
Side Sensing Options
Lateral sensing requires only one switch for speed detection and is suitable for back-biased applications such as motorcycle wheel speed sensors. Lateral sensing also enables surface mount device (SMD) solutions that typically require traditional through-hole components. Reflow soldering offers more advantages over other solutions. Typical applications include blowers, cooling fans, and pumps.
In addition, side-to-side sensing increases the flexibility of magnetic design, enabling smaller motor designs regardless of pitch. Typical applications include window regulators and sunroofs in the automotive industry.
Dual chip option
The dual-chip option can provide another programmable/preset output, that is, the chip contains two independent outputs in total, and the chip has more functions, which can realize applications such as reverse output, alarm detection and loss of field detection. In addition, a more compact PCB design can be achieved. Typical applications include transmissions, electronic latches and window regulators.
The sensor chip has the characteristics of “micropower”, which combines the advantages of low-Voltage operation and low current consumption, and is especially suitable for battery-powered equipment. The device employs an internally managed sleep/wake strategy that significantly reduces power consumption. Typical applications include door locks for safes, etc.
Latches and switch chips can be programmed by customers to meet their various needs. BRP and BOP can be adjusted according to specific applications. The chip is directly programmable with high flexibility and accuracy.
Since different types of magnets (ferrite magnets, neodymium magnets, etc.) behave differently at the same temperature, thermal compensation (TC) is a critical parameter for some applications. Therefore, it is very important to choose a chip with an appropriate TC or programmable TC.
Float Switch Options
The floating switch chip uses an isolated output (SOI technology), which can directly drive the load just by powering up. Typical applications include level gauge applications, push buttons and direct drives of loads, etc. In addition, floating switch chips can also directly replace mechanical reed switches.
Options to meet Automotive Safety Integrity Level (ASIL) requirements
ASIL-compliant chips are designed according to the ISO 26262 standard. Such chips are suitable for automotive safety applications such as seat belt buckles.
What are the different magnetic chips for different applications?
Melexis’ latch and switch chips support a variety of applications. The main products are described below. When selecting a sensor chip, carefully consider the magnetic chips that can be applied to the design, and choose the most suitable product for the application to achieve the best results.
The slide switch chip works only in one magnetic field range. In the example below, the South magnetic field has been selected. The magnetic field strength is at its maximum when the magnet is directly in front of the chip. As long as the magnet moves in a direction parallel to the chip, it weakens the magnetic signal. In this configuration, a unipolar sensor chip is recommended.
Proximity switch chips also only work within one magnetic field range. In the example below, the magnetic field strength is at its maximum when the magnet is as close as possible to the chip. As long as the magnet is moved in the direction of the chip, it will enhance the magnetic signal. For this configuration, a unipolar switch should be used.
Rotary Encoder Magnet
Disc-shaped magnets are commonly used in index counting applications. The sensor chip will be enabled by the south magnetic field and disabled by the north magnetic field. For each pole pair, the sensor signal will switch twice. For these configurations, a latch chip is a good choice, since such chips operate and release under opposing magnetic fields. Orientation sensors can be an advantage in situations where orientation information is requested.
Ferrous blades can be used to interrupt magnetic signals. The blade changes the strength of the magnetic field and triggers the output of the sensor chip. A weaker magnetic field may still be applied to the sensor chip. For such applications, unipolar sensor chips are preferred.
Back Bias Application
For this configuration, various implementations are possible. The first solution can be implemented using expensive zero-Gaussian magnets, creating an actual zero-field region at the location of the sensor’s sensing point. This scheme supports the use of pre-programmed chips.
The second solution can be implemented using conventional magnets that can sense lateral magnetic field components. The solution has been implemented with Melexis’ IMC technology. With this scheme, ordinary magnets can be used; when the lateral magnetic field component is close to zero, the magnet’s behavior remains stable over the entire temperature range. For this configuration, simple programming after mounting the chip on the magnet is recommended to ensure smooth operation.
The figure below shows the magnetic components Bx and Bz of a common magnet configuration. The Bz (green line) shows a large excursion (100 mT) in the application, indicating that temperature changes can have a large effect and that it is very challenging to achieve good operation over the entire temperature range. For these types of applications, dynamically compensated sensor chips are often used, resulting in higher bill-of-materials costs.
The lateral component (Bx) of the magnetic field is symmetric around 0 mT, resulting in excellent temperature characteristics and improved design stability. With this scheme (after 0 mT), a static switch sensor chip can be used to control the desired duty cycle. The chip can be programmed after it is mounted, which is critical for improving accuracy. A small hysteresis of +/- 1 mT is recommended for this configuration.
When paired with a row of float switches, magnets can be used to detect the liquid level. In addition, floating switch chips can also directly replace mechanical reed switches.
Melexis Latch and Switch IC Portfolio
We offer a wide range of latches and switch chips to support a variety of applications
Latches and switch chips are increasingly used in a variety of applications across a wide range of industries to detect position, distance, velocity or directional motion. In addition, the latches and switch chips are low maintenance and robust in design for non-contact, wear-free operation and are immune to water, dust and vibration. Melexis designs, develops and manufactures high quality latch and switch chips.