APPLICATION STORY

          

          



Hall-effect sensor switching
for low-power applications

Allegro MicroSystems has introduced a new Hall-effect switch, the A3210, which meets the demand for an accurate, small and relatively low-cost devices that offers the added benefit of consuming minimal power. The key design requirements for modern consumer electronic equipment can be summarised as high reliability, small size, low cost, and low power consumption. One result of these requirements is that product development engineers have had to look at alternatives to traditional electromechanical switching devices. Because mechanical switches (such as reed switches) tend to have low reliability without offering cost or packaging advantages, the consumer electronics industry has been forced to re-evaluate their long-term viability. One result is an increased interest in Hall-effect switches to provide a low-cost, reliable solution. Because Hall-effect sensors have no electromechanical contacts, one of the main causes of unreliability in traditional switches is removed. In fact, Hall-effect devices typically outlive the product in which they are used, making them reliable substitutes for mechanical switches. The new Hall-effect device described in this article meets the demand for an accurate, small and relatively low-cost devices that offers the added benefit of consuming minimal power. Accuracy and sensitivity The new A3210 device is fabricated using an advanced BiCMOS process, which allows component matching with low-input offset errors and provides small geometry chopping and logic circuitry. The ultrasensitive Hall-effect switch allows for accurate, low magnetic switch-points over a wide range of air gaps. A patented chopper stabilisation technique reduces offset drift caused by temperature and stress, allowing for sensitive and stable switch points. The typical offset drift is ±5 Gs. A switch-point specification of less than 50 Gs means that a small magnet is sufficient for proper operation at air gaps between two and four times greater than the typical operating air gap of other Hall-effect switches. The new device also operates on low power, with a typical current consumption of only 12 µA at 3 V, compared with 5 mA at 12 V for earlier Hall-effect switches. The device is a single-chip, chopper-stabilised, unipolar switch with an on-chip oscillator that maintains a 0.1% duty cycle for 'on' time and latched open-drain output. The signal is sampled every 60 ms, with an 'awake' time of 60 µs for sampling. The output of the Hall-effect sensor during sample time is latched and held until the next sample occurs. This technique allows for an average supply current of 12 µA, making the device a perfect fit for products that require low-power consumption. Pole-independent sensing The design of the new device offers another advantage by using a novel pole-independent sensing technique. This makes it an easy 'drop-in' replacement for reed switches. The pole-independent chip orientation allows for operation with either a north or south pole magnet, making the device easier to manufacture. The new technology gives the same output polarity for either pole. A north pole is typically thought of as a negative field, and a south pole is typically a positive field. Applications Hall effect sensors provide an electrical voltage when excited by a perpendicular magnetic field. Most Hall-effect devices have their own signal conditioning circuitry. Both analogue and digital output Hall devices are available; their specifications are focused on measuring various magnetic configurations. One such configuration is in 'flip top' mobile telephones. A typical open-or-closed detection scheme is shown in Fig.1. The magnet orientation (i.e. the pole facing the sensor face) is typical for Hall-effect sensors. Because of the new device's pole-independent sensing technique, however, the device will work with a reed switch magnet with a 90° orientation from a typical Hall-effect magnet. The device does require a regulated power supply to protect against transients on the supply line. The external circuitry required, however, is minimal. The switch is available in two packages: a SOHED package for surface-mount applications, and a single in-line package for through-hole mounting. Picture. This mobile telephone's 'open or closed' detection scheme uses the A3210 sensor. A switch point of below 50 Gs allows a small magnet to deliver proper operation at air gaps 2-4 times greater than the typical operating air gap of other Hall-effect switches _____________________________________________ Article - by Dominikus Maisl, Allegro MicroSystems Europe June 2000
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