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A Low-Power Hall-Effect Switch
The following article appeared in the June, 1999 edition of Sensors Magazine. It has been reprinted with their permission. For more information, visit their website at http://www.sensorsmag.com/
Highly reliable, very compact, and nicely priced, Hall-effect sensors are becoming the switch of choice. Allegro's latest delivers another bonus: low power consumption.
Christine Graham, Allegro MicroSystems, Inc., USA
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Figure 1. In this power-supply timing diagram, a sampling time of only 60 µs every 60 ms maintains a 0.1% duty cycle and a low average supply current of <20 µA. |
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Figure 2. The pole-independent chip orientation allows operation with either a north or south pole magnet. Allegro's pole-independent sensing technique makes the sensor an easy drop-in replacement for reed switches. |
Today's big four design requirements for consumer goods-high reliability, small packaging, low costs, and low-power consumption-have driven product engineers from mechanical switches to active sensors. Because mechanical switches, (e.g., reed switches) tend to have low reliability without offering cost or packaging advantages, the commercial industry has been forced to reevaluate their long-term viability. In response, industry has turned to Hall-effect switches, for a low-cost, reliable solution. Hall-effect sensors, having no finite number of contacts, typically outlive the product in which they are used, making them reliable substitutes for mechanical switches.
Built for the Job
Allegro Microsystems has introduced a Hall-effect sensor, the A3210, that meets the demand for accurate, small, less expensive devices that consume minimal power. The A3210 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. Allegro's patented chopper-stabilization technique reduces offset drift caused by temperature and stress, allowing for sensitive and stable switch points. The typical offset drift is ±5 Gauss. A switch-point specification of <50 Gauss lets a small magnet to be sufficient for proper operation at air gaps 2 to 4 times the typical operating air gap of other Hall-effect switches.
| TABLE 1 | |
| Device | 1cc Avg. |
| Low-Power Hall-Effect Switch | 12 µA (3 V) |
| Typical Hall-Effect Switch | 5 mA (12 V) |
The A3210 also operates on low power (see Table 1). Allegro's single-chip, chopper-stabilized, unipolar switch has an onchip 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 (see Figure 1). 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.
A Replacement for Reed Switches
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Figure 3. This cell phone open-or-closed detection scheme uses the A3210 sensor. A switch point of <50 gauss allows a small magnet to deliver proper operation at air gaps 24 * the typical operating air gap of other Hall effect switches. |
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Figure 4. The A3210 has an NMOS output for direct digital interfacing with no further signal processing required. |
The A3210's design sweetens the pot with another advantage. Allegro's pole-independent sensing technique (patent pending) makes this device 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 state-of-the-art 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 (see Figure 2). Allegro's switch is available in two packages (see Table 2).
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 analog and digital output Hall devices are available; their specifications are focused on measuring various magnetic configurations.
One such configuration is in cell phones. A typical open-or-closed detection scheme is shown in Figure 3. The magnet orientation (i.e., the pole facing the sensor face) is typical for Hall-effect sensors. Because of the A3210's pole-independent sensing technique, however, the device will work with a reed switch magnet with a 90 degree orientation from a typical Hall-effect magnet.
The device does require a regulated power supply to protect against transients seen on the supply line. The external circuitry overall is minimal (see Figure 4).
| TABLE 2 | |||
| Package Name | Suffix | Temperature Range | Type |
| SOHED | LH | -40°C to 85°C | Surface Mount (SC-59A) Industry Standard Footprint) |
| Single-In-Line (SIP) | UA | -40°C to 85°C | Though-Hole Mounting |
In Closing
The A3210 is availabile in a SOHED package for surface-mount applications, and in a single in-line package for through-hole mounting. The advantages offered by the A3210 take Hall-effect switches to a new level for active sensors.
Christine Graham is a Systems Engineer with the Allegro MicroSystems, Inc., Manchester Facility, 955 Perimeter Road Manchester, NH 03103-3353, USA, TEL: 603-626-2434, E-MAIL: cgraham@allegromicro.com