April 9, 1998

Hall-Effect Sensor ICs Sport Magnetic Personalities

BILL TRAVIS, SENIOR TECHNICAL EDITOR

Innovative stabilization and trim techniques are yielding dramatic improvements in Hall-effect sensor ICs.

Magnets provide a convenient way of sensing linear or angular position. Hall-effect devices and other magnetic sensors (see box "Hall's not the only magnetic effect") provide signals that allow you to implement various measurement and control functions. Discrete Hall-effect sensors, coupled with current-excitation and signal-conditioning blocks, provide a voltage output in the presence of a magnetic field. A number of integrated sensor ICs ease your design task by combining Hall sensors and peripheral circuitry to provide linear or switched outputs.

A vast array of applications exists for Hall-effect sensors. The allure of contactless sensing (totally devoid of mechanical wear), low parts cost, and easy design-in make Hall devices the sensors of choice in hundreds of automotive, aircraft, appliance, and tool applications. The first class of Hall-effect sensor ICs includes linear devices, which find use in magnetometers (Reference 1), current-measurement systems, and gaussmeters, for example. The second group includes digital-output switching ICs, which comprise the majority of Hall ICs produced because of the greater number of applications.

The two subclasses of digital-output switching ICs include switches and latches (Reference 2). A unipolar switch operates with a single magnetic pole and is guaranteed not to switch on in the absence of a magnetic field. An opposite-polarity magnetic field has no effect on the switch. A bipolar latch, however, responds to both magnetic poles. Bipolar latches turn on in the presence of a north or south pole and turn off only if the opposite-pole field is strong enough. "Bipolar switch" is a term that applies to latch fallouts. Such switches turn on in the presence of a field and may turn off with a smaller field of the same polarity, with no field, or with an opposite-polarity field.

Table 1, which is derived from Melexis (formerly, USMikroChips) data sheets, provides ample clues that a bipolar switch is simply a test-fallout device from a batch of bipolar latches. The typical figures for the operate (turn-on) and release (turn-off) points are identical for both classes of sensor, as are the hysteresis (operate-to-release spread) figures. These figures reflect more tightly controlled trip points (for example, 5 to 45 gauss) for the latch vs the wide latitude of trip points (for example, ­20 to +60 gauss) for the switch. Note also that the latch guarantees true bipolar operation: The release operation requires an opposite-polarity magnetic field--which is not a requirement for the switch.

Figure 1a shows a unipolar Hall-effect switch used as a proximity switch. The switch turns on when the magnet's south pole ap-proaches the IC, and it turns off when the pole recedes a stipulated distance. Applications for proximity switches include seat-belt, air-bag-ejection, power-window, door-ajar, and refrigerator-door sensors, for example. Figures 1b and 1c show slide-by switch configurations in which the activating pole "slides by" the sensor. The configuration in Figure 1b suits an exercise-machine counter, for example; the rotary slide-by schemes in Figure 1c are useful in tachometers and speed indicators.

The proximity configuration is the simplest design, although it requires the greatest amount of physical movement. It is also the least precise with regard to the position that turns the switch on and off. The magnetic-field intensity is greatest when the magnet touches the IC, decreasing exponentially as the magnet moves away. The slide-   by configurations involve switching magnetic fields from north to south and provide a well-defined position-switching relationship. The required motion may be as little as 1 or 2 mm.

Figure 2 shows linear (a) and rotational (b) interrupt-switch configurations. A ferrous vane, interposed between the magnet's pole and the Hall-effect IC, shunts the magnetic field or reduces it to a small fraction of its maximum value, thereby turning off the switch. Interrupt switches are useful in position sensing. The configuration in Figure 2a suits security-door sensor systems, for example. The rotary configuration in Figure 2b finds common use in automotive-ignition systems as well as many industrial applications.

When designing any assembly using Hall-effect sensor ICs, you must consider five variables: cost, temperature range, position tolerance, position switching accuracy, and tolerance buildup. The cost of the Hall IC is primarily a function of the temperature specs of the turn-on (operate, or B_OP) and turn-off (release, or B_RP) magnetic parameters, and the hysteresis between them (B_HYS). By using stronger magnets, you can sometimes use more loosely specified Hall ICs and thus save money. You must weigh the cost trade-off between using more expensive magnets (for example, rare-earth units) and more or less tightly specified Hall ICs.

Hall ICs are categorized in different temperature ranges for use in specific applications. Be sure to consider the temperature range of the system you're designing, and don't waste money by overspecifying. Position tolerance and position switching accuracy depend on the application--as well as how the sensor is assembled. Some systems, for example, are more tolerant of changes in air gap and lateral motion than are others. The position switching accuracy is the most crucial spec for the magnetic circuit and Hall IC. For example, if switching must occur within ±0.125° or ±0.1 mm, the Hall IC's spec must be much tighter than if the requirement were ±1° or ±1 mm. Tolerance buildup is the sum of all the variables that determine the operate and release points of a Hall IC: position tolerance, temperature coefficient, assembly wear and aging, and magnet variations.

Piezoresistive effects (resistive changes) arising from stress cause random offsets (V_HOFF) in the Hall plates in silicon Hall-effect sensors. You can reduce the offset from, say, ±500 gauss to approximately ±20 gauss by connecting multiple Hall cells in parallel and by using active trimming. However, CMOS technology offers a less expensive solution--both in silicon area and manufacturing time. Figure 3 shows the chopper-stabilization technique Melexis uses in its Hall-effect sensor ICs (Reference 3).

R_OFF in Figure 3a is a piezoresistive stress fault in the Hall plate, configured as a Wheatstone bridge. The CMOS switches essentially flip the bridge by 90° at a rate exceeding 100 kHz. The resulting signal is the stress-induced offset, V_HOFF, with the desired magnet-induced V_H riding as an ac signal on V_HOFF. Subsequent demodulation and lowpass-filtering circuitry then extracts the useful V_H Hall-effect signal.

CMOS again comes to the rescue for op-amp offset. In Figure 3b, the op amp's positive and negative inputs and outputs switch synchronously, so the amplifier's offset becomes an ac signal riding on the amplified input voltage. In addition to cost-cutting, chopper stabilization takes care of piezoresistive stresses in the IC that develop after production and shipping. The $0.53 (1000) US2881 bipolar-latch IC from Melexis uses such chopper-stabilization techniques. The $0.51 (1000) US2882 bipolar-switch sensor also uses chopper stabilization.

The US2881 is available in SOT-23 (US2881SO) or in a conventional three-pin radial package (US2881UA). The SOT-23 version has reversed polarity from the conventional south-pole-operate, north-pole-release: It turns on with ­20 gauss (north) and turns off with 20 gauss (south). The HAL501 to HAL508 Hall-effect ICs from Micronas Intermetall (formerly, ITT Semiconductors) also use chopper offset cancellation for their Hall plates.

The seven parts of the HAL500 family offer a choice of switching points, sensitivity, and hysteresis. For example, the HAL501 turns on at 0.63 mT (milli-teslas) and turns off at ­0.63 mT (1 tesla=10,000 gauss). The reversed-polarity HAL505 turns on at ­14 mT and turns off at 14 mT for 28-mT hysteresis. The HAL508 switches on at 18 mT and switches off at 16 mT for 2-mT hysteresis. HAL500 family devices cost $0.85 (10,000).

A two-wire device from Micronas Intermetall also uses chopper cancellation for its Hall plate. The HAL556 (Figure 4) draws a high current with a magnetic south pole applied to its branded side, and it draws a low current with the magnet removed. The HAL566 offers the inverse function. A temperature-dependent bias ci  rcuit in the IC increases the supply voltage to the Hall plates and adjusts the switching points to compensate for the reduced field from most inexpensive magnets at higher temperatures. The HAL556 sensor costs $0.65 (10,000).

Some degree of hysteresis, which is the same as the gap between switching points, is usually desirable in a sensor circuit. Without hysteresis, a circuit would tend to "chatter" with stimuli at or near the threshold level. Also, as Table 1 shows, Hall ICs' operate and release points are usually fairly loosely specified, so many sensor circuits depend on the difference between the two actuating opposing poles. Panasonic's DN8762, for example, specs 1.1 to 5 mT (11 to 50 gauss) for low-to-high switching and 0.1 to 4 mT (1 to 40 gauss) for high-to-low switching. Its hysteresis ranges from 1 to 3 mT (10 to 30 gauss). The IC is clearly destined to operate on a differential basis with alternating magnetic fields. One way of using the DN8762 is to place a magnet behind it; the IC can then detect ferromagnetic material passing by its front face.

Several Hall-effect ICs use two Hall plates to provide a differential function. Allegro Microsystems' A3421 and A3422 (Figure 5), for example, contain two independent Hall cells separated by 1.5 mm. The cells connect to independent latch circuits using CMOS-based logic circuitry that decodes the speed and direction of a ring magnet. The A3421 is a high-hysteresis (typically, 335 gauss) device designed for low-resolution pulse counting. The low-hysteresis (typically, 46 gauss) A3422 provides higher sensitivity for applications using high-density magnets.

The dual-Hall-plate HAL-300 from Micronas Intermetall uses chopper magnetic-offset compensation and temperature-dependent bias to compensate for magnets' negative temperature coefficient. Its Hall plates are separated by 2 mm. The HAL300, available in SOT-89A or radial TO-92UA packages, operates with alternating magnetic fields from dc to 10 kHz. The TLE4921-3 from Siemens (Figure 6) has two Hall plates separated by 2.5 mm. The chip is intended for use with a biasing magnet. You can attach a magnet whose field ranges anywhere within ±5000 gauss. A ferromagnetic material (such as a gear tooth) passing by the device alters the flux and generates a differential signal in the IC.

This biasing-magnet arrangement is a convenient method for sensing gearwheel rotation. You can use the scheme with any of the dual-Hall-plate sensor ICs, providing that the magnet's field is within the full-scale flux range of the Hall cells. It's only natural, then, for a Hall-IC manufacturer to offer a range of self-contained magnet-IC assemblies. A large family of gear-tooth sensors from Allegro Microsystems offers a variety of bells and whistles for gearwheel position sensing:

• ATS610LSA and ATS611LSB: dynamic, peak-detecting, differential gear-tooth sensors

• ATS612LSB: a dynamic, self-calibrating, peak-detecting, differential gear-tooth sensor

• ATS632LSC: a zero-speed, self-calibrating, nonoriented gear-tooth sensor

• ATS630LSA and ATS631LSA: zero-speed, self-calibrating, true-power-on gear-tooth sensors.

Prices for the ATS6xx-family sensors start at $5.13 (10,000).

The devices discussed thus far provide a switching function--in other words, either a high or a low output. Many applications, however, can benefit from a linear output-vs-field transfer function. Typical applications for a linear sensor include current sensing, motor control, position sensing, magnetic-code reading, rotary encoders, ferrous-metal detection, vibration sensing, liquid-level sensing, and weight sensing.

Recently announced Hall-effect linear sensors from Honeywell Micro Switch and Allegro Microsystems provide a ratiometric sensing function. "Ratiometric" means that the output voltage is proportional to the supply voltage. This proportionality is useful when you couple a sensor with a ratiometric A/D converter. Because the size of the LSB in the ADC is proportional to the supply voltage, the marriage results in a transfer function that does not vary with supply level.

Honeywell's SS490 Series provides a quiescent output that equals half the supply voltage (within ±3%). Its sensitivity to magnetic fields equals 3.125 mV/gauss, ±3 or ±4%, depending on grade. The IC accommodates magnetic fields over a ±670-gauss full-scale range. The sensitivity, or gain, has a +0.02%/°C temperature coefficient, which provides compensation for the mirror-image temperature coefficient of inexpensive permanent magnets. The SS490 costs $5.65.

The A3515 and A3516 from Allegro Microsystems provide similar transfer functions of 5 and 2.5 mV/gauss, respectively. The ICs provide linear operation with fields of approximately ±800 gauss. The devices cost $2.22 and $2.13 (10,000), respectively. Maximum variation of sensitivity over temperature is 7%, ­9%. Both the Honeywell and Allegro devices are available in versions that operate from ­40 to +150°C, indicating that they're applicable to automotive applications. Note, however, that you need to design-in overvoltage protection against "load-dump" automotive conditions--the ICs spec absolute maximum supply ratings of 11 and 8V, respectively.

CMOS provides the ideal vehicle for combining digital and linear functions. Logic, memory, and D/A converters can provide compensation for the often less-than-ideal performance of CMOS linear circuitry. (Recall an oft-quoted saying: "CMOS makes great switches, lousy linear.") The MLX-90215VA (Figure 7, Reference 4) from Melexis offers complete digital programmability of every useful parameter in a linear Hall-effect sensor: quiescent output voltage, magnetic sensitivity, and temperature coefficient. The IC contains PROM, RAM, and three DACs.

The MLX90215VA IC accepts 37 bits of data through its supply and output pins. The data programs the on-chip DACs. You can adjust the offset voltage in 4.8-mV increments, thereby eliminating the need to make position adjustments for the associated magnet. Sensitivity is programmable from 0.5 to 16 mV/gauss. You can also program the temperature coefficient from 0 to 2500 ppm/°C with 5-bit (one part in 32) resolution. An automotive-grade sensor costs $3 (10,000).

To sum up, magnetic sensing is cost-effective, reliable, and easy to implement. ICs incorporating Hall-effect cells are making continuous improvements in stability, accuracy, and flexibility. Their application base (especially automotive) is expanding rapidly with no slackening in sight.

References

1. Travis, Bill, "Electromagnetic sensors put a spin on compasses," EDN, March 14, 1996, pg 77.

2. "Hall Effect IC Solutions," 1998 Catalog, Melexis Inc, pg 53.

3. Travis, Bill, "CMOS tricks stabilize "Hall-effect sensor," EDN, Sept 1, 1997, pg 22.

4. Travis, Bill, "Hall-effect sensor has programmable everything," EDN, Jan 1, 1998, pg 12.

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