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Unipolar Hall-Effect Sensors

by Joe Gilbert, Sensor Application Engineer

Unipolar Hall-effect sensors, often referred to as Hall effect switches, operate using only a positive magnetic field. A single south pole magnet, with sufficient magnetic strength (flux density) greater than the sensor's operate point (B_OP), will cause the device to switch to its on state. Once on, the unipolar sensor will remain on until the magnetic field is reduced to below the sensors release point (B_RP). When the magnetic field reaches B_RP, or flux densities less than B_RP, the unipolar sensor will switch off (see figure 1).

Magnetic Parameters

B_OP — magnetic operate point. A positive magnetic field > B_OP will switch the sensor on (output low).

B_RP — release point. Removal of the magnetic field < B_RP will switch the sensor off (output high).

B_hys — hysteresis. B_hys = |B_OP| + |B_RP| (Hysteresis is designed into every Hall-effect switch).

Typical Operation

Figure 1 is a pictorial showing the two possible states of the unipolar digital switch. When reading the chart from left to right, at the far left the magnetic field is less than B_RP, the sensor is off, and the output voltage is high (full supply voltage). Moving to the right, at some point the magnetic field will become greater than B_OP and the sensor will then switch on. Output voltage will transition from high to low, with the low voltage (V_sat) being typically <200 mV with a normal data sheet maximum of 400 mV. As long as the magnetic field remains above B_RP the output remains low. When the magnetic field again drops below B_RP, the sensor will transition from low to high. (As stated earlier, note the designed-in hysteresis.)

Pull-Up Resistance Value

A pull-up resistor from a positive supply to the output pin is required. Common values for pull-up resistors are 1K to 10K ohms. The minimum pull-up resistance is a function of the sensors maximum output current (sink current) and the actual supply voltage. Twenty milliamps is a typical maximum output current and in this case the minimum pull-up would be V_CC/0.020 A. In some cases, where current consumption is a concern, the pull-up resistance could be as large as 50K to 100K ohms. Caution: With large pull-up values it is possible to invite external leakage currents to ground, which are high enough to drop the output voltage even when the sensor is magnetically off. This is not a sensor problem rather a leakage issue in the conductors between the pull-up resistor and the sensors output pin. Taken to the extremes this can drop the sensors output voltage enough to inhibit proper external logic function.

Power Dissipation

Total power dissipation is the sum of two factors.

1. Power consumed by the sensor IC, excluding power dissipated in the output. This value is Vcc times the supply current. V_CC is the sensor supply voltage and the supply current is specified on the data sheet. For example V_CC = 12 V, Supply current = 9 mA. Power dissipation = 12 X .009 or 108 mW.
2. Power consumed in the output transistor. This value is Vsat times the output current (set by the pull-up). If V_sat is 0.4 V (worst case) and the output current is 20 mA (often worst case), the power dissipated is 0.4 x 0.02 = 8 mW. As you can see, because of the very low saturation voltage the power dissipated in the output is not a huge concern.

Total power dissipation for this example is 108 + 8 = 116 mW. Take this number to the derating chart for the package in question and check to see if the maximum allowable operational temperature must be reduced.

Power-Up State

An important characteristic of a unipolar sensor is that the sensor will always power-up* with its output in a known state. For a magnetic field less than B_RP the sensor is normally off, and for a magnetic field greater than B_OP the sensor is normally on. This characteristic is referred to as "true power-on state" or TPOS.

Other Technical Information

Bandwidth

The bandwidth of Hall-effect sensors is typically 25 kHz to 30 kHz. In this era of 2 GHz microprocessors 25 kHz appears to be rather slow. In reality it is very rare for bandwidth to be a concern. Few mechanical systems will require or are capable of moving or spinning magnets fast enough to approach 25 kHz.

Power-Up Time

Power-up time depends to some extent on the sensor design. Digital output sensors, such as the unipolar sensor, reach stability on initial power up in the following times.

Sensor type	Power-up time
Non-chopped designs	<1 µs
Chopper-stabilized	< 40 µs

Allegro unipolar digital output data sheets often list the power-up time as <50 µs. Basically this says that prior to this time the sensors output may not be in the correct state and that after this time the output is guaranteed to be in a correct state.

Use of Bypass Capacitors

Non-chopper stabilized designs

It is recommended that 0.01 µF capacitors be placed across the output-to-ground and supply to ground pins.

Chopper-stabilized designs

A 0.1 µF capacitor must be placed across the supply-to-ground pins, and a 0.01 µF capacitor is recommended for the output to ground pins.

Frequently asked questions

Q: How do I direct the magnet?
A: The south pole is directed towards the branded face of the sensor. The branded face is where you will find the Allegro "A" as well as a partial part number and temperature code.

Q: How does the north pole or negative magnetic field affect a Unipolar sensor?
A: A north pole or negative field has no effect on a unipolar sensor.

Q: Can a very large field damage a Hall-effect sensor.
A: No. A very large field will not damage or affect an Allegro Hall-effect sensor nor will such a field add additional hysteresis (other than the designed hysteresis).

Q: Can I approach the sensor backside with the magnet?
A: Yes, however now the north pole must be directed towards the back side of the sensor. Note: The orientation of the flux field through the sensor is identical for the above two scenarios.

Q: Are there trade-offs to approaching the sensor backside.
A: Yes. The "active area depth", or the location of the Hall-element, is closer to the package front-side. For example, for the "UA" package, the sensor IC is 0.018" from the front side or 0.042" from the package backside.

Q: Why would I want a chopper-stabilized sensor?
A: Chopper-stabilized sensors have greater sensitivity with more tightly controlled switch points than non-chopped designs. This may also allow higher operational temperatures. Most new sensor designs utilize a chopped hall element.

Suggested Devices

Allegro Part Number	Temperature Range(s)	Package Type(s)	Tape & Reel Available	Comments
A1142	E, L	LH, UA	Yes	two-wire unipolar
A1143	E, L	LH, UA	Yes	two-wire unipolar
A3212	E, L	LH, UA	Yes	low power unipolar
A3240	E, L	LH, UA	Yes	unipolar
A3250	J, L	LT, UA	Yes	programmable unipolar
A3251	J, L	LT, UA	Yes	programmable unipolar

Possible Applications

• Vane interrupter switch
• Proximity sensor
• Seat position sensing
• Seat belt buckle sensor
• Open/close door or lid sensor
• Pulse counter
• Cell phone flip switch
• Speed sensor

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