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Latching Hall-Effect Sensors
by Joe Gilbert, Sensor Application Engineer
Latching
Hall-effect sensors, often referred to as
"latches", are digital output, Hall-effect switches which
switch on with a positive magnetic field and switch off with a negative
magnetic field. Since these sensors latch on and latch off both magnetic
polarities are required for operation.
Magnetic Parameters
BOP — magnetic operate point. A positive magnetic field > BOP will switch the sensor on (output low).
BRP — release point. Removal of the magnetic field < BRP will switch the sensor off (output high).
Bhys — hysteresis. Bhys = |BOP| + |BRP| (Hysteresis is designed into every Hall-effect switch).
Typical Operation
As stated above, a positive magnetic
field >BOP will latch the sensor on. The sensor will
remain on even as the positive field is reduced to zero. Indeed, the
sensor will remain on until the field is reversed, and the negative
field reaches BRP. Once turned off at BRP the
sensor remains off until the field is again reversed and Bop is reached.
Because zero field must be crossed, hysteresis is a much larger value for latching sensors than it is for unipolar switches.
Figure 1 is a pictorial showing the two possible states of the latching sensor. Note that the center of the magnetic scale is zero gauss. Reading the chart from left to right the sensor is initially off and the output is shown to be high, which will be the full supply voltage. The output remains high until the positive field magnitude reaches BOP at which time the output switches on and the voltage drops to its low state (Vsat), typically <200 mV. If the magnetic field is reduced the sensor remains on and will remain on until the negative field magnitude reaches BRP.
Magnets
If multiple magnetic poles are desired, individual magnets may be used. However, it is usually more cost effective to use ring or strip magnet material. Ring and strip magnets are magnetized with alternating poles whose spacing is specified. A ring magnet is a donut or disc shaped assembly with alternating radially magnetized poles. Magnets can be obtained that allow radial and/or axial sensing. A strip magnet is a flat strip with alternating magnetic poles. Ring magnets are available in a variety of materials including ceramic, Rare Earth, and flexible materials. Strip magnets nearly always utilize flexible materials such as Nitrile rubber binder containing oriented Barium Ferrite, or higher energy Rare-Earth materials.
Ring magnets normally are specified as having a number of poles while strip magnets are normally specified in poles-per-inch. A four-pole ring magnet contains two north and two south oriented poles (NSNS) while an 11 pole-per-inch strip magnet has alternate poles spaced on 0.0909" centers. A variety of pole spacings are typically available from magnet manufacturers.
Pull-Up Resistor
A pull-up resistor must be connected between a positive supply and the output pin. 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; in this case the minimum pull-up would be VCC/0.020 mA. In 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 but is rather a leakage that occurs in the conductors between the pull-up resistor and the sensors output pin. Taken to the extreme, 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.
- Power consumed by the sensor IC, excluding power dissipated in the output. This value is Vcc times the supply current. VCC is the sensor supply voltage and the supply current is specified on the data sheet. For example VCC = 12 V, Supply current = 9 mA. Power dissipation = 12 x 0.009 or 108 mW.
- Power consumed in the output transistor. This value is Vsat times the output current (set by the pull-up). If Vsat 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 latching sensor is that the sensor will power-up in a valid state only if the magnetic field is >|BOP| or >|BRP|. If the magnetic field is in the hysteresis band, <|BOP| or <|BRP|, the sensor can assume either an on or off output state. The only exception to this rule is a sensor designed to power up with its output off until a magnetic transition occurs. The only latching sensor I'm aware of which meets this design criteria is the Allegro A3197.
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 latching sensor, reach stability on initial power up in the following times.
| Sensor type | Power-up time |
| Non-chopped designs (such as A3187 family) | <1 µs |
| Chopper-stabilized (such as A3280 family) | < 40 µs |
Allegro latching 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 correct (also note "Power-up state" information provided above).
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 magnet poles are 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: Can I approach the sensor backside with the magnet?
A: Yes, however, the north pole now generates a positive field
while the south pole generates the negative field. 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: Can a very large field damage a Hall-effect sensor?
A: No. A very large field will not damage an Allegro Hall-effect
sensor nor will such a field add additional hysteresis (other than
the designed hysteresis).
Q: Why would I want a chopper-stabilized sensor?
A: Chopper-stabilized sensors allow 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 |
|---|---|---|---|
| A1212 | E, L | LH, UA | Yes |
| A1214 | E, L | LH, UA | Yes |
| A3280 | E, L | LH, UA | Yes |
| A3281 | E, L | LH, UA | Yes |
Possible Applications
- Speed sensor
- Rotary Encoder
- Revolution counting
- Flow meter
- Brushless motor commutation
- Anti-pinch sunroof / window lift motor commutation