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A3295 Datasheet(PDF) 5 Page - Allegro MicroSystems |
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A3295 Datasheet(HTML) 5 Page - Allegro MicroSystems |
5 / 9 page Chopper Stabilized, Precision Hall Effect Switch for Consumer and Industrial Applications A3295 5 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com Functional Description Amp Regulator Low- Pass Filter Figure 2. Chopper stabilization circuit (dynamic quadrature offset cancellation) Chopper-Stabilized Technique The Hall element can be considered as a resistor array similar to a Wheatstone bridge. A basic circuit is shown in figure 1, demonstrating the effect of the magnetic field flux density, B, impinging on the Hall element. When using Hall effect tech- nology, a limiting factor for switchpoint accuracy is the small signal voltage, VHALL, developed across the Hall element. This voltage is disproportionally small relative to the offset that can be produced at the output of the Hall device, caused by device overmolding, temperature dependencies, and thermal stress. A large portion of the offset is a result of the mismatching of these resistors. The A3295 uses a proprietary dynamic offset cancellation technique, with an internal high-frequency clock, to reduce the residual offset, see figure 2. The chopper-stabilizing technique cancels the mismatching of the resistor circuit by changing the direction of the current flowing through the Hall element. To do so, CMOS switches and Hall voltage measure- ment taps are used, while maintaining VHALL signal that is induced by the external magnetic flux. The signal is then captured by a sample-and-hold circuit and fur- ther processed using low-offset bipolar circuitry. This technique produces devices that have an extremely stable quiescent Hall output voltage, are immune to thermal stress, and have precise recoverability after temperature cycling. This technique will also slightly degrade the device output repeatability. A relatively high sampling frequency is used in order to process faster signals. More detailed descriptions of the circuit operation can be found on the Allegro Web site, including: Technical Paper STP 97-10, Monolithic Magnetic Hall Sensing Using Dynamic Quadrature Offset Cancellation, and Technical Paper STP 99-1, Chopper- Stabilized Amplifiers with a Track-and-Hold Signal Demodula- tor. Operation The output of the A3295 switches low (turns on) when a mag- netic field perpendicular to the Hall element transitions through and exceeds the Operate Point threshold, BOP. This is illus- trated in figure 3. After turn-on, the output is capable of sinking 25 mA, and the output voltage reaches VOUT(SAT). BHYS VOUT(off) VOUT(on)(sat) B+ V+ Hysteresis of ΔVOUT Switching Due to ΔB Figure 3. Output voltage responds to magnetic flux density. Figure 1. Hall element, basic circuit operation +V CC B –VHALL +VHALL |
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