AFullyIntegrated,RobustSolutionforInterfacingwithHall-Effect

A Fully Integrated, Robust Solution for Interfacing with Hall-Effect Sensors
  Maurizio Gavardoni
  Abstract: This artle provides an overview of Hall-effect sensors, particularly the use of two-wire, Hall-effect digital switches in automotive and industrial applications. It then details a method for adding diagnostic capabilities and robust protection to automotive and industrial applications using a fully integrated Hall-effect sensor interface. Test results for immunity to ground shifts are also provided.
  
  IntroductionThe Hall effect refers to the principle that when a magnetic field is applied perpendicularly to a current-carrying conductor, a difference in potential appears src="/data/attachment/portal/201007/ET33822201007230417421.gif">
  Figure 1. A block diagram and typical application circuit using the MAX9921 is shown.
  
  A robust IC, the MAX9921 is designed to withstand harsh operating environments such as the src="/data/attachment/portal/201007/ET33822201007230417422.gif">
  Figure 2. A circuit used for testing the MAX9921's immunity to ground shifts is shown.
  
  
  Figure 3. The MAX9921's current thresholds and hysteresis are shown.
  
  In the first experiment, the current flowing in the programmable load was set to 9.05mA, which is only 50µA below the threshold that would switch the MAX9921 output from low (~0V) to high (~5V with a pullup resistor connected to 5V). The ground of the programmable load was shifted up and down versus the MAX9921's ground with a DC voltage generator (V). As shown in Table 1, with VBAT = 12V, the MAX9921's threshold was immune to ground shifts up to ±8V!
  
  Table 1. Immunity to Ground Shifts with a 9.05mA Current Threshold ISENSOR (mA)VGND (V)VIN (V)VOUT (V)9.05-810.80-4-20+2+4+810.9
  A similar experiment was conducted to test the robustness of the 8.2mA high-to-low current threshold. The current was programmed to 8.25mA, and again the MAX9921 output did not switch even for ground shifts up to ±8V (Table 2).
  
  Table 2. Immunity to Ground Shifts with an 8.2mA Current Threshold ISENSOR (mA)VGND (V)VIN (V)VOUT (V)8.25-810.85-4-20+2+4+810.9
  The MAX9921's high-side current-sense topology makes the device immune to ground shifts up to ±8V between the sensor and the interface. This capability is particularly important in applications where the Hall-effect sensor is far away from the place where the logic (MAX9921 interface and microcontroller) resides. In cases like this, the MAX9921 eliminates the need for a ground wire between itself and the Hall-effect sensor, thus saving cost and space.
  
  ConclusionThe MAX9921's ground-shift immunity, input protection, ability to withstand up to 60V, and diagnostic capabilities allow the MAX9921 to provide a fully integrated, compact, and robust solution for interfacing Hall-effect sensors to microcontrollers in harsh environments, such as those found in automotive and industrial applications.