Low Power and Turns Count Sensing with the Angle Sensor IC

By | April 1, 2017

This article discusses the uses of the A1337 in battery-powered automotive and non-automotive applications with the need of the sensor to function in multiple mission modes.

Automotive Systems Requiring Sensor Operation Even in Key-Off Conditions

Certain automotive angle sensing applications require the ability to track angular position even in key-off conditions. In the key-off state, most voltage regulators in the vehicle are not operational. Therefore, sensors that must operate in the key-off state are often powered directly from the car battery (12 V). Examples of such applications include:

  • Seat-belt passive safety systems
  • EPS motor position

Often, these motor and seat-belt systems are geared down so that multiple angle sensor rotations need to be counted by the angle sensor IC. For this reason, the A1337 includes a circuit that counts the rotational turns of a magnet. When sensor ICs are connected to the car battery, they must also have low-power modes that enable efficient battery usage. Very often, a sensor IC must track the turns-count (TCs) of the magnet ecen when the vehicle is in the key-off state. The A1337 monitors and keeps track of TCs, even when set to Low Power Mode. This will ensure that the system can accurately and consistently track steering wheel position or seat-belt extension when using the A1337 in a key-on or key-off mode. Traditionally, this key-off requirement is achieved by a combination of relatively complex mechanical and electronic components. The A1337 can reduce system-level complexity and eliminate many system components by performing both the absolute angle measurement and the tracking of TCs, while maintaining low battery power consumption (85 µA) at vehicle key-off.

Figure 1

Overview of A1337

The A1337 is a 360° angle sensor IC that provides contactless high-resolution angular position information based on magnetic Circular Vertical Hall (CVH) technology. It has a system-on-chip (SoC) architecture that includes a CVH front-end, digital signal processing, and SPI, SENT, and PWM outputs. It also includes on-chip EEPROM technology for flexible end-of-line programming of calibration parameters. The A1337 is ideal for automotive applications requiring 0° to 360° angle measurements, such as electronic power steering (EPS), rotary shifters (PRNDLs), seat-belt tensioners, and throttle systems. The A1337 angle sensor device is designed to support a wide variety of applications and has multiple operating modes, organized by output format or by power consumption. By output format, the A1337 supports SPI, SENT, PWM, and Manchester interfaces. For each of these interfaces, the A1337 has the ability to report either the direct angle output (12-bit digital angle output reported over the selected output interface) or a Turns Count (TC) output, which is a quantized tracking count of the number of turns made by the magnetic target in either the clockwise or counter-clockwise direction. By power consumption, the A1337 features a Normal Power Mode, a Low Power Mode, and an ultra-low power Transport Mode.

Normal Power Mode

In Normal Power Mode, the IC draws maximum current (nominally 8.25 mA—see Normal Mode Supply Current specification in the A1337 datasheet for more details) to operate its full feature set, and updates the angle output register at the fastest rate as selected by RPM mode and AVG settings (see the A1337 programming reference for more details).

Low Power Mode

Low Power Mode is useful for battery-powered applications where the task of tracking the target’s rotation can be delineated into one of two mission modes. The first mission mode would be similar to an angle tracking mode, where the sensor tracks the output at full bandwidth and provides its measure of the angular output at full resolution. The second mission mode can be considered as a turns-tracking mode. In this mode, the sensor does not need to track the angle at full resolution—it is sufficient to track the Turns Count value of the target. The size of one turnscount unit can be preselected via EEPROM setting in the A1337 to be either 180 or 45 degrees. The A1337 tracks ±1280 turns in both directions. In Low Power Mode, the A1337 is mostly held in a lower quiescent current consumption state. The IC does not provide normal angle readings over the SPI, SENT, or PWM interfaces, but wakes up periodically to check for the occurrence of Turns Counts. The off-time of the Low Power Mode operation can be adjusted by the user based on the application, by programming on-chip EEPROM memory. Figure 2 shows Average ICC in μA versus the programmable off-time tOFF.

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