SLVSDN2E January 2018 – March 2021 DRV10974
During operation of the DRV10974 device, the FG pin provides an indication of the speed of the motor. The FG pin toggles at a rate of one time during an electrical cycle. Using this information and the number of pole pairs in the motor, use Equation 3 to calculate the mechanical speed of the motor.
During open-loop acceleration the FG pin indicates the frequency of the signal that is driving the motor. The lock condition of the motor is unknown during open-loop acceleration and therefore the FG pin could toggle during this time even though the motor is not moving.
During spin down, the DRV10974 device continues to provide speed feedback on the FG pin. The DRV10974 device provides the output of the U-phase comparator on the FG pin until the motor speed drops below 10 Hz (fFG_MIN). When the motor speed falls below 10 Hz, the device enters into the low-power mode and the FG output is held at a logic high.