4.3 Zero-Cross Detection

The zero-cross point must be detected with high precision to ensure an accurate switching point. The amplitude of the voltage present on each phase is reduced using a voltage divider and is then filtered with a low-pass RC filter.

One internal comparator (AC1) is used to compare the filtered voltages with the virtual neutral of the motor, which is provided as a sum of all the phase voltages.

The AC1 comparator used here has multiple inverting and non-inverting inputs selectable with the help of a built-in multiplexer. This feature is used to switch the AC1 inputs function by a state machine, allowing the use of one AC for all three phases and thus, reducing the number of peripherals required. Figure 4-5 describes the equivalent circuit used to detect the zero-cross point.