Shown here is a block diagram of the back-EMF detection function. On the right hand side, a half bridge topology is setup. Note that there are three such configurations for a 3-phase motor but one half bridge is shown for simplicity. The high-side and a low-side driver configuration is connected to supply and ground respectively, and the center tap voltage (phase voltage) is measured and fed back to the L99ASC03. The phase to be sampled can be selected via the microcontroller BC pin (shown as an input to the device), or via SPI. This sample is then processed by one of three comparators. A ground comparator that compares the voltage sample to ground, a supply comparator that compares to supply via SMS, or one that compares to half of the supply. If the high-side switch is being PWM, while keeping the low-side switch ON, then it is best to use the ground comparator. If the low side is being switched while keeping the high side ON, it is best to compare to supply voltage, the Vsms. This applies to low duty cycle applications. For very high duty cycle applications, whether high side or low side switch is being PWM, half supply comparator is recommended. The comparator selection can be made via SPI, after which this information is sent to a flip-flop which is triggered by either a rising edge or a falling edge. This allows back-EMF detection to occur either on turn-on or turn-off. Finally, the back-EMF value is fed back to the microcontroller via the pin “BACK-EMF OUT”. All of this processing is done internally in the L99ASC03.