DRV11873: Time to start switching/COM pin

Hi Shota,

The DC signal that you see in phase voltage is when commutation logic starts to drive the motor with one phase high, one phase low, and the third shut off. If a zero-cross is detected on the shut off phase, commutation logic advances to the next step and starts switching. After the commutation logic receives 4 continuous successful zero-crossings, it switches to normal operation.As you can see in your current signal, after 4 successful zero crossings, device starts to operate in closed loop. 

1. The time between startup and switching actually depends on how quickly the zero crossing is detected. Variables like rotor position, the phases that are chosen to switch high, low and shut-off, number of poles in the motor can impact zero-crossing detection. For example, startup time for low speed motors (high pole pairs) will be less as the angle between each poles is less and the zero-crossing can quickly be detected and commutation logic can quickly advance to the next step. However, on high speed motors (low pole pairs), startup time may be longer. When the FS pin is tied to Vcc, the DRV11873 is optimized for startup for high speed fan motors with low motor winding resistance and high inductance. When the FS pin is tied to GND, the DRV11873 is set for optimized startup for low fan speed motors with high motor winding resistance and high inductance. Setting the FS pin LOW or HIGH may impact startup time. 

2. COM pin voltage of the motor is compared to voltage on each phase of the motor. Output of this phase to COM comparator decide where to commutate the motor. See the functional block diagram in section 8.2 in datasheet.

3. There will be some leakage current flowing in COM pin depending on how the phases of the motor are balanced. 

Regards,

Vishnu