[Mariss Freimanis]

dandumit,

The only switching options are "fast decay" and "slow decay". For "fast decay" the polarity on the winding is reversed and current decays at di/dt = (V supply) / (motor L). For "slow decay" the winding is shorted and current decays at di/dt = (V motor) / (motor L). Because (V supply) can be 20 times (V motor), "fast decay" can be up to 20 times faster than "slow decay".

We use "fast decay" because winding currents track the sine-cosine reference far more quickly than "slow decay".

"Mixed-Mode" means different things to different people. For Allegro, "mixed-mode" means "slow decay" from sine 0 to sine 90 and "fast decay" from sine 90 to sine 180. This introduces an unacceptable distortion in the motor current. Let's say at sine 89.9 (fast decay) current ripples between 0.8A and 1.0A making the average current 0.9A. At sine 90 the mode is changed to "slow decay" and ripple becomes 20 times less, 0.99A to 1.0A for an average current of 0.995A. The motor "sees" a near 0.1A squarewave offset current which introduces a terrible harmonic distortion (and vibration)!

For us, "mixed-mode" means using both continuously. This introduces no distortion and the motor is significantly cooler while stopped or turning very slowly. It is also a significantly more complex circuit (4 comparators instead of 2), so it must deliver a significantly greater advantage to justify the complexity. Sadly it doesn't when looking at the bigger picture. See the attached pdf.

Mariss