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The diode isn't needed if you drive directly from the ULN3808, but with the P-MOSFET buffer, you do need to protect the MOSFET.
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The diode should be connected between the drain and GROUND to improve the decay characteristics.
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The FET internal body diode, as a classic diode, isn't in the correct place.IThe FET internal body diode provides all the necessary protection in this stepper circuit.
Directly across the coil is the slow decay mode; the faster decay occurs with a resistance in series with the diode. Classically we put the resistor (or a zener) directly in series with the diode for fastest decay, but in this circuit we already have a resistor to re-use. It's the same resistor we used to decrease the time constant of the current attack time.Also incorrect, putting the diode from stepper coil to ground int his circuit puts the motor recirculating current in slow decay mode, this severely restricts the higher speeds of the motor which is what the OP requires.
The FET internal body diode, as a classic diode, isn't in the correct place.
Alain,It would be nice if you could share your new drawing with the way you think the circuit should be designed.
Alain
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