There are basically two types of PWM: the phase chopping and the inhibition chopping. The former acts only upon one of the stepper's A,B,C and D inputs, whereas the latter acts upon all. Let's see if it's clearer with an example: if we are feeding one of the bobbins of a bipolar stepper and we have input A low and input B high, then current will flow in the bobbin. If at this point there is a need to interrupt this current, for example in case of PWM, input B will be brought to a low logic level (let's suppose by a chopper) permitting the accumulated energy by the inductance to be discharged  through the circuit formed by d3 and Q1 (which is still active).

The discharge time is, in this case, quite fast for the little resistance given by the discharge circuit. But it is also possible to keep the Q4 active and turn off Q1, that is bringing A to a low level instead of B. In this case, the current generated by the bobbin would re-circulate through Q4 in order to then go through Rs and D2 and, naturally, the decadence of the current would be longer. But there is another consequence and that is that it rises dissipation (therefore the temperature) of the Rs resistor. Consequently the latter will have to be chosen of higher quality (so it will also be more expensive).

Allegro's integrated 2918 circuit (an evolution of the integrated 2917) is a motor driver which can pilot both bobbins of a single bipolar stepper motor or two motors in direct. Both driver bridges can support tensions up to 45V and currents of 1.5A which can be modulated through the PWM included in both bridges. For current modulation, the maximum value can be determined by the user through a reference tension created  by a resistance connected to an earth.

The PHASE step determines the direction of the load current while a low input in the ENABLE step controls the activation of the bridge's drivers. Bridge s include diodes for the protection of the inductive transients and delays generated internally prevent commutation over current. The chip is also provided with a circuit that will disable it in case of temperature and finally no special chip feed sequence is required. As we have already mentioned the integrated's bridges were designed to pilot both bobbins of a bipolar stepper motor, the driver's output current is adjusted for each bridge by an external adjusting resistor connected to an internal comparator that controls a flip-flop (which is also internal).

November.2001