A really interesting ic to drive, even with microstepping, a unipolar stepper motor is producted by Sanyo, the STK672-080. This is not a real ic but it is a small module, using small surface components inside a resin case. It is really interesting because can manage upto 2.8 amp/phase without heatsink (3.3 amp/phase with heatsink).

It is a common component used mainly for japanese copy-machines. It is characterized by:

• microstepping (upto 1/8 step);
• upto 45V supply voltage;
• upto 3.3A (2.8 without heatsink)
• step - dir - enable command pins (internal traslator);
• built-in sensing resistors;
• pwm;

and it needs almost no external components at all, only a couple of capacitors (for filtering) and a pair of resistors (for Vref settings) will complete the circuitry.

There are some limitations:

• 1/16 of steps it is not a true 'microstepping' because it is obtained generating a step on the rising and the falling edges of the signal pulse. In our environment (CNC machines), therefore, it is useless (we can consider, for this reason, a max of 1/8 step);
• max input pulse frequency is 50Khz. It means that using a 1/8 microstepping with a 200 steps motor we can obtain, at max, a theoretical 31,25 turns per second.

This is an honest driver anyway, very simple to manage, useful in many circumstances.

This is the introduction schematic proposed by Sanyo in the datasheet of the component.

To obtain a safer and pratical schematic we are going to propose a few modifications according to the  suggestions proposed by the application note.

The max input voltage, for safety, has been reduced to 40V, the phase/current is set through the Vref voltage level (from 0 to 2.5V).

The led shows the excitation state monitor.

Excitation table: