Chopper drive circuits are referred to as constant current drives because they generate a somewhat constant current in each winding rather than applying a constant voltage. Chopper operation enables microstepping.
Typical stepper motors for chopper operation are bipolar, 2-phase stepper motors. Bipolar means that both ends of each of the two coils are accessible to the driver, and the motor comes with 4 wires. To control the current, each phase connects to one MOSFET half-bridge, which can switch either end to supply voltage or ground.
The currents through both motor coils are controlled with choppers. For each chopper cycle, a very high voltage is initially applied to the winding. This causes current in the winding to rise quickly, since dI/dt = V/L. Controllers monitor current in each winding, usually by measuring voltage across a small sense resistor in series with each winding. When the current exceeds specified limits, voltage is turned off, or "chopped". When current drops below specified limits, voltage is turned back on. This approach is able to maintain relatively constant current particular step positions.
Because modern microstepping drivers implement this control loop, additional controller interaction is eliminated.