When the reference is decreased further, the minimum duty cycle is still applied to the output, therefore no more reduction in regulated current is possible. This introduces a non-linearity in the transfer function as shown by the orange line. If a value between zero and the minimum current is needed, the circuit will basically respond with the minimum current. In full step and half step applications, this will limit the minimum current that can be delivered to a lower value resulting in a reduction in holding torque. In microstepping, this means that it is not possible to get zero current simply by setting Vref to 0V.