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Small Motor Drivers Slide 20

Another similar device, the L6258, is specifically well suited for microstepping applications. The current control in the L6258 senses the current directly in the motor winding by inserting a sense resistor in series with the motor winding on the output of the bridge. The voltage across the sense resistor then shows both the magnitude of the current and the polarity. Using this sensing technique the true average current can be regulated so that currents at the zero crossing can be accurately regulated as well. The two DACs on the input, independently set the current in each phase so that the phase currents are independently controlled by the micro. The transfer function of the DACs are not the typical straight line linear transfer function. They are actually scaled to implement a quarter sine wave on the transfer function. This allows the micro to simply output an angle, in steps of 1/16 of 90 degrees, and the DAC multiplies that angle by the sine to get the current setting for the current control.

PTM Published on: 2011-12-14