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Small Motor Drivers Slide 30

When the left transistor turns off, the voltage on it’s drain will fly back to the clamp voltage and be clamped by the diode/zener clamp. The voltage on the collector of the right transistor will go down at the same time so that the voltage across the two halves of the coil is the same. Since the clamp voltage is more than twice the supply voltage the voltage at the collector of the right transistor will go negative. In Darlington arrays, like the ULN2000 series, taking one of the collector output pins below the chip ground can cause substrate injection which can make the device operate improperly or may even lead to catastrophic failure in some devices. Such Darlington arrays that are not specifically designed to drive steppers are not recommended for this application.

PTM Published on: 2011-12-14