UCN5804 O/P series diodes

Hey guys, if you look at the datasheet of Allegro's UCN5804 typical application shows 4 diodes in series from the 4 o/p's to the 4 windings with anode towards the windings and the cathode towards the chip's o/p .....

im a bit comfused with how this works.....i know the datasheet says sumthing about substrate current but this does not make much sense to me!!!

how can the o/p from the chip even reach the windings if the diode is reverse biased???

Thanks Nav

Reply to
Nav2u
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As you mentioned, the datasheet states, "During normal commutation of a unipolar stepper motor, mutual coupling between the motor windings can force the outputs of the UCN5804B below ground. This condition will cause forward biasing of the collector-to- substrate junction and source current from the output. For many L/R applications, this substrate current is high enough to adversely affect the logic circuitry and cause misstepping. External series diodes (Schottky are recommended for increased efficiency at low-voltage operation) will prevent substrate current from being sourced through the outputs."

On the first page, it states, "The bipolar outputs are capable of sinking up to 1.5 A..."

It then appears that the diodes are shown properly since normal operation is for the output to sink, not source, current.

Hope this helps.

Richard

Reply to
Richard

It is a good idea to provide a link to the data sheet you are asking about. I hope this one is what you are asking about:

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On page 5 they show two methods that can be used to prevent the motor emf (described at the bottom of page 4) from driving the outputs more negative than the chip's negative supply rail. Since the common lead in the motor is tied to the positive supply rail, the diodes in series with each winding end are forward biased when any output pulls negative. but reverse biased if any winding produces a voltage more negative than that.

The second version, instead, places the diodes between the outputs and the negative rail to clamp any negative voltages to no more than one forward biased diode drop. I don't see any maximum negative voltage the chip can stand in the specs, but on page 4 they do recommend Schottky diodes for this method to keep the forward drop lower than the collector to substrate forward voltage drop.

Reply to
John Popelish

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