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UCN5804 O/P series diodes

Discussion in 'Electronic Basics' started by Nav2u, Nov 9, 2005.

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  1. Nav2u

    Nav2u Guest

    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
     
  2. Richard

    Richard Guest

    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
     
  3. 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:
    http://www.alltronics.com/download/5804.pdf

    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.
     
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