| Author |
Message |
Nav2u
Guest
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 Posted:
Wed Nov 09, 2005 9:35 am Post subject:
UCN5804 O/P series diodes |
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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
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Richard
Guest
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| Posted:
Wed Nov 09, 2005 4:40 pm Post subject:
Re: UCN5804 O/P series diodes |
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"Nav2u" <navdeep2u@gmail.com> wrote in message
news:1131519069.732434.298560@g43g2000cwa.googlegroups.com...
| Quote: | 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
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 |
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John Popelish
Guest
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| Posted:
Wed Nov 09, 2005 11:50 pm Post subject:
Re: UCN5804 O/P series diodes |
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Nav2u wrote:
| Quote: | 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
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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