Re: Allegro 3977SED questions

Paul,

The A3977 chip depends on the normal chopping action to prevent
excess current from harming the chip. A short circuit will kill the
chip because the chopping circuit is not fast enough to prevent a
deadly current spike. This killer spike can be less than 1
microsecond if the inductance is low enough. Normally the motor
inductance will limit the current rise time and give the chopping
circuit time to react. A short circuit of course has almost no
inductance. That being said, 3.6 mH inductance in within the normal
operating range and we have not seen problems with such motors.

The A3977 can also be easily damaged by overvoltage. While we usually
assume that chips are conservatively rated the voltage rating on the
A3977 is pretty tight. Allegro specifies 35 volts maximum and in our
testing the "smoke comes out" pretty dependably at 36.5 volts. Given
that the typical AC mains voltage can vary 10% from nominal, one
should be careful not to push the supply voltage too high without
overvoltage protection.

Having different top speeds for different step modes (half, quarter,
and eight) is common when top speed is limited by motor resonance.
Eight step would be expected to allow faster operation in this case.

Another issue that will affect top speed differently for different
step modes is the ability of the driving system to generate
accurately timed step pulses. A slow computer trying to generate
step pulses in software will have more difficulty with eight step
mode than for half step mode for the same motor RPM. This is because
it has to generate four times as many pulses in the same time frame
while not distorting the pulse to pulse timing. It may even reach its
limit and not be able to generate the pulses as fast as required. I
have seen this happen with 20 TPI lead screws and eight step mode on
a 1 GHz Pentium computer.

Regards,
Steve Stallings
www.PMDX.com


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Paul Kelly" <tech@c...>
wrote:

>
> My 2c worth here. I recently built 4 identical 3 axis boards using

these

> IC's. They work quite well.. I have however, been able to destroy a

chip by

> hooking up 3.6mH motor and winding the current limits right up.

I've also

> found that I just can't get the same top speeds out of them when

running in

> half, quarter and eighth step modes..
>
> PK