Re: L297 L298 sense circuitry

Hi,

I just built a prototype 297/298 stepper driver, and spent a weekend
changing parameters and watching waveforms with a 'scope. What I
observed is that you do not need the filter on the sense path. Yes,
there is a lot of overshoot / undershoot, but the duration of the
noise is shorter than the low going portion of the set pulse to the
flip flop, so false resetting is avoided.

Daya.

--- In CAD_CAM_EDM_DRO@y..., "mariss92705" <mariss92705@y...> wrote:
> JJ
>
> Anytime a bridge switches, there will be a current spike on the
> leading edge of the change of state. Some of this will be reverse
> recovery shoot-thru diode currents, capacitive line charge and
> discharge currents, etc.
>
> The result is what happens in the first 200nS after the edge is
ugly;
> current are spikes easily 10 times what the ultimate value will be.
>
> The RC filter softens the spikes to some degree so as not to fool
the
> sense comparator into prematurely terminating the switching cycle.
>
> This works to a greater or lesser degree if the filter is chosen
> carefully. What works much better is having a blanking period. This
> means the comparator is disabled for a period of time (2uS to 5us)
> after the edge. When it re-enables, all the transients have passed
> and the sense signal is steady again.
>
> Mariss
>
> --- In CAD_CAM_EDM_DRO@y..., "JJ" <jj5412@e...> wrote:
> > Some schematics for the L297/L298 show a resistor and cap from
the
> sense
> > resistors to the L297, others show a direct connection. Can
anyone
> tell
> > me what the R/C do for you, and what values to use?
> >
> > Any help appreciated!
> >
> > ADVthanksANCE,
> > JJ
> >
> > Be Kind, Be Careful, Be Yourself