Re: Allegro 3977SED questions

So basically the PFD is something that is selected by me? I could
use 0.21V or 0.6V or something in between to set it to whatever
decay speed I want? I thought that maybe it automatically sensed a
voltage in the circuit and then adjusted the operation on its
own...but if its a steady state signal then I just pick the voltage
I need to set it in that mode. Am I understanding it correctly now?

Thank you also for the resistor values. I'm just trying to make this
as simple as possible. I'll be driving bipolar 2.0Amp motors...so I
should be able to make everything work with your suggestions and
Allegro's values for the chip.

I know there is a company out there making PCBs for this board...but
I want to do it myself. I would like the satisfaction of figuring
all this out! Its much more fun that way and makes me feel like I've
accomplished something!

Michael



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Steve Stallings"
<stevesng@n...> wrote:

>
> The PFD (percent fast decay) pin is an input to the A3977 chip. It
> has three possible modes depending on the voltage. The voltage can

be

> established using a resistor divider, but must have a 0.1 uF or
> larger capacitor to suppress noise on the input. If the voltage is
> less than 0.21 volts, the chip uses fast decay mode. If the

voltage

> is greater than 0.6 volts the chip uses slow decay mode. If the
> voltage is between these two values the chip uses mixed decay

mode.

> Think of it as a "trinary" logic input signal. It is a steady

state

> input, not a signal that must be changed as the circuit operates.
>
> The value of the current sense resistors is determined by the

maximum

> input voltage on the SENSE pins. This is 0.5 volts absolute

maximum.

> Higher voltages are more immune to noise, but will result in more
> power dissipation in the sense resistors. Selecting 0.125 ohms

would

> result in 0.3125 volts at the SENSE input for 2.5 amperes of coil
> current and 0.78125 watts maximum dissipation in the sense

resistor.

> This allows the use of a 1 watt resistor and keeps the sense

voltage

> well inside the safe zone.
>
> Regards,
> Steve Stallings
> www.PMDX.com
>
>
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "lomcevaktumble"
> <lomcevaktumble@y...> wrote:
> >
> > I'm hoping someone can help me figure all this out. I've figured
> out
> > quite a bit on this chip...but I have some questions that I

can't

> > answer myself.
> >
> > I went ahead and bought myself a few Allegro 3977 SED chips.

Most

> of
> > the stuff on the diagram I understand. But there are some things
> > that I need elaboration on.
> >
> > There is something called Percent Fast Decay or PFD. It shows a
> > place for Vpfd to be entered...and the info tells you the

allowable

> > voltage range...but the info also makes it sound like this is a
> > voltage that comes from monitoring something. I just can't

figure

> > out where to connect the Vpfd so it works correctly.
> >
> > Also Allegro is pretty good at telling you what components you
> > need...but there are 2 sense circuits that don't show what value

of

> > sense resistor or capacitor you need. Does anyone have a clue on
> > this?
> >
> > http://www.allegromicro.com/sf/3977/ has the functional diagram
> that
> > I've been looking at.
> >
> > Any help, clues, suggestions much appreciated!!!
> >
> > Thanks!
> >
>