Re: Allegro 3977SED questions

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!
>