Re: [EE]: PWM stepper driver with external bridge

On 8 Jan, CAD_CAM_EDM_DRO@egroups.com wrote:

> 0) its cheap!

A dual comparator is cheaper.

> 1) it has internal d/a for microstepping (although not perfect, its close)
> and using the internal d/a ccts makes the pic software much simpler.

No it doesn't. A simple combination of resistors connected to three
output pins on the PIC, with those pins programmed hi, lo, or tristate,
makes just as good a D2A

> 2) it can be run from up to 50v supply range which makes my buffered bridge
> circuit happy. The bridge likes to see pwm drive to within 3.5 volts of the
> power and ground rail. (no charge pumps needed for high side drive).
> see attached preliminary schematic.

Schematic removed from email, so comment difficult.

Note that from the block diagram of the a3955, it appears as if Allegro
is using the term PWM VERY loosely. It sure looks like this device uses
standard simple chopper current control.

> 3) it has bridge drive options with respect to recirculating current that
> can be handled in 3 different ways. Since this is an r&d project and my
> first high current fet design, any options can be useful in helping get the
> best performance. I believe the sequenced switching of the A and B outputs
> of the 3955 will translate to the attached bridge cct which has gate drive
> impedances selected to encourage fast turn off and slow turn on to minimize
> cross conduction shoot-through currents.

Not seeing your schematic of a3955 to output devices, I'm going to be
VERY interested in how you control the four output transistors in all
the various combinations with just the two outputs.

Remember, there are FIVE possible states of the output bridge.

Assuming the transistors are ABCD,

+
A ----- C

- motor -

B ----- D
-

that is, AB is one totempole pair, CD the other, the combinations are

ABCD Off
AD on BC off
AC on BD off
BC on AD off
BD on AC off

Note too that if the PIC directly controls the bridge drivers, a couple
of NOPs inside the PIC handles the crossconduction issues very
adequately. Also, since the PIC is outputting the reference voltage
via those few resistors mentioned above, the PIC can easily decide what
current decay strategy to use.

> 4) it has all the circuitry for current feedback and pulse width modulation
> generation internally. This saves a lot of opamp, oscillator and high
> voltage driver design.

Current feedback is a low value resistor, the oscillator is a timing
loop in the PIC, there is no need for an opamp, simply a comparator to
assist in the chopping, and bridge design is already done and available
in any decent EE reference text.

This is not that hard .. careful study of the issues and a bit of
ROUTINE PIC programming is all that's required. The PIC mostly sits in
a loop going

loop:

another step pulse ?
goto next step code

comparator gone high ?
goto current decay code

goto loop


Note that the code is small enough that I don't even try to compact
things by keeping track of what state I'm in ( 0 degrees, 45 degrees,
90 degrees, 135 degrees,, etc. ). I simply replicate the loops as
similar blocks of code ( with the appropriate changes for current
setting and driver enabling ) over and over.

When a step comes in, I check the direction bit and then jump to either
the block of code following or preceding the current block of code.

( Unfortunately, no, this code is not currently public domain.)

> 5) it is not a full bridge driver (4 outputs) but its 2 outputs to +-rail
> should drive the attached bridge cct quite well without exposing the 3955 to
> capacitive loads.

Again, put the circuit somewhere so we can all look. Still can't see
how you'll get what you want without having access to the BASEs of the
output devices inside the chip.

Alan

--

Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta