[jkpugh]

Update: I probably posted this in the wrong place: I just put it in the electronics forum

Greetings,

I am designing a 3 axis CNC motor driver board with PC interface using your A3979 chip and have a few questions. Data sheet is: http://www.allegromicro.com/datafile/3979.pdf

* PFD, page 3, pin 5; (percent fast-decay). I'm not what to hook it to. After reading the application notes, it sounds like it needs to go to a resistor/pot network so that turning the pot gives 0 to 1 volt to control the decay. So at <.21V gives fast decay; .21-.6mV gives mixed decay, and, over .6mV gives slow decay.
+ Is this right? Can I just pull it high to 5VDD or low, to gnd? Is this just for tuning? How would I use this?

* Fixed off-time, page 9 seem easy to calculate but what values should I start with? It looks like these various RC combination gives a MOSFET off-time of 5.6uS to 150uS.
+ Is this a one-time tuning value or should I put in a pot? Is there a "magic-bullet" off-time that will work for any situation? How can I tune it?

Last question, I promise...

* RC blanking using RC1 & RC2 says Tblank = 1400CT, page 9. I understand that the comparator needs blanking time until voltages stabilize but don't understand the formula. Same question as above:
+ Is this a one-time tuning value or should I put in a pot? Is there a "magic-bullet" blanking-time that will work for any situation? How can I tune it?

If you have any other documents on tuning this Allegro A3979 prototype board, I would like to see them, or if you know of any forums where I could post driver questions, that would be awesome. Thanks in advance for your help!!

Jeff Pugh
Field Service Engineer
www.Matrical.com
Spokane, Wa
509 998-8608
LAT 47-40-58.1452N
LON 117-19-21.2103W

[pastera]

The percent fast decay sets how rapidliy the driver can reduce the current in the coil. Fast decay pumps energy back into the power supply rapidly reducing current but increasing current ripple. At low speed you would want a higher percent of slow decay raising to 100% fast decay at high speed.

Fixed off time - will set your max chopping frequency - must be longer than your blanking time. Shorter off times will more closely regulate current at the expense of higher switching losses.

Blanking time - depends on your reverse recovery time and layout. The data sheet figure of 950ns seems a little long. This limits your minimum motor inductance.

The data sheet does not specify how the bridge is set up when the coil current is commanded to zero - this should be slow decay (shorted) for maximum resonance damping.

Aaron

[Sychad]

Does aynbody have a circuit schematic to drive this IC?

[jkpugh]

I am working on a circuit schematic and 2 layer board for an all-in-one parallel interface + 3 axis stepper driver (using the A3979 Allegro) if you are interested. What exactly do you need?

Jeff

[qgs]

Does aynbody have a circuit schematic to drive the a3979 in 3d Axis CNC bipolar stepmotor driver? Please send directly to my email address: [email protected]

Thanks

[joco10]

Please send directly to my email address: [email protected]

Thanks
joco10

[pminmo]

I built one, but found that power dissipation of the a 3979 was problematic. The pad on the bottom chip on a two layer pcb just doesn't lend itself to enough surface area to be able to get signals in and out, while allowing copper conductivity for thermal management.

[mechatronicsman]

Does aynbody have a circuit schematic to drive the a3979 in 3d Axis CNC bipolar stepmotor driver? Please send directly to my email address: [email protected]

Thanks

Hi,
I have made a stepper driver design based on A3979. See http://hobbymechatronics.com/electro...g-motor-driver where you will find complete schematics, partslist and PCB layout. These drivers where used in my sherline mill CNC conversion http://hobbymechatronics.com/project...cnc-conversion