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Electronics for CNC apparates

Last update: 31/07/2015

(L'A3986 is considered obsolete, replaced by Allegro A4989)
 The Allegro A4989 can be considered the natural replacement of the 'old' A3986 (even pin to pin compatibility).  Inside a really small package (TSSOP38) we have a functional step-dir translator and a mosfet gate driver. Using external N-channel mosfets it can be considered perfect to power really big bipolar stepper motors (upto 500W).

Main characteristics:

  • two fixed PWM current controllers;
  • step-dir interface;
  • microstepping up to 1/16 of step;
  • mixed, fast or slow decay;
  • power supply upto 50V (plus 3-5V for the logic).

The problem of this integrated driver is the package; exactly like the A3986, a TSSOP38: 36 pins on 9.6x6.3 mm. Very probably too small to create, using the traditional milling process, an in-home pcb (better to consider the use of an adapter).

The schematic is the same of the A3986.


Honestly speaking we were not sure to publish this project or not because the Allegro A3986 is a nice dual full bridge mosfet driver but it is sold only in a 38 pins TSSOP package. The size of this package is so small that it is really difficult solder it on the board without professional instruments.

Anyway this chip include only the driver for eight external MOSFET IRL540 (including current control). This fact permits to realize a controller even for quite big bipolar stepper motors (according to the datasheet from 30 upto 500W). In the repository can be downloaded a very interesting application note regarding gate driver hexfet mosfets. 

The schematics is quite traditional for an Allegro design and can be considered a base schematic to start with:

allegro a3986


a3986 brd

The board is quite small and all mosfets have been positioned aligned just to facilitate the use of a metallic heatsink (a must).

R12-R13, R14-R15 are the current sensing resistors. We have used two resistor (metallic film) on any channel because theyr values, really low, are quite difficult to find. While a 0.1ohm can be generally purchased without difficulty it is not the same for an 0.05ohm resistor.

Anyway, according to the formula, using two 0.1ohm and a Vref of 2V (ITRIP(max) = VREF / (8 × RSENSE )) we should be able to control a 5A/phase stepper motor. For higher power we have to search resistors with lower values.

While ceramic C10-C11-C12-C13 (10-20 times the mosfet gate capacitance) are capacitors whose value is connected to the mosfets used, 27nf in our case, the CREF (C9) is 1uF. These values will probably be different using other mosfets.

Microstepping and decay can be chosen according to the following table:

 Microstepping Decay 
 MS2  MS1  Microstep    PFD2  PFD1  Fast Decay
 0  0  Full step     0  0   0%
 0  1  Half step    0  1  8%
 1  0  Quarter step    1  0  26%
 1  1  Sixteenth step    1  1  100%



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