AVR-Chat

/* my comments like this. */

----- Original Message -----
From: Wagner Lipnharski <wagner@ustr.net>
To: <AVR-Chat@yahoogroups.com>
Sent: Sunday, January 25, 2004 17:30
Subject: [AVR-Chat] Driving FET.


Hi, I am building a 4.2V @ 6A stepper motor (6 wires -
unipolar) driver, using the IRL530N or IRLZ24 power N FET.
I actually using the Alegro 2398 (twin bridge bipolar
driver) dedicated chip for small motors, that chip doesn't
stand such 6A current.

As a protection scheme, I am designing a feedback current
limit for the FETs, so it won't fry the stepper in case of a
mechanical lock or even during steady holding current.  The
main idea is to measure a 0.05 ohm resistor small voltage
drop, connecting FETs drain to ground.  The small vdrop is
to be compared with an adjusted trimpot voltage at a LM339
comparator.  The comparator will cut off the respective FET
that had overcurrent. This approach is being used with the
2398, since it has an enable/disable input per side of each
bridge.

I already came up with several possibilities, using
opto-couplers, or PNP or P Channel (BS110) small switching
transistor to fire the FETs, everything under control of an
AVR of course, but also with the current limit control by
the LM339 and additional circuitry.

Then I thought about the possibility to use some already
small dedicated chip for the job of firing the external FET.
Something that has current limit sensing, that I can connect
to the FET drain resistors, and will cut me some slack in
the board and make things better.

Perhaps some stepper motor driver, or a multiple solenoid or
small motor driver, enough to have the output as open
collector or floating emitter, enough to deal with a pull-up
or pull-down resistor at the gate of the IRL530N.

Of course that those small dedicated chips that has "step
forward, backward, etc" do not serve, since all the control
is being done already by the AVR.  All I need is something
that can deal with higher voltage at output (20V) and has
voltage sensing comparator for current limiting.

I always have the other solutions, using LM339, etc, but why
not try something ready...?

/* Why are you returning the LM339 output to +12V when
everything else is +5V?   LM339 is happy with +5V.
One other suggestion:  Place a 120R resistor in series with
the FET gate.  This suppresses oscillations. */

+AVRDrive         +5V   +12V    +5V
-------.           |      |      |
       |           |      |      |
     R10k          |     R1k    COIL
       |        |¯¯¯¯¯|   |      |
 300mV o-----(+)|     |   |   |--'  IRF530
       |        | LM  |---o---|--.  IRL530N
     R620       | 339 |       |<-|  IRLZ24
       |   .-(-)|     |          |  w/clamp diode
      _|_  |    |_____|          |
           |                     |
           '---------------------o
              300mV              |
                               R0.05
                                 |
                                _|_

R620 could be a trimpot adjusting full span current of 6A
over R0.05.
One LM339 houses 4 comparators, so one chip per unipolar
motor (4 phases & FETs).

/* So you're going to have 4 trimpots? */

Any known dedicated IC to substitute the LM339 with
advantages?

/* LM339 is OK.  This might also work and you only need one
trimpot and fewer resistors:
AVRdrive must be high at the same time the comparator is
turned OFF for the FET to turn on.

                      +AVRDrive
                          |
                  +5V     |          +5V          +5V
                   |      |           |            |
                   |      |           |            |
                   |     1k0         COIL         10k
                |¯¯¯¯¯|   |           |            |
    Vref-----(+)|     |   |        |--'  IRF530    |
                | LM  |---o--120R--|--.  IRL530N   o---Vref
                | 339 |            |<-|  IRLZ24    |  (4
places_
           +-(-)|     |               |            |
           |    |_____|               |            |
           |                          |           1k0<-----o
           '--------------------------o            |       |
                                      |            |       |
                                    R0.05         _|_
_|_
                                      |
                                     _|_
*/

Thanks.
Wagner.






Erikc  - firewevr@airmail.net
///
"An Fhirinne in aghaidh an tSaoil."
"The Truth against the World."
        -- Bardic Motto