AVR-Chat

-----Original Message-----
From: Wagner Lipnharski [mailto:wagner@ustr.net]
Sent: Monday, 26 January 2004 1:31 AM
To: AVR-Chat@yahoogroups.com
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...?

+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).

Any known dedicated IC to substitute the LM339 with advantages?

Thanks.

Wagner.

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