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LMD18200: Temperature flag pin behavior

Philippe Maletto
Prodigy 10 points
Part Number: LMD18200

Hello,

I use a LMD18200 motor driver to drive a motor.  The motor is rated 3A.  My aplication is basically a positionning application.  The motor is coupled to a gearbox and there is an encoder at the output of the gearbox.  The controlled part(gearbox's output) can go from -20 degres to 20 degres.  My application is not something very demanding, the motor runs not very often and for a short period of time.  But the temperature flag pin is going ON and OFF as soon as the motor moves.  I touch the driver and it is completely cold to the touch.  Is that normal?

Thanks

  • 0
    TI__Guru 54765 points

    Philippe,

    Does this occur on more than one device?  Just trying to rule out a damaged device.  

    You say it is going "ON" and "OFF".  Is there a particular frequency to this?  Is the output also shutting down or still running?  

    Is the tab attached to a heat sink of some sort?

    Regards,

    Ryan

  • 0 in reply to Ryan Kehr
    Prodigy 10 points

    Hello Ryan,

    It happens on more than one device yes.  I can't really say if there is a particular frequency, but I can tell that it goes ON and OFF  because in the program of my MCU I monitor that temperature flag pin and send a message over the UART when that pin become active.  As soon as the motor runs I can see a bunch of that UART message.  So the pin goes ON and OFF, if it was too hot and that pin just went ON all the time I would see only one message.

    The output is not shuting down.  The motor does its complete motion, i can confirm that it is not shuting down the output.

    It is properly attach to a heatsink yes

    thanks

  • 0 in reply to Philippe Maletto
    TI__Guru 54765 points

    Philippe,

    Can you put a scope on the pin?  Is the GND connection solid to the device and same GND as MCU?  Would be good to rule out some ground bounce issue causing this.  From what you have stated, seems like noise on that pin that is not a real thermal issue. 

    Regards,

    Ryan

  • 0 in reply to Ryan Kehr
    Prodigy 10 points

    Hi,

    I would have checked it with a scope first thing if I could have.  Unfortunately it is something I noticed on a client's board remotely, fortunately it doesn't stop anything, it only logs the messages.  It is not the end of the world, the machine still works.  Before we didn't monitor that thermal flag.  It is just that when I redesigned a board with better more powerful MCU, I intergrated all the diagnostic stuff of many componant, this thermal flag is part of those.  Everything is well grounded, I believe.  They have the same GND as well, it is a 4 layers PCB with a GND plane in middle, all GND connects to that plane.

    I noticed in the doc that it says "Junction temperature".  Where is that "junction"?  Is it possible that because it drive a kinda big motor (rated 3A continuously) the temperature at the "junction", wherever that is, nears the 145c(hense the On/OFF) but the body of the LMD18200 stays cold because it moves the motor for a short time?  Heat doesn't have time to propagate...

    I do have more of the same board but not the same motor and same setup, so I can't really put a scope on it.  The motors I have on hand are much smaller and with those the thermal flag does't go ON at all.  Checking with a scope is something I'll do when I have the same setup available.

    Thanks

  • 0 in reply to Philippe Maletto
    TI__Guru 54765 points

    Philippe,

    Yes, it is possible for a portion of the die, the power FET, to quickly hit high temp without propagating to plastic part of the package.  The monitors are built into the die itself, so if there is a hot spot...it can happen quickly.

    Still concerns me that it cycles unless it is just right there on the edge of thermal warning.  Since the device does not shutdown, it is not getting to the thermal shutdown. 

    There is no action taken by the device when hitting thermal warning other than throwing a flag.  Does the software respond to this and lower PWM for example?

    Regards,

    Ryan

  • 0 in reply to Ryan Kehr
    Prodigy 10 points

    Hello Ryan,

    No the software does not respond to this input.  It only logs it as a warning.

    I don't know if it can help you to know what is the motor I'm actually using.  But here it is:

    https://www.faulhaber.com/en/products/series/3863cr/

    It is a faulhaber 3863 serie, the 24 volts version.

    And I forgot to mention, but I also limit the current on the motor.  It can't go more than 2.5Amp.  With the MCU(pic33ep) there is function with the PWM module to compare with a current feedback and a setpoint and it automatically slow the PWM or something to keep at the current at the setpoint.

    I find it annoying also that it cycles like that because, it doesn't seems normal and it is polluting a little my log files with a bunch of those messages each time the motor is moving.

    Thank you.

  • 0 in reply to Philippe Maletto
    TI__Expert 4955 points

    Hello Phillippe, 

    Allow us to investigate and we will get back to you within 2 business days. 

    Best,

    Pedro Arango Ramirez

  • 0 in reply to Pedro Arango Ramirez
    TI__Guru 54765 points

    Philippe,

    I did not realize before that you are current limiting.  Perhaps as part of this loop, it is a real thermal warning you are receiving as the loop ramps up/down the current to hold this set point.  One test to try would be to lower this set point and see if the thermal flag goes away.

    Regards,

    Ryan

  • 0 in reply to Ryan Kehr
    TI__Guru 54765 points

    Philippe,

    Is there any more information we can help provide on this?

    Regards,

    Ryan

  • 0 in reply to Ryan Kehr
    Prodigy 10 points

    Not really I think, until I can get a setup with the same motors to replicate the issue I don't think we can do more for now.  But there is also Pedro two post above who said to allow him some time to investigate and still no answer.  I should have a complete setup in the following months, I requested to the purchasing department to buy some stuffs to recreate the same setup.

    Thanks for all the help you provided so far.

  • 0 in reply to Philippe Maletto
    TI__Guru 54765 points

    Philippe,

    Sorry about that.  I was out of the office and Pedro was covering my forum posts.  I responded when I returned.  

    I will look forward to your results.  For now, I am going to close this thread.  You can re-open it with a reply when you have a setup.  If the thread is locked due to inactivity, you can click the "Ask a related question" and it will start a new thread.  

    Regards,

    Ryan