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Problem with DRV8825 Stepper Driver

Angel Viera
Prodigy 30 points
Other Parts Discussed in Thread: DRV8825

Greetings fellow TI commuters!

We're a group of college students competing in the TI Analog Design Contest.  In our project we have to use some 400 step/revolution, 3V stepper motors with 1.7A/phase (datasheet).  We then decided to use the DRV8825 Stepper Motor Driver, which seemed like a good choice.  In the datasheet it says the DRV8825 supports up to 2.5A. Does this mean 2.5A per package or 2.5A per coil (5A total)? I ask this for a couple of reasons.  First we tried running a smaller 100 step/rev, 12V, 450mA/phase(1A-ish total) motor, which works just fine when I run it at a decent speed (less current), but when I try to do a single step or to run very slowly it, for some reason, functions weirdly and the fault condition is set at seemingly random times (which goes to about 2V even though my logic and both VREF is 3.3 volts, not sure if that's normal).

Here's what worries me. When I tried connecting the 1.7A/phase motor, everything just went kinda insane.  I was using a current limiting power supply and was slowly setting the limit closer to the motors rated current but it never actually ran (not sure if that damages the IC).  Eventually, for some unknown reason, the driver gave away and truncated VM to 8 volts, started getting hot and was probably fried at that point. This happened twice.

The circuit was built the same as in the datasheet, and the IC's had heatsinks. So was anything done wrong? Did we misinterpret the capabilities of the DRV8825? If so, what other drivers (not necessarily micro-stepping) would you recommend? 

We appreciate all your help and will take all the advice we can get.

Thanks

  • 0
    TI__Guru 55325 points

    Angel,

    Do you have a pull-up resistor on the nFAULT output?  It is an open-drain output, so will not go up to 3.3V without a pull-up.  

    What do you have your Rsense set to for current regulation?  There is an equation in the datasheet to calculate regulation current based on this value.

    The device is rated at 1.75Vrms per phase.  This is at 24V and 25C.  At elevated temperatures or higher voltages, you may see a thermal trip when driving too much current.  

  • 0 in reply to Ryan Kehr
    Prodigy 30 points

    I forgot about the pullup resistor on nFAULT, I'll be sure to add it. 

    I was using a potentiometer in Rsense trying to keep it below 1ohm but I never saw any change in the circuit.  It always faulted when I dialed the pot higher than that.  Also, I wasn't using the formula as I thought you could just calibrate it on the fly, and even though I was using this heatsink, I wasn't soldering the thermal pad to anything. (tisk,tisk).

    Now I just finished designing a new pcb following the guidelines in slma004b.  I also calculated the Rsense I should use to be 390mΩ with 3.3V VREF (both connected to 3P3VOUT) for an output of 1.69A.  Now, I have some questions:

    Should I use Rsense so that it gives me a bit more than the motors rated current or a bit less? (or exactly the same?)

    Could I use one resistor for both Isense or do they each need one?

    I attached the gerber files and eagle files for the PCB I'm making in Eagle 6.  If you could take a look at them and tell me if there's anything I should change I would really appreciate it.

    6232.DRV8825_Gerber.zip

    Thanks for the reply.