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DRV8842Eval minimum current issue

Ahmad Zaatari
Prodigy 20 points

Hey all, 

I am using the DRV8842eval board, and I just wanted to mention an issue and ask a question regarding some current railing that I am observing.

One issue I had with this board is that it doesn't have a protection diode. I switched the polarity on the power and poof there goes my chip. I have since replaced the chip without attaching the heat sink. I was wondering why TI wouldn't include a diode for stupid mistakes like these...

On another issue, I have had problems running this drive at low currents. It seems that if I try to drive the current to low PWM, it always switches to a 0.05 A and I can't seem to drop it below that level, even if I change the Vref using the application. That is concerning to me because I am trying to control an underactuated double pendulum problem, and .05A is a lot of torque on the system which ends up oscillating quite hard. 

Thanks,

Ahmad

  • TI__Expert 7455 points

    Hi Amad,

    With regards to your low current setting, what you are experiencing is the effects of the MINIMUM TIME ON or TBLANK. What happens is that although you are telling the system to regulate to a lower current by making VREF smaller, this is only possible as long as the respective duty cycle at the outputs is such that its TIME ON period is larger than the MIN TIME ON period. The TBLANK function cannot be skipped or removed.

    With the parameters as on the board, to reach a smaller current is not practical (or possible). Remember we cannot design an EVM to tackle every possible scenario there is. However, tailoring your EVM to your particular needs is rather simple. If you want to obtain good quality microstepping at a smaller current, what you need to do is increase the SENSE resistor. By doing this, you are limiting the maximum current you can reach with the same VREF range.

    What I would do is change the SENSE resistor to something like 500 mOhms or 1 Ohm. Make sure you are measuring the current going into the motor and not the current going into the device, as these are not the same thing.

    Increasing the SENSE resistor has the implication of increasing losses on the resistor, but since the current you are wanting to supply is so tiny, this should not be a problem at all.

    Hope the info helps. Best regards,

    Jose Quinones