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DRV8899-Q1: Problem with driver operation in cold environment

SlawomirM
Prodigy 50 points
Part Number: DRV8899-Q1

Dear all,
I have some riddle with the system utilising DRV8899. I am using it to drive small stepper motor, and as a part of CubeSat subsystem, we were performing some TVAC testing. Motor driver works good in room temperature, and during actuation with some load it consumes 800 mA Irms. For hot case scenario (around 60 deg C), Irms goes up to around 1-1.1A. For cold case, around -5 deg C, Irms drops to around 550-600 mA and we lose a lot of torque - mechanism is stuck. 

As far I know, Irms is affected by environment temperature (Rds ON of the j bridge), and with some calculation, it looks that change in Irms is reasonable. Is this Irms drop and drop in torque caused only by mosfet temperature or is there anything else? We tried to just hold the torque and warm up the switches but it didn't solved the issue. I am thinking about adding small heater on top of the chip to warm it up before operation (if needed). Would it be typical practice, or I am missing something? The goal is to be able to operate when environment temperature is around -40 deg C. 

For the hot case, around 60 deg when Irms rise above 1 A, we get undervoltage error. In real situation, in LEO environment we won't get to this temperature inside satellite, also heatsing is quite sufficient so we shouldn't get that high in nominal operation (also I am switching with new iteration to DRV8889 which has bigger package and heatsink. Problem here is the undervoltage - we have to supply the motor driver with 5V line, and undervoltage starts to act around 4.5V. Also, this problem is strongly affected with longer cables (which is reasonable). My question here is, if the bus voltage is low, and we operate in nominal range, just keepiong bulk capacitance high, would solve the issue? Or utilising higher voltage is unevitable? In the new iteration i changed ceramic bulk capacitance to polymer tantalum but I still need to test it. 

I will be glad for any suggestions!
Best regards,
Slawomir

  • 0
    TI__Mastermind 18865 points

    Hi Slawomir,

    Thank you for posting on this forum. How is the VREF input sourced? If it was from DVDD using a resistive divider it might vary a lot with temperature compared to an external stable voltage source for VREF. For example you can input stable 3.3V to VREF and control the IFS using the TRQ_DAC setting. The IFS would remain stable over temperature changes as long as the VREF is stable over these temps. If there is sufficient head room with VM voltage vs. voltage required to push the target IFS through the windings, changes in Rdson with temperature would not impact the output current much. 

    Worst case UVLO threshold is pretty close to 4.5 V, the max. spec in the datasheet is 4.35 V for falling voltage and 4.45 V for rising voltage. Larger bulk capacitor would help if it can keep the supply above 4.5V while the motor is driven. If not, using higher voltage to overcome this issue is inevitable.

    Regards, Murugavel

      

  • 0 in reply to Murugavel4637
    Prodigy 50 points

    Hello Murugavel,
    first of all thank You for Your reply! For this Vref pin, that may be the source of problem. Actually, what I am using for Vref pin is DAC output from the microcontroller. We had some problems with resistive divider, so it was removed - dac output goes directly on Vref pin through 0 ohm resistor. As far I remember this Vref was actually controlled by current, maybe some swings of the resistance of this resistor influence this? Actually, I did some test here and attached probe to the Vref pin and I got it out from the vacuum chamber. I measured DAC output pin from the MCU, and voltage swing from room temperature to 65 deg C was ~100mV (3.4V) - I was curious here maybe internal reference for DAC is floating. Do You think that it may be this resistor? I will do some tests today.

    Regarding controlling Vref pin - TI also recommends to instead using DAC, use RC filter and PWM signal - isn't this solution even worst in regards to temperature dependence? It looks like most reliable solution will be high stability external Vref .

    Thank You for Your advice!
    Best regards,
    Slawomir

  • +1 in reply to SlawomirM
    TI__Mastermind 18865 points

    Hi Slawomir,

    I agree with your comment "It looks like most reliable solution will be high stability external Vref ". For this application use case that would be the best fit. Dedicated references come with built-in thermal compensation and their drift with temperature is very low. You can pick a standard value such as 2.5 V assume it covers your IFS easily and manage actual current using the internal TRQ_DAC via the DRV8899-Q1 register settings.

    Vref does get converted to current internally but based on our design parameters it has a very low temp coefficient single digit ppm/C. We don't expect that to cause the kind of IFS drop you were experiencing like 25 to 30 % drop in IFS from room temp to -5 deg C. There could be other reasons but this will not be one of them.  

    You mentioned "I measured DAC output pin from the MCU, and voltage swing from room temperature to 65 deg C was ~100mV (3.4V)". The current sunk on the Vref pin would be fraction of mA. I'm guessing the voltage DAC itself is drifting by 100 mV. You could measure the Vref source after disconnecting it from the Vref pin of the DRV8899. A stable dedicated voltage reference should give better results if you won't have to vary the Vref voltage with the MCU..

    If that does not address your use casec, having an heated chamber would be the last option. I hope this helps. Thanks.

    Regards, Murugavel

  • 0 in reply to Murugavel4637
    Prodigy 50 points

    Thank You for Your advice, I will focus on making sure that Vref is correct and everything around it. 
    Best Regards,
    Slawomir