ULN2003A: Voltage spikes from stepper motor

Part Number: ULN2003A

Hi all,

In our basic stepper motor board we are using the ULN2003 to perform little steps with a small stepper motor. Motor runs at 5V total ~200mA for only the first 4 channels (see schematic below), 5,6,7 are floating (input grounded). Runs on PWM signals from 3.3V MCU. This works perfectly fine.

However we are observing some voltage peaks in our power supply (induced emf?) then the motor is performing/or done with a step. When running from steady 5.0V...it increases to 5.8V! This is unwanted because of other components on the same 5V power rail. I prefer the 5V-line to be very stable.

Regarding the datasheet there is no need for additional diodes. I have tried to add an additional, big aluminum electrolytic capacitor from COM to GND. But no real effect.


Please advice!

Thanks in advance!
- Sander

  • Just found in the datasheet (chapter 4, page 3) for the COM port "Common cathode node for flyback diodes (required for inductive loads)". Since I work with an inductive load: Would just a simple diode between COM -> VMOT do the job?

  • Hi Sander,

    ULN2003A has integrated flyback diodes from each C to the COM pin - all you need to do in your case is connect COM directly to VMOT. Please let me know the results of this, and if you have additional questions.

    Thanks,

    Patrick

  • Hi Patrick,

    Yes, thanks for your reply. Indeed I assumed the same (integrated flybacks). So, on our board there are no additional diodes. COM is connected directly to the VMOT (5v) already.

    However, are observing voltage peaks of 5.8V from inductive load on the power rail. How to remove/mitigate this?

    Thanks, Sander

  • Hi Sander,

    What is your power source?

    And, if you disconnect COM from VMOT, do the flyback spikes on the C pins exceed 50V? If so, can you add a cap on the C pins to reduce the magnitude of the spikes? My thought is that your VMOT source is not great at sinking current, and that's why you are seeing the spikes.

    Thanks,

    Patrick

  • Hi Patrick,

    Thanks for your suggestion. The power source (on the board) is a Fixed 5.0V LDO (LP3874EMP-5.0). Gives a nice 4.995V when motor (pwm) is off.

    Have disconnected COM from the 5V VMOT power rail.
    A) Measured OUT1 and gives 5.76V pwm (max on dmm)
    B) Measured disconnected COM and that gives even 6.88V!
    C) Measured IN1 and stays nicely under 3.3V.

    What would you advice? Adding caps to the every Channel?

    Thanks!
    Sander

  • Dear Patrick,

    It turned out (after we slowly tested/removed components) it was our power supply LP387 that was generating this weird output. We replaced it, and now we got a extremely stable 5.007V output. With or without ULN2003.

    Thanks for your help with our board issue! 
    Sander

  • Hi Sander,

    Great to hear this issue is resolved! Please feel free to let me know or open a new thread if you have any additional questions!

    Thanks,

    Patrick