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DRV8885: Bypass capacitor capacity

user5287685
Prodigy 160 points
Part Number: DRV8885
Other Parts Discussed in Thread: DRV8812, DRV8824
When the bypass capacitor capacity is increased, step-out occurs.
Please tell me the mechanism.
What percentage is acceptable from 0.01uF?ask.xlsx
  • 0
    TI__Mastermind 37683 points

    Hello,

    Let me research this and I will get back to you by Nov. 18 (United States time).

  • 0 in reply to Pablo Armet
    TI__Mastermind 37683 points

    Hello,

    I tried to replicate your results in the lab but was unsuccessful. The motor was running normally even with a 0.03uF bypass capacitorv (/cfs-file/__key/communityserver-discussions-components-files/38/bypass_5F00_cap.pdf). I believe the reason for difference results could be from having different motor and set-up/equipment. 

    I want to confirm whether the behavior you are seeing is due to the bypass capacitor value or if it's due to a test setup or layout issue.

    Could you kindly provide me with the following information:

    • picture/diagram of your setup.
    • board layout/schematic (can send via private message)
    • Information on the motor used (resistance. inductance, set angle)

  • 0 in reply to Pablo Armet
    Prodigy 160 points

    Hello

    About the board layout and the motor information please reference the insert file

    I tried the 1KHz and 14.5KHz (STEP)  however, the motor was running normally

    Did the capacitor value relate to STEP?

    Could you kindly tell me why the recommended VM capacitance value is 0.01uF (mechanism)

    If  increase that value, motor will be running abnormally?

    Thank you!bypass.xlsx

  • 0 in reply to user5287685
    TI__Mastermind 37683 points

    Hello,

    Thank you for providing the information.

    I looked at your set up and the information on the file. I have a couple things I want to point out:

    • the DVDD seems to be connected to GND. can you confirm if DVDD is indeed connected to GND? If it is, then it can cause potential damage to the device. 
    • There seems to be some soldering residue on the board. This residue can cause unwanted shorting and additional resistances which can affect the performance of device. 
    • The bulk capacitor is not shown in the image. Is it connected somewhere outside of the testboard? The bulk capacitor is recommended to be placed near the supply inputs on the board.

    I suggest trying the following:

    • Clean up the residue on the board, disconnect DVDD from GND, and place the bulk capacitor near the supply input on the testboard. Then, retake your measurements to see if that helps the motor run normally.

    [Q]: Could you kindly tell me why the recommended VM capacitance value is 0.01uF (mechanism). If increase that value, motor will be running abnormally?

    [A]: The Vm capacitance value is recommended from the design team. They run multiple simulations to find the value which will work best. In your case, increasing the value from 0.01uF to 0.03uF should not cause the motor to run abnormally.

  • 0 in reply to Pablo Armet
    Prodigy 160 points
    Hello!
    Thank you for your advice

    I am sorry but the DVDD is not connected to GND (angle of the picture)

    I replaced the electrolytic capacitor(220uF) to C1 from mainboard(VM line)
    then, the motor run normally even  increasing the bypass value to 0.1uF　(C4＆C5)
    【Questions】
         1. Does the reason of the abnormality is electrolytic capacitor placed on other board?
         2. The value of bypass(C4&C5) does not affect the motor movement?
         3. What is the definition about <near the supply input> (Within ?cm)
         4. The location of electrolytic capacitor also should near by DRV8885? (Within ?cm)

    Thank you!
  • 0 in reply to user5287685
    TI__Mastermind 37683 points

    Hello,

    I am glad to hear the motor is running normally. I have the answer to your questions below:

         1. Does the reason of the abnormality is electrolytic capacitor placed on other board?
    Usually, having the bulk electrolytic capacitor farther away from the IC can add additional inductance due to longer traces/wires. This additional inductance can prevent the current from rising or decaying fast enough as required by the IC. You can observe this behavior by measuring voltage at Vm pin of the IC with bulk electrolytic capacitor at C1 and on your mainboard. There will be larger spikes in VM voltage with capacitor at mainboard (farther away from IC) when compared to capacitor at C1 (closer to IC). 
    Furthermore, you can add a local bulk capacitor (~10uF either electrolytic or ceramic) to C1 with the 220uF electrolytic cap on your main board. The local bulk will help minimize the effect of the additional inductance on Vm.
         2. The value of bypass(C4&C5) does not affect the motor movement?
    The bypass caps should not affect the motor movement if the capacitance is within the recommended value (0.01uF). In your case, using 0.03uF should not affect the motor movement.
         3. What is the definition about <near the supply input> (Within ?cm)
    The bulk capacitor should be placed near the power entry point of the board. However, if the driver is far away from power entry point of the board, then you can move the bulk capacitor closer to the driver or add an additional local bulk cap near the local bypass ceramic capacitors.
         4. The location of electrolytic capacitor also should near by DRV8885? (Within ?cm)
    Yes; the capacitor should be near the driver. The distance between the capacitor and driver has to be short enough such that the additional trace inductance is minimized.
  • 0 in reply to Pablo Armet
    Prodigy 160 points

    Hello,
    Thank you for your answer

    And i have another question about VCP PIN
    On the datasheet of DRV8812(We using) there is a 1MΩ parallel with 0.1uF in VCP PIN
    Does the 1MΩ no necessary in DRV8885？ (please refer the Excel file)
    If it is no necessary,  please tell me the reason

    Thank you!
  • 0 in reply to user5287685
    TI__Mastermind 37683 points

    Hello,

    Please take a look at this thread (e2e.ti.com/.../1908513. This thread explains the reason for the 1Mohm resistor. The post is for the DRV8824 but the concept still applies to the DRV8812.

    In short summary, adding the resistor is optional to the DRV8812. There will be no change to the device's performance or design functionality if the resistor is not added. Likewise, the resistor is also optional for the DRV8885 for the same reason.