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DRV8811 Filtering

James Johnston
Prodigy 50 points
Other Parts Discussed in Thread: DRV8811, DRV8811EVM

Hi!  We're using the DRV8811 in a 3-axis device and I have a few basic questions about designing in the DRV8811...

  • What kind of filtering is typically used / advised on the AOUT and BOUT pins on the DRV8811 device in order to reduce EMI problems?  The datasheet makes no mention of this but common sense seems like something would be advisable.  We would like to defensively put this in now just to reduce the chance of problems later on down the road.
  • What value would be advised for a bulk capacitor on the power supply (VMA / VMB pins)?  Obviously it will depend on the motor current; what is the typical advice here?
  • Is it advisable / necessary to put overvoltage protection on the power supply (VMA / VMB) to suppress overvoltage transients above the supply voltage?
  • We did not get a hold of the DRV8811EVM due to availability problems; unfortunately TI did not post the schematic for this EVM on the web site so the only reference we found to use was this one: http://www.avayanelectronics.com/AE-MDL-STPR1_Schematic.pdf .  Are there any obvious shortcomings / problems with that design?  (Or better, can I get a copy of the schematic from the EVM, e.g. via e-mail so I have the proven TI reference design?)
  • Drive current for our application is going to be 1.27 amps; supply of 24 volts.  Our device will be operated in a typical air-conditioned office environment and no other significant sources of heat apart from the three DRV8811 chips will be present on the application circuit board.  Thermally, can we expect to be ok using synchronous rectification, a 4-layer board with 2 oz outer layers, 1 oz inner layers and ground planes on all 4 layers with the required thermal vias - or is that pushing the limits?

Thanks in advance for any help!

  • 0
    TI__Expert 7455 points

    Hi James,

    I'll try to answer these questions in the same order they were posted:

    1. For EMI purposes a snubber consisting of a resistor in series with a cap can be placed in parallel with each of the stepper windings. Resistor value can be 10 ohms and capacitor about 470 pf or 1000 pf. At the same time, 470 pf ceramic caps can be placed from each output to GND. We have heard good results about this combination.

    2. Bulk capacitors at the VM lines could be anything from 220 uF to 470 uF.

    3. We have not seen a need to apply voltage protection at the input voltage rail. If your power supply grows close to the 40 Abs Max limit, it may make sense to add some kind of voltage supression scheme. But if your power supply is in the 24V range, there is no need to add this circuitry.

    4. We are in the works of posting the entire EVM CD content on the respective device web page. The CD contains all the information regarding the EVM including its schematic.  I could also send you a copy of the schematic to your email.  

    Avayan Electronic's design is electrically speaking a good representation of how to design around the DRV8811, but the bare board they offer lacks a series of aspects you would require if trying to run the driver at high current. For starters, the board is only two layers, where 4 is often recommended. Their board is also 1 ounce copper, when 2 ounce copper is optimal. The size of the board does not help either. Ideally, you would have a large board space to aid with the thermal impedance. If you will be running below 1A current per phase, the board may be suitable. Above this level, I do not foresee the board operating properly for prolonged periods of time without entering thermal shutdown.

    5. Your design requirements are what I would consider pure beauty! You are by no means pushing the limits. In fact, with the four layers and external layer density of 2 ounces of copper, I would anticipate it being fairly possible to reach the 2A mark without suffering from thermal shutdown.

    Please let me know if there is anything else I can help you with. Best regards,

    Jose I Quinones

  • 0 in reply to Jose Quinones
    Prodigy 50 points

    Thanks for the response; that clears things up a good bit!

    1. I would assume that ferrites are also helpful / advisable in addition to the RC snubber? (e.g. ferrite chips on each output, and/or ferrite around the cable assembly to the motor, placed near the controller)
    2. Looks like I'll use three 220 uF capacitors: one per motor controller.
    3. I added a 24 volt TVS for ESD purposes (starts clamping at around 27 or 28 volts or so).  I suppose this can safely double as a protection here at any rate.
    4. Thanks! - sending you an e-mail....
    5. Good to know. :)  My prototype motors had 1.5A RMS and on the Avayan boards I had to attach small BGA heatsinks to the boards to get them to run that much.  But we don't run high RPMs for our application so I think a different, lower-current motor winding is in order: we need by far the most torque when running at only 1 or 2 RPS.

    Best regards,

    James Johnston