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Problems in step loss with DRV8811

Other Parts Discussed in Thread: DRV8811, DRV8811EVM, DRV8825

Hi,

we're using DRV8811 for a new project driving Sonceboz bipolar stepper motor (1,5A/phase max).

In particular we purchased DRV8811 evaluation board from TI, and all work fine. After that we've realized our customized PCB board, with the same components value and a similar layout, and some probems appear.

In the same conditions (current per phase, integer step, step ratio....), it seems that with our circuit the motor have frequently some step loss; also if we perform a sequence of single step we ear a different noise and steps appear not perfectly equals. In particular after each four steps sequence, it seems that motor have a position not perfectly stable, and  we ear higher noise level. In all our test we are not able to reach the same behaviuor with DRV8811EVM and our board. Have you got some suggestion about it? Thank you very much.

  • Hi Lorenzo,

    It is a little bit hard to safely determine what may be going on here, with the information provided. However, first thing that is clearly different is the layout, which may be causing the device not miss steps because it is missing steps (e.g. the logic is not seeing a transition at the STEP input and hence the output does not commutate) but because it is entering thermal shutdown in which case the H Bridge is tri stated during the duration of the high temperature occurrence. This will of course result in steps not being commutated into the motor. So the question we must ask is what kind of thermal impedance do you have? I am not trying to get a number from you as you would require thermal simulation to gather this data point, but let me ask you this:

    1. How many layers is your custom board? two or four? Our EVM is four.

    2. What copper density is the customer board fabricated under? 1 oz or 2 oz copper? Our EVM is 2 oz copper on both external (top and bottom) layers.

    3. Did you follow the SLMA002 guidelines when designing the custom PCB? It is very important to make sure you have enough copper on your layers as this will work as a heat sink for the device. Also the vias play a role.

    4. Was the device properly soldered? A failure to solder the power pad will ensure TSD at lower current settings. Also, this device has an MSL3 rating which means it must be soldered into the board no later than a week after it has been removed from the sealed bag. If the devices have been exposed to humidity for over a week, delamination can occur. In order to bring devices back into safe levels, they need to be baked at 125C for about 8 hours.

    With regards to noise there are many contributors to audible noise on any stepper driver. You say the levels are different on the custom board than on the TI EVM. Are you sure the components are the same? Something is different. It could be:

    1. The switching frequency: You will want this frequency to be as high as possible without incurring into too much switching losses. The R and C components at the RCx pins, along with the motor inductance and power supply (VM) will determine the final frequency.

    2. The decay mode: Slow decay mode offers less audible noise due to smaller current ripple, but if you are microstepping you will then need some form of mixed decay mode.

    3. Mixed Decay Mode ratio: You will need to play with the mixed decay mode ratio to enhance the sine wave generation; you will hear the difference!

    4. Asynchronous versus Synchronous current regulation: IIf SRn is HI, the device will be configured to use the external Schottky diodes which by default implies fast decay. Fast decay is the noisiest of the three current decay modes.

    5. Current Magnitude: The higher the current, the louder the motor should sound. So make sure the VREF and the RSENSE are similar than on both cases. Also, if the layout is not properly designed, it may affect the actual RSENSE, which may be a reason to see different currents on different boards. This may be unnoticeable, though.

    At the end it is very important to understand that on any current chopper, some audible noise will exist. All you can do is minimize it to acceptable levels.

    If you want to send us a board or the Gerber files we can evaluate what else may be going on. However, in my experience, the suggestions above should solve the great majority of problems with this device.

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

    Jose Quinones

  • Hi Jose

    Thank you very much for your answer.

    Understanding your tips,I think that we may have a layout problem...

    We actually use a 2 layer PCB, 2 oz copper; thermal dissipation is made by one big vias from top to copper (that's because for the prototipe we have to manually solder components, so, in this way, we're able to completely fill vias with tin, contacting the powerpad package to bottom layer). The bottom layer transfer heat direct to to aluminium box (5mm thick), thru thermal pad of 1mm thickness.

    I suspect that our problem may be the pcb routing, in particular for R sense traces from 0,1 ohm resistors to DRV8811 pin; as you can see from attached gerber, we use a "long" way for these traces...

    May be that non simmetrycal traces is causing problems in driving the two phases?

    Is there some traces sensitive to noise created by hig current flowing in other traces?

    Please. look at attached gerber files to better understand it.

    Thank you

    7041.Gerber.zip

  • Hi Lorenzo,

    Thanks for posting the Gerbers. Any chance you can post the schematic as well? If this data is too sensitive you can send to my personal email at jquinones@ti.com

    I was not able to see the drill drawing but if you only have a single via from top to bottom, this is not going to help the thermal characteristics. I understand you are thinking about the prototyping stage, but have in mind that if it is easy to solder, that is because heat has a hard time going away from the copper structure. As much as it helps the solder to melt and stay molten as you place the device on the board, it will also not help to take the heat away from the device.

    At 1.5A I can imagine you are experiencing consecutive TSD events and that is why you are losing steps. There are a few ways to validate this. If you can add more heat sinking by either adding a large piece of metal to the top of the device or add some air flow, this may prove the problem is thermals and not noise induced by the routing.

    I do want to point out that this design is being considerably challenged by the fact this board is quite small in area, is only 2 layers and the copper are is rather small. To reach 1.5A with a design this small, you may need a lower RDSon device. Perhaps the DRV8825 can get you here, but I find it very hard to believe the DRV8811 will.

    I am not certain where the RSENSEs are, but I can not think of why this would bring the device to miss steps. It could make the current go crazy, but to miss a step is seems you will need much more iregularities. I could of course be mistaken, so I am willing to entertain this option after we rule out thermals.

    Hope the info helps. Best regards,

    Jose Quinones