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NEMA24 Stepper with DRV8829

Lucas Ruiz Diaz
Prodigy 20 points
Other Parts Discussed in Thread: DRV8829

Hello we are a group of Engineering students currently working on a CNC. For the project we are using two NEMA24 motors that need from 2A/Phase to 4A/Phase as maximun current (I attach some detailed specs ) and looking at Texas solutions we found DRV8829 a preatty tempting IC, since it supports 5A per phase.

Before PCB production, we needed to test the chips on a real motor to see full compliance of the specifications. So made some pcb prototypes trying to make them as simple as posible, and adding a shift register to control logic inputs, allowing us to use SPI communication protocols. 

The thing is that we manage to build everything up to test it, trying to take into consideration every detailed specification on the datasheet, successfully making the ICs to work. The thing is, after some tests they keep on failing on unknown causes. We already burned 5 of these on different boards and still not clue of what could possibly be happening.  Test conditions are:

-VM = 12V,

-Bipolar Resp.Bipolar Serial configuration (so we dont, by any chance, hit the OCP).

-Using V3P3OUT to supply VREF.

-0.4Ohm ISENSE resistance.

-and Fast decay configuration.

-Ground plane is as big as 50mmx100mm and is solder to thermal pad.

What we are experiencing is that after some time, the ICs just stop working, they dont even turn on, we stop getting any output and V3P3OUT stops working. 

Some questions.

-If it's a thermal problem is it posible the ICs burns up before the TSD jumps up?

-Is it posible that noise over OUT1/2 (product of the motor) lines may kill the ICs so easily and some more protection is needed (We only have some diodes for over and under tension)? 

-Can Glitches on VM may be cause of the problem?

-Is it posible that using ENABLE instead of Ix pins to accomplish HALF-STEP waveforms be of concern?

-What can I do or where should I look to see whats going on?

Any help will be deeply appreciated,

Lucas RD.

  • TI__Expert 7455 points

    Hi Lucas,

    My best guess is that the devices are dying because of hot spots. Although the devices are protected against high temperatures at the die level, for this protection to work the device has to be soldered into a very good thermal impedance board. The prototypes you are using may not be able to offer this path to heat. A few question to corroborate what is going on:

    1. How many layers is the board?

    2. If two or more, do you have enough vias connecting the multiple GND planes?

    3. What current are you trying to source?

    4. Can you share schematics and gerbers? If you send to jquinones@ti.com I'll be glad to take a look and see what else may be going on. I do not anticipate glitches on VM, using ENABLE to generate half stepping (AKA open loop PWM microstep generation) or noise on the outputs to cause the problems you are seeing. The device is protected against all of these occurrences.

    I have personally used this device on a CNC router I built some time ago and I know the device will be able to power up a motor of this size,  but at those currents the board's thermal impedance is beyond crucial.

    Hope the info helps. Best regards,

    Jose Quinones

  • in reply to Jose Quinones
    Prodigy 20 points

    Hi Jose,

    Im really thankful for your answer on this topic, and began to be more concern about heat dissipation on the board. Let me explain a little bit how we are making the board. Our main problem here is the elevated manufacturing PCBs costs, so prototypes are made by hand and in this case in particular, for mayor simplicity, its only a single layer board. From data-sheet I'd realized heat may be of concern thats why we left a big ground plane, (couldn't get gerber, but Im sending you some Altium Designer and schematics pictures so you can get an idea) but never thought it could be so important. Because this boards are made by hand, soldering the dissipation pad and also making some vias beneath, can be a little bit tricky, and even though the board gets warm, we have no means to check how good thermal impedance is.

    Never the less (just for testing) we are trying to source only 1.7A (placing a 0.4Ohm resistance on ISENSE and using V3P3OUT as VREF) so we don't need to dissipate that much. 

    Some questions i want to ask you:

    - Is it ok to use V3POUT as input for VREF or is straightly necessary to source it with an DigitalAnalog converter?

    - Following your line of thinking that heat its most likely to be the problem (even though I'll wait your comments on our PCBs), and taking into account our complication to make and solder proper boards, is it too crazy to solder the ICs upside down and place a huge dissipator on top of it? Would that help? or is strictly necessary to solder it to a ground plane on the PCB?

    Again, thank you very much in advance. Best regards, 

    Lucas Ruiz Diaz.

  • in reply to Lucas Ruiz Diaz
    TI__Expert 7455 points

    Hi Luacs,

    Thanks for sending the material. Seems to me your layout is pretty good for this current (1.65A which gives about 0.9W of power dissipation per drive stage). A few questions:

    1. Why do you think these devices are damaged? What behavior are you seeing which corresponds to damage?

    2. How are you soldering the Power Pad into the board? You said by hand, but are you using a hot air gun? Is the Power Pad soldered into the exposed copper slug? It is crucial for the Power Pad to be soldered into the copper with at least 50% solder coverage. If you remove the device with a hot air gun, you should be able to see some solder left over on the Power Pad. If the Power Pad is clean, the thermal impedance is that of air and that is not going to help you. At 1.7A per phase I would start expect possible bad things to happen.

    Now, you also mentioned fliping the chip and using a heat sink which interfaces directly to the Power Pad. If you can accomplish this without too much effor, this would have to be the optimal solution! You could also make a hole on the board and attach a heat sink though the board aperture. I have seen boards like this in which a rectangular aperture is routed below the Power pad and then a silicon gap pad is used so the heat sink makes good connection with the Power Pad. It may require a little bit more manufacturing, but I imagine it would be simpler than flipping the chip and bending the leads. 

    You can and should use the V3P3 output to drive your VREF inputs. This regulator was designed specifically for this function. There is no need to use an external rail, although that is perfectly fine as well. The DAC becomes only important if you want to obtain more than 32 degrees of microstepping. If you want to use the internal DAC, then V3P3OUT (and possibly a potentiometer) is all that you need.

    Hope this helps you to figure out what is going on with your system!

    Best regards,

    Jose Quinones