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Original question:

DRV8704EVM: Does DRV8704EVM can work with 24V, 30A motor?

DRV8704EVM: 775 Motor Contol

Michael David
Intellectual 460 points
Part Number: DRV8704EVM

Hello,

I am looking at EVAL board for my development, but I have a few questions. 

1. The board two PWM generator's. I assume there a individual controlled by the on board POTs. Is there a configuration to drive both motor outputs with one speed control POT?

2. Are the direction jumpers for forward and reverse motor direction?

3. Are the launchpad headers for any DSP eval board that also has launchpad? Meaning is the headers standard across the board?            

4. What does slow and fast decay mean? Is this similar to soft-start or soft-stop?  

Best Regards,

  • 0
    TI__Guru 55605 points

    Michael,

    1)  There is not a way to do this on the board.  You could achieve this by cutting on the POT traces and soldering over from the over POT.  This should be pretty straight forward and the schematic of the board is available online under the "Design Files" heading in the tool folder.  It is called "DRV8704EVM Hardware Files".  

    2)  There are 3 possible settings for each input and jumpers are provided for each scenario.  The 3 settings are "HIGH", "LOW", and "PWM".  The PWM duty is controlled by the POTs.  Table 3 in the User Guide shows the resulting action for the motor based on the settings for each input.  

    3)  The board is only meant to work with the MSP430 Launchpad.  See link below:

    4)  Please see figure 9 in the datasheet.  "Slow Decay" is when current is recirculating around the low-side FETs during the PWM off state (not driving).  "Fast Decay" is when current is recirculating through a low-side FET and the alternate high-side FET.  The current decays out of the load more quickly in this state, so that is where the term "fast" comes from.  Figure 10 shows the current decaying.  Typically, with a brushed motor where to desire to hold torque more steady, "Slow decay" is used.  

  • 0
    TI__Guru 56650 points

    Hello, Michael:

    1. Yes. I think we need to modify the board connection to short BINx to AINx.

    2. Yes. The direction jumpers are for forward and reverse motor direction.

    3.  The board is only meant to work with the MSP430 Launchpad.

    4. After the motor driver drives the output current reaching the current regulation point, it needs to stop driving the output current up. There are many ways to control the output current down. Slow decay shorts the output load terminal by turning on both half-bridge's low side FETs or both high side FETs. In slow decay, the winding current drops very slow since only a small voltage is across its terminal. Fast decay applies a reverse voltage on the winding terminal to make fast current drop. For brushed DC motors, the FET control would be similar between the coast mode and fast decay or between brake mode and slow decay.

  • 0 in reply to Wang5577
    Intellectual 460 points

    Hello,

    Thanks for the information. Is this EVM available for sampling?

    Best Regards,

  • 0 in reply to Michael David
    TI__Guru 55605 points

    Michael,

    Yes it is.  Link below to order.

  • 0 in reply to Ryan Kehr
    Intellectual 460 points

     did not see any mounting holes. does the board have any holes for standoffs?

  • 0 in reply to Michael David
    TI__Guru 55605 points

    No, unfortunately it does not.  

  • 0 in reply to Ryan Kehr
    Intellectual 460 points

    Is there a similar EVM that has mounting holes?

  • 0 in reply to Michael David
    TI__Guru 56650 points

    David,

    In general, the integrated FET motor driver device has a small solution size. The EVM board size is also small. I don't remember the recent release EVMs have the mounting holes. Some EVMs may have bumpons on the bottom side to lift the board up.

  • 0 in reply to Wang5577
    Intellectual 460 points

    Hello Wang,

    What are bumpons?

  • 0 in reply to Michael David
    Intellectual 460 points

    Also what is the POT control voltage range?

  • 0 in reply to Michael David
    TI__Guru 55605 points

    Michael,

    Something like this can be mounted on the bottom of the board to raise it up above your bench.  That is what Wang was referring to.

    The POT is used as a variable resistor to adjust the duty cycle of the input PWM which varies from 0-5V.  The hardware files are available online if you would like to look at the schematic.

  • 0 in reply to Ryan Kehr
    Intellectual 460 points

    Hello Ryan,

    Thanks for showing that to me. I think the bumpons will work for me. Looking at the layout file, all the components are on one side. So their is room for the adhesive to stick to the board. Is that correct?

    I was not able to truly understand fast and slow decay. Are these just two different switching schemes?  Is there a training video or white paper that will help explain the concept?  

  • 0 in reply to Michael David
    TI__Guru 55605 points

    Michael,

    That is correct.  Can easily put bumpons on bottom of board.

    Please see the following application note:

  • 0 in reply to Michael David
    Intellectual 460 points

    Hello,

    Looking at the user guide in table 3, there are only four cases where you can use the POT to control the speed. I circled them in red shown in the image below. Is this correct?

  • 0 in reply to Michael David
    TI__Guru 55605 points

    Michael,

    Yes, these are the cases where you would use the POT for speed control.  The other settings where the POT is NOT used result in full speed forward or reverse or the coast or brake states as shown in the first 4 rows of the table.  

  • 0 in reply to Ryan Kehr
    Intellectual 460 points

    Hello Ryan,

    I need a way of starting and stopping both motors at the same time. What happens if the device is in reset for long periods of time? Can this be a way to achieve this?

  • 0 in reply to Michael David
    Intellectual 460 points

    Hello,

    I need a way to start and stop both motor outputs at the same time. What happens if the device is keep in reset for long periods of time? Can reset be used in this way?

  • 0 in reply to Michael David
    TI__Guru 56650 points

    Michael,

    The output control logic table is listed in the datasheet. If they synchronizes the A bridge and B bridge's input signal, both motors can be start and stop at the same time.

    "What happens if the device is keep in reset for long periods of time?"

    Let us assume the winding current is zero after long periods rest time. Then, the condition should be same as the start-up condition.

    BTW, you can start a new thread if the thread is too long and the new question is not related to the original question. 

  • 0 in reply to Wang5577
    Intellectual 460 points

    Hello Wang,

    I understand. I was having trouble finding previous answers. Reading back through this thread found repeated questions. OK so I will close this thread out and start new ones for each question. Thank you to you and your team for answers my questions. I really appreciate your time.

    Best Regards,