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DRV8704: Gate drive PWMing when Torque set above load limited current

Part Number: DRV8704

Hello,

I am seeing some odd behavior with bridge A of my DRV8704. When firing coil A at the same chopping current in the "on" direction the high and low gates PWM frantically. As a result I get a slightly lower average current to my load. Firing in the "off" direction, all four gates are completely on as expected and current is limited by the load resistance. My circuit is current limited by the resistance of the coil well below the commanded chopping current. I only write to the torque register and leave the 4 data MSBs set as 0's. I still noticed this issue when commenting out the code writing the Torque register. All other registers are left as the default values. I do not see this issue using the same load on coil B in either firing direction. 

/cfs-file/__key/communityserver-components-mult/cfs-file/__key/communityserver-discussions-components-files/38/Coil-Controller-Rev-1.brd

ipleuploadfilemanager/38b446fa_2D00_5478_2D00_4adf_2D00_addf_2D00_cd24969ebdfd-223229-complete/Coil-Controller-Rev-1.sch

My scope plots have 3 channels captured in them. Ch1 is a trigger signal from the controller's MCU to trigger the oscope. Channel 2 is a current probe clamped to the load coil. Channel 3 is the gate signal.

On Pulse A1HS gate signal. 

On Pulse A1LS

On Pulse A2HS

On Pulse A2LS

And an image of the off pulse for A2LS to show it isn't toggling and the difference in delivered current to the load. 

Any suggestions? In my application, I need the maximum current in order to get the actuator to fully travel. That slight degradation in the on-pulse is enough to make the actuator unreliable. Any suggestions hardware or software would be greatly appreciated. This hardware has already been delivered to my customer, I'm hoping a quick software change will fix it.   

  • Ryan,

    I just downloaded and installed the allegro 17.2 viewer. But, it cannot open your brd file and sch file. It can open my previous brd file.

    I want to check:

    1. What is the current sense resistor value and current regulation setting. 

    2. From your waveforms, around positive 11A, the DRV8704 runs into PWM mode. Around negative 12A, DRV8704 doesn't reach the current regulation point. I want to know

    a. how far the current setting point to 11A or 12A

    b. "I only write to the torque register and leave the 4 data MSBs set as 0's." If you don't change torque register setting, do you get this issue?

    c. can you try the lower ISGAIN (ISENSE amplifier gain) setting from 40V/V to other value? After the change, do you see any improvement?

  • Ryan,

    Our senior engineer found out the layout could be imported to Altium tool. Also, pointed the potential root cause of this issue.

    On the layout, the current sensing circuit should use a Kelvin connection: small current sensing trace should be connected to the sense resistor pad directly.

    xISENP or xISENS trace should be separated from main power ground copper or low side FET's source pad's copper pour. They should be connected to the sense resistor directly with separated traces.

     

    A Bridge AISENP sensing point is far away from the sense resistor. This Image shows the sense resistor in yellow circle and arrow pointing to AISENP sensing point.