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DRV8328: DRV8328ARUYR pre-drive chip working issue

Cherry Zhou
Mastermind 22235 points
Part Number: DRV8328

Hi team,

Here's an issue from the customer may need your help:

DRV8328ARUYR pre-drive chip does not work properly. The schematic is designed by reference the EVM and as per the datasheet, when INHx INLx  all are logic 0, SHx is in a high-impedance state. During test there're 2 problems:

Q1: When the PVDD is powered at 24 V, the INHx INLx is not signaled, and all are low (power-on state), the SHx has 24.44 V to ground, is it OK?

Q2: The lower bridge is open normally, when the control signal on the upper bridge is turned on, the internal high-side PMOS is not turned on, and fault is pulled low. Because shx has voltage to ground, VBSTx has completed its bootstrap with 33 V to ground when the lower bridge is open. What's the possible reason for the fault?

Could you help check this case? Thanks.

Best Regards,

Cherry

  • 0
    TI__Mastermind 26466 points

    Hi Cherry,

    Thanks for your questions! Let me try to help answer these questions.

    1. This is not an issue, as there can be a little bit of leakage current on the SHx pin that will flow through the body diode of the high side MOSFET, pulling SHx to PVDD + diode drop. As long as the high side VGS voltage is 0V, that means the MOSFET is off and this shouldn't cause any issues to have all the sources pulled to 24.44V. 

    2. When starting the motor, you will first need to turn on the low side MOSFET before turning on the high side MOSFET in order to charge up the bootstrap capacitor to enhance the high side MOSFET. The SHx node must be pulled to ground through the low side MOSFET to properly charge the bootstrap capacitor to about 12V, and then the high side MOSFET should be able to turn on without getting a fault.

    Regards,

    Anthony Lodi

  • 0 in reply to Anthony Lodi
    TI__Mastermind 22235 points

    Hi Anthony Lodi,

    Thanks for your help.

    Anthony Lodi said:
    as there can be a little bit of leakage current on the SHx pin that will flow through the body diode of the high side MOSFET, pulling SHx to PVDD + diode drop

    Is "high-side MOSFET" referred to the external high-side NMOS? "Leakage current" refers to the GHx---SHX-Cgs of the external Uplowed NMOS, and then some current is returned to its D-side?

    Anthony Lodi said:
    2. When starting the motor, you will first need to turn on the low side MOSFET before turning on the high side MOSFET in order to charge up the bootstrap capacitor to enhance the high side MOSFET. The SHx node must be pulled to ground through the low side MOSFET to properly charge the bootstrap capacitor to about 12V, and then the high side MOSFET should be able to turn on without getting a fault.

    The lower bridge is turned on first, the Cbst is charged to GVDD-1.6V, and then the lower bridge is turned off, when the Cbst bootstrap to GVDD-1.6V+Vshx is about 32V due to problem 1. At this point, turn on the control signal to the high side and find that the Cbst falls from 33 V to 29 V, with fault pulling low.

    It is possible that Vbst undervoltage is triggered by the discharge of 10K from Rpu in Rgs(datasheet). However, the lowest value given by dataheet is 3.6 V, which is only 3 V used by the customer. After removing Rgs, fault does not pull low and the upper tube remains open. Of all the protection values, what are the rising and falling thresholds? Is Rpu necessary, need a resistor greater than 10k?

    The overcurrent protection pin is connected to GVDD with a 100K resistor, and the VDSLVL voltage tested is 6.7V greater than the maximum value given in the manual. Is it OK?

    Thanks and regards,

    Cherry

  • 0 in reply to Cherry Zhou
    TI__Mastermind 26466 points

    Hi Cherry,

    Yes, high-side MOSFET is referring to the external high side NMOS. There can be some small leakage current from the DRV out of the SHx pins that flows through the body diode of the external high side NMOS, which pulls the SHx voltage high since the leakage current is flowing from the source to the drain through the body diode. This is expected and shouldn't be a problem.

    What value for Cbst is the customer using? What is the Qg of the MOSFET? Would the customer be able to provide their schematic? 

    The external gate to source resistor is optional, and is mainly to add some passive pulldown resistance on the gate to help the gate stay off. If the customer choses to keep the pulldown resistor, I would recommend increasing it significantly instead of using the lowest value since it seems that the 10k value is putting too much demand on the bootstrap capacitor.

    The bootstrap undervoltage threshold is minimum 3.9V, with a typical value of 4.45V. You can check out the Protection Circuits section of the Electrical Characteristics table in the datasheet for more information.

    Let me check with my coworker regarding the VDSLVL pin voltage.

    Regards,

    Anthony Lodi

  • 0 in reply to Cherry Zhou
    TI__Mastermind 26466 points

    Hi Cherry, 

    What is the exact voltage that the customer is measuring at the VDSLVL pin? Can the customer confirm that there is a 100k resistor to GVDD with no other pulldown resistors to ground on the VDSLVL pin? Based on the datasheet you should expect to see a voltage on the VDSLVL pin between 3V and 5.5V with a 100k pullup to GVDD.

    Regards,

    Anthony Lodi