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DRV8711: Predriver fault in indexer mode

Ruixin Feng
Intellectual 1885 points
Part Number: DRV8711

Hi team,

when device will indicate predriver fault when device is in indexer mode? In datasheet description, when in indexer mode, H bridge are disabled and xPDF will be set. Does it mean that in indexer mode, when power up, predriver fault will occur? 

Customer use indexer mode and mosfet spec as attached, and configure register 0x06 as 0x005F, and then found device indicates predriver fault sometimes. could you pls help to check if customer configuration is wrong? Customer want to know why device indicated predriver fault. I also attached sch for your reference.

SPEC-00X-000XXX-00-M00133-1.pdf

Thanks~

Rayna

  • 0
    TI__Mastermind 21690 points

    Hey Rayna, 

    This answer details how the Predriver Fault function works:

    1. We monitor the external FET’s Vgs during on and off. We have an analog comparator to detect the voltage to see if the Vgs over or under 1V. 
    2. If predriver is to command the external FET to on, then after tDRIVE time(us), we will look the comparator output, if output is high, then no fault, otherwise flag a fault. 
    3. If predriver is to command the external FET to off, then after tDRIVE time(us), we will look the comparator output, if output is low, then no fault, otherwise flag a fault. 

    Then IF a fault occurs in Indexer Mode it will disable both H-bridges.  The difference between Predriver fault in Indexer mode vs PWM mode is that All H-bridges turn off, vs in PWM mode only the H-bridge experiencing the fault is turned off.  

    Here's another good question about predriver fault conditions:  https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/749003/drv8711-predriver-fault-xpdf-detection

    And some threads discussing fixes for predriver faults on this specific chip: Thread 1    Thread 2

    Do you know the circumstances where customer is experiencing predriver fault?  Is it when they are under load, accelerating, stopping, holding torque, etc? 

    Regards,

    Jacob 

  • 0
    TI__Mastermind 37683 points

    Hi Rayna,

    Is this issue observed in multiple boards? Once one driver shows the pre-driver fault, does it always flag the PDF fault? Or is the PDF fault occasionally occurs in one IC? 

    Can you provide waveform of XXHS and XXLS voltages with and without PDF fault condition. In the same capture, include the nFAULT signal? Similar to the waveform below:

    Can you try the following:

    • Set TRDIVE to the max value (µs). Typically, increasing TDRIVE helps prevent PDF faults.
    • lower IDRIVEP to 50ms and IDRIVEN to 100mA and observe if the PDF fault disappears. It is not a good idea to have the gate drive current to the highest value specially if the Q_GD is very low. This will cause a very fast rise time on the switching node which generates noise and instability. The Q_GD of your FET is 5.2nC typ and at 400mA IDRIVE, the rise time is about 13ns (this is very fast!).
    • Use equation (3) in the datasheet to check if the Q_G of your external FET is within the acceptable range. 

    Regards,

    Pablo Armet