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DRV8704: Trouble driving over 3 MOSFET's per leg

Matthew Menze
Prodigy 30 points
Part Number: DRV8704

Hello,

I am using a DRV8704 to drive an H-Bridge consisting of 5 NVMFS5C426N Mosfets per leg. Each MOSFET has a total gate charge of 65nC, for a total charge of 325nC per leg. The bridge PWM frequency is going to bet set between 4000 and 8000 Hertz (based on final testing).

According to section 7.3.8, with the most aggressive settings I should be able to drive just over 700nC:

Q < (20mA * (2*DTIME + TBLANK + TOFF))/4 = (20mA*(2*880nS + 5.880uS + 133.8uS))/4 = 707.2nC

Yet, even with those settings and the relatively slow PWM frequency, I am immediately triggering a pre gate fault at anything except fully on or off.

When I depopulate the prototype to use only 3 MOSFETs per leg, it seems to work just fine, even with the default settings.

Here is a dump of the DRV's memory, for reference, with bit 11 being the leftmost bit:

DRV Crtl:   111111111011
DRV Torque: 001111111111
DRV Off:    000111111111
DRV Blank:  000011111111
DRV Decay:  001000000000
DRV Drive:  111111111111
DRV Status: 000000000000

I get a status bit 3 (APDF) = 1 as soon as I attempt to start the H-Bridge, no matter what load is attached.

Thank you for your help!
Matt





 

  • 0
    TI__Guru** 114905 points
    Hi Matt,

    Are you experiencing the predriver fault when turning on the high and low FETs?
    Can you provide the schematic of the connections from the DRV8704 to the FETs?
    Do you have series resistors between the xyLS and gates of the FETs?
    Can you provide a scope shot of the xyLS output when the FETs are turned on with 3 and 5 FETs (zoomed in to ~2 to 5us/div, and ~20us/div)?
  • 0 in reply to Rick Duncan
    Prodigy 30 points
    Hello,

    Sorry for the slow reply.

    Yes, when we turn on the FETs with anything except a DC signal (100% duty cycle). Frequency does not seem to impact this behavior at all.

    I've attached schematics of to the main post now as PDFs.

    Still working on getting you the scope shots and resistor question from the hardware team (I'm only responsible for the firmware operation, and not extremely familiar with the hardware for this board).

    Thank you!
    Matt
  • 0 in reply to Matthew Menze
    TI__Guru** 114905 points
    Hi Matt,

    I haven’t heard back from you, I’m assuming you were able to resolve your issue. If not, just post a reply below (or create a new thread if the thread has locked due to time-out)