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DRV8402 Duty Cycle

Chris B
Prodigy 120 points
Other Parts Discussed in Thread: DRV8412

Can the DRV8402 be used with a PWM duty cycle of 100%?  Whenever I hit 100% duty cycle, the outputs drop to zero.

  • 0
    Genius 5885 points

    High Chris

    Read the manual chapter "Bootstrap Capacitor Under Voltage Protection". If you are not familiar with bootstrap principle I recommend reading "Design And Application Guide for High Speed MOSFET Gate Drive Circuits" by Laszlo Balogh.

     

    Best regards, Mitja

  • 0 in reply to Mitja Nemec
    TI__Expert 4320 points

    Chris

    as Mitja writes the DRV8402 uses bootstrap drive for the high side mosfets, you can therefore not run with 100% duty cycle high. you need to limit the PWM between 2-4% and 96-98% depending on switch frequency.

    If you switch to the DRV8412 or DRV8432 you can have a constant 0 on the output - but you still need to limit the max dutycycle in order to have the bootstrap capacitor charged.

    rgds,

    K. N. Madsen

  • 0 in reply to Kim Nordtorp Madsen
    Prodigy 120 points

    I have read the datasheet, and was under the impression that the Bootstrap Capacitor Undervoltage Protection would automatically reduce the duty cycle slightly, in order to keep the bootstrap capacitors charged.  Is this not the case?  The datasheet specifically mentions that it is designed to keep the bootstrap capacitors charged in situations of low switching frequencies, and with a duty cycle of 99.5% or higher.

  • 0 in reply to Chris B
    Prodigy 30 points

    I just purchased the eval board for this part and am working with it.  I find that the board is acting as described in the datasheet as per the section you mention above.  In other words, I can run the input PWM duty cycle to 100% (PWM_A on 100%, PWM_B on 0%) and the chip seems to automatically limit the duty cycle as described.  Board is running as a full bridge, parallel operation with 48 V rails.  When the chip duty-cycle limits, the output of the board is a little over 47 volts.  Note that the data sheet says "... bootstrap capacitor [may] not charge if no load is present at output"