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DRV8412: Low Frequency Bootstrapping

Daniel Morejon
Prodigy 20 points
Part Number: DRV8412

While performing some testing on the DRV8412 and attempting to drive a single phase (A) with a 5Hz PWM signal, I noticed the below behavior (yellow signal). After the output goes HIGH, it remains HIGH for about 2.3ms before the shown downward spikes occur.

 

 

I strongly suspect that this behavior is caused by the bootstrap capacitor value and the spikes are a result of the DRV8412 BST_UVP condition.

 

I believe it to be the BST_UVP condition because the above capture was recorded for a DRV8412 with 1uF bootstrap capacitors, and a subsequent capture with 2uF bootstrap capacitors showed a 2x longer HIGH duration prior to the spikes (~4.7ms). Additionally, the spikes occur at ½ the frequency with 2uF capacitors (1kHz) as with 1uF capacitors (2kHz). See below for 2uF capture.

 

 

A couple of questions:

  • Can you please confirm if my suspicion about the BST_UVP mode is correct?
  • What is the largest possible bootstrap capacitor value?
    • Note that I already have 5.1Ω resistors for each of the 12V GVDD pins
    • Note that I am required to drive a single phase (A) from 5Hz up to 10kHz for a heater, and drive both phases (A and B) at 25kHz for driving a stepper motor

Thank you.

  • +1
    TI__Mastermind 23421 points

    Hey Daniel,

    It looks like you've followed 7.3.2.1 Bootstrap Capacitor Undervoltage Protection very well.  Agreed that it looks like the BST_UVP condition.

    In 6.3 Recommended Operating Conditions it says the bootstrap capacitor range is 33nF to 220nF.  

    Also it says the minimum PWM pulse duration on the low side to charge the bootstrap capacitor is 50ns.  So if you're trying to drive a motor at 100% duty cycle it won't work.  For 5kHz this would translate to an upper limit of 99.975% duty cycle if I did my math right.  50ns = 1/5000s * 0.00025.  So try this with a 99.9% duty cycle and see if it works for you.  

    Regards,

    Jacob Thompson