• State Resolved
  • Locked Locked
  • Replies 7 replies
  • Subscribers 29 subscribers
  • Views 1028 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

DRV8332 questions (Bootstrap Cap Voltage Rating and some more)

Anderson Vinco
Prodigy 40 points
Other Parts Discussed in Thread: DRV8332

Hi for everyone,

This is the first time that i use the TI forum. After many searching, there are some questions didn't answered in my head. Please help me.


I'm using the DRV8332 to drive a BLDC blower. This motor has a peak of power on 192W (32V/6Amps). On my application, I'm using a PWM frequency of 72Khz, my bootstrap capacitor value is 100nF (indicated by the datasheet) and my OC_ADJ resistor is 22Kohm. If necessary, i can put here the detail schematic here.

So, my questions are:

 - What is the better voltage rating of the bootstrap capacitor?

 - At 72Khz, the value of 100nF (bootstrap capacitor) is the better choice?

 - On all of the PVDD_x pins, i'm using an 100nF ceramic cap with 50V of voltage rating, Is this the better choice? I mean, what is the better voltage rating for a cap on the PVDD pins?

An early answer will be appreciated,


Best Regards,

Anderson.

  • 0
    TI__Guru** 114905 points

    Hi Anderson,

    The bootstrap capacitor needs a minimum voltage rating of 16V. Typically the voltage across the bootstrap is approximately 12V.

    The 100nF cap is recommended per the datasheet. Please use 100nF unless the switching frequency is being reduced to less than 10kHz.

    The voltage rating on the PVDD_x pins depends on the voltage excursions on PVDD.  It is best to measure the voltage excusions on PVDD. You can start with a higher voltage rated capacitor, then lower as you learn more about your system performance.

  • 0 in reply to Rick Duncan
    Prodigy 40 points

    Hi Rick Duncan,

    First of all, thanks so much for the answers. 

    I have one more question:

     - I know that it is a good practice put an inductor or a ferrite bead between the OUT_x pin and the load. I would like to put a ferrite bead (because the space requirements are limited), but for this level of current (6Amps), what is the better choice, an inductor or a ferrite bead? 

    Thanks!

    Anderson

  • 0 in reply to Anderson Vinco
    TI__Guru** 114905 points

    Hi Anderson,

    Please refer to the section labeled "Output Inductor Selection" on page 20 of the datasheet. This section provides guidance on your question.

    Assuming your current requirements can be met, a ferrite bead is generally the preferred choice. If a ferrite bead does not met the requirements, putting two in series or an inductor is the next choice. That choice depends on size and cost.

  • 0 in reply to Rick Duncan
    Prodigy 40 points

    Hi Rick,

    I read this topic before writing the question. But when they said: "The current rating of the ferrite bead has to be higher than the RMS current of the system at normal operation", another questions arises. My motor runs at 3 Amps in the most of time. Sometime, the motor can run at 6 Amps for 2 or 3 seconds. So, my question are:

    - Do I have to consider 6 Amps as my RMS current? Or can I consider 3 Amps? 

    - For this level of current, what must be the max DC resistance of the ferrite bead?

    Best regards,

    Anderson.

  • 0 in reply to Anderson Vinco
    TI__Guru** 114905 points

    Hi Anderson,

    I thought you had read the topic prior. Thank you for doing so.

    If the motor is running at 6A for 2 to 3 seconds, you should consider the 6A as your RMS current.

    The max DC resistance is really what you are willing to accept in power losses. For example a quick search of 10A ferrite beads yielded DC resistances from 0.75mOhm to 10mOhm. At 6A, that ranges from 27mW to 360mW per output.

     

  • 0 in reply to Rick Duncan
    Prodigy 40 points

    Hi Rick,

    Just for knowledge, If the motor is running at 6 Amps for just 1 second, Or can I consider 3 Amps as RMS current? 

    I have some more questions about system design and PCB design recommendations:

    - I know that there are a lot of good practices that we can do. Like put decoupling capacitors, put ferrite bead, ground plane, PCB copper (2oz.) and some more. But i don't know if it is necessary to put an digital isolator (like the ISO7140) between the DRV8332 and the microcontroller. What you think about it? 

    Thanks so much for the answers. 

    Best regards,

    Anderson.

  • 0 in reply to Anderson Vinco
    TI__Guru** 114905 points

    Hi Anderson,

    Your question should be directed to the ferrite bead manufacturer. I suggest you contact them for clarification.

    In general, we have not seen the need for digital isolation if all signals, powers and grounds are on the same PCB. If multiple boards PCBs with separated powers and grounds are used, then isolation should be considered.