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LM5101C: driver PWM input voltage maximum rating

Yang Zhou76
Prodigy 60 points
Part Number: LM5101C
Other Parts Discussed in Thread: UCC27524A

Hi, my design contains an uncommon condition, so that I want to confirm with the designer of this chip. The driver power supply VDD sometimes will disappear firstly, and in the mean time the 5V LI or HI PWM input is still there, which means when VDD=0V and LI/HI=5V, obviously this violates the spec in the table below, could you explain if this is really an issue I need to worry about. Another point here, when VDD lowers than 6.5V, the chip already in UVLO status, I can't make the decision without knowing the details of the internal structure. Thank you.  

  • 0
    TI__Mastermind 20950 points

    Hello Yang,

    I will have to research the details of the internal structure of the IC to confirm the answer to your question. But generally if the driver inputs can be higher than VDD, without concern, this will be stated in the datasheet. The example below of the UCC27524A ratings indicates the driver inputs can be higher than VDD with no concern.

    While I confirm the input pin voltage ratings and restrictions I have a couple of questions, to resolve the concern in the application.

    Do you have any resistance in series with the input pins to the driver? If the driver inputs are indeed voltage limited with a limited range to the VDD pin, one solution would be to add resistance in series with the input pins and small schottky diodes from the inputs to the VDD pin.

    Regards,

    Richard Herring

  • 0 in reply to Richard Herring
    Prodigy 60 points
    Thanks, Richard. Please do follow-up with me with your further research. Unfortunately my board doesn't have the resistor from MCU PWM to the driver input, also not able to populate the diode. If my concern is really an issue, I guess we have to redesign the PCB a little bit or figure out a way to prevent VDD from reducing to 0V earlier than input.

    Yang
  • 0 in reply to Yang Zhou76
    TI__Mastermind 20950 points

    Hello Yang,

    I will confirm with either device bench testing to see if there is a diode path from input pins to VDD, design investigation, or ideally both. If I do confirm there is a diode path we will need to determine the best reasonable method to avoid concerns with the input pins. I will certainly investigate the part today.

    Can you confirm a couple of details? What is the amplitude of the input signals to the driver inputs? Does the controller have current limited drive to the driver inputs? Usually if this is a microcontroller, the signal outputs do not have high current capability.

    Regards,

    Richard Herring

  • 0 in reply to Richard Herring
    Prodigy 60 points

    Richard,

    The amplitude PWM is 5V to the driver input from MCU, MCU io max output is limited at 5mA internally. Thanks.

    Yang 

  • 0 in reply to Yang Zhou76
    TI__Mastermind 20950 points
    Hello Yang,
    I wanted to keep you up to date on the findings. I am testing the part and so far cannot confirm there is a diode drop from input pins to VDD. I need to do more testing based on consultation, and continue to look into the structure details.
    We need to make sure there is no concern with your input voltage condition, since it does not meet the abs max table ratings.

    Regards,
    Richard Herring
  • 0 in reply to Yang Zhou76
    TI__Mastermind 20950 points

    Hello Yang,

    After some consultation with colleges who have access to the design details, the conclusion is that the LI and HI inputs higher than VDD should not be an issue.

    As general practice we recommend staying within the datasheet ratings when using TI device. Please strive to adhere to the datasheet ratings with future applications with TI devices.

    If this answers your concern, please confirm by selecting the green button on the thread.

    Regards,

    Richard Herring