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ISO7761: Inquiry about circuit configuration.

deukki shin
Prodigy 40 points

Part Number: ISO7761
Other Parts Discussed in Thread: ISO6461

Hello.

I am currently using a gate driver and an isolation IC as follows:

 - Gate driver: DRV8350RH

 - Isolation IC: ISO7761FDBQR

I have a question regarding the implementation of the nFAULT circuit.

I would like to confirm whether pull-up resistors are required on both sides of the isolation IC for the nFAULT signal.

My understanding is as follows:

Screenshot_20260221_062337_Samsung Notes.jpg

Could you please review and confirm?

Thank you.

Best regards,

  • 0
    TI__Mastermind 21225 points

    Hello,

    The OUTF channel does not need to have a pull-up to VCC. Since the INF signal will already be HIGH from the pull-up resistor, the output will also be HIGH, regardless of pullup. Another solution is to use a non-F device, such as the ISO6461 or ISO7761 (non-F), which has default HIGH.

    Regards,
    Aaditya

  • 0 in reply to Aaditya V.
    Prodigy 40 points

    Hello,

    What would happen if there is a pull-up resistor only on OUTF but not on INF in the current state?

    Would nFault fail to operate correctly?

    Thank you.

    Best regards,

  • 0 in reply to deukki shin
    TI__Mastermind 21225 points

    Hello,

    Thanks for your response. 

    This primarily depends on your system requirements and the default condition that the MCU requires in relation to the nFault signal. Having a pull-up on the output pins can add load. The resistor size should be chosen such that the device output load is in the range specified in the datasheet and well within the absolute maximum ratings. This allows there to be no damage from this setup on the isolator as well.

    I would highly suggest using a device that matches the default state the MCU or peripheral device requires. Not only will this reduce the devices and components on the board, but it will also reduce future complications. if a device change is not possible, keeping a default HIGH buffer between the output pin and the peripheral device is also a good option.

    Regards,
    Aaditya

  • 0 in reply to Aaditya V.
    Prodigy 40 points

    Hello.

    Thank you for the quick confirm.

    I have a few questions. Could you please review them for me?

    ​1. Since the gate driver's nFault output is an open-drain structure, is it possible that signal transmission fails because it cannot properly exceed the input logic threshold  if there is no external pull-up on the ISO7761F's INF pin?
    ​

    2. I have resolved this by applying a 10kΩ pull-up, but would it be more advantageous to lower the resistance to 4.7kΩ for better noise immunity in a high-power switching environment?
    ​

    3.Given the characteristics of the ISO7761F (Fail-safe Low) model, what are your thoughts on the possibility of malfunction when the input-side power supply is unstable?

    Thank you.

    Best regards,

  • 0 in reply to deukki shin
    TI__Mastermind 21225 points

    Hi,

    deukki shin said:
    Since the gate driver's nFault output is an open-drain structure, is it possible that signal transmission fails because it cannot properly exceed the input logic threshold  if there is no external pull-up on the ISO7761F's INF pin?

    Yes, your understanding is correct. That is why having a pull-up on an open drain structure is needed, and therefore the input side having a pull-up is fine. 

    deukki shin said:
    I have resolved this by applying a 10kΩ pull-up, but would it be more advantageous to lower the resistance to 4.7kΩ for better noise immunity in a high-power switching environment?

    I do not believe that changing eh resistor size would have an effect on noise performance.

    deukki shin said:
    Given the characteristics of the ISO7761F (Fail-safe Low) model, what are your thoughts on the possibility of malfunction when the input-side power supply is unstable?

    When the supply is unstable, there can be noise on the supply that momentarily could power-off the isolator or cause the supply to cross UVLO thresholds, which also could cause signal issues and bit loss. keeping the supply stable (increased filtering) will help make sure that the supply is more stable.

    Regards,
    Aaditya