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LMH1983 - SDA/SCL I2C pull up voltage when VDD is off

Kawai
Guru 19775 points
Other Parts Discussed in Thread: LMH1983

Hi Team,

Please allow me to ask you about the SDA and SCL pin voltage when VDD is off, in case if there is an exception.

Our customer will use different power supply for LMH1983 VDD and pull up voltage for I2C bus lines. They will turn off LMH1983 VDD when the device is not used in their application to lower system power consumption. However, I2C pull up voltage would be kept on.

I believe absolute maximum ratings of SDA and SCL pins are also VDD+0.3V. So would it be not allowed to apply voltage to the SDA and SCL pins when VDD is OFF ?

Thanks and Best Regards,
Kawai

  • 0
    Guru 19775 points
    We would like to know if I2C bus is still available when LMH1983 VDD is switched off.

    Thanks in advance.

    Best Regards,
    Kawai
  • 0 in reply to Kawai
    TI__Guru* 88835 points
    Kawai-San,

    SDA and SCL SMBus data and clock signals have open drain Input/Output structure. Since they are open drain or open collector, you can have some devices power off while others powered on. This will not cause a problem.

    Also, the ESD structure on these two pins are different from a regular input/output device. Therefore when one device is power off the other device that is powered on cannot forward bias the ESD diode on the device that is not power on.

    Regards,,nasser
  • 0 in reply to Nasser Mohammadi
    Guru 19775 points

    Hello Nasser-san,

    Thanks for your support.

    So, even if there is LMH1983, which does not have power supplied, connected to I2C bus, the other devices on the I2C bus could communicate correctly and LMH1983 would not have any damage. Am I correct ?

    I believe it would be better to have dedicated "Absolute maximum ratings" description for SDA and SCL pins, because, user would think VDD+0.3V would be the required spec as it says all input pins / output pins.

    Does SDA and SCL pins only have bottom side ESD protection diode ? What would be the absolute maximum rating for these pins ?

    Best Regards,
    Kawai

  • 0 in reply to Kawai
    TI__Guru* 88835 points
    Kawai-San,

    1). Your understanding is correct. Powered LMH1983 devices can communicate using SMBus even when there are other un-powered LMH1983 devices.

    2). SCL and SDA are the only signals with bottom side ESD protection. Please refer to the data sheet for the absolute maximum ratings.

    Regards,,nasser
  • 0 in reply to Nasser Mohammadi
    Guru 19775 points

    Hi Nasser-san,

    I greatly appreciate for your support. I am almost clear.

    What do you mean by refer to the data sheet for the absolute maximum ratings ?

    Regarding the absolute maximum rating specification in the data sheet, I thought there are top side ESD protection for SDA and SCL pins, too. What would be the absolute maximum ratings for these pins ? Is it 3.6V like VDD pin ?

    Best Regards,
    Kawai

  • 0 in reply to Kawai
    TI__Guru* 88835 points
    Hi Kawai-San,

    Yes it is 3.6V for the absolute maximum rating on SDA and SCL pins.

    Regards,,nasser