LTC2991 I2C VIH ?

Can the LTC2991 properly detect the I2C transactions (voltage levels) if the I2C SCL and SDA lines are pulled up to 3.3V while the device is being supplied (Vcc) by 5V?

Per the datasheet, the minimum I2C VIH is 0.7*Vcc, which will be 3.5V in my application (Vcc at 5V).

But I have been communicating with the LTC2991 device properly through I2C, even when it is pulled up to 3.3V.

Is there a typo in the datasheet? Should the minimum I2C VIH be 0.7*Vcc min?

Per the datasheet, the Vcc min is 2.9V, and the Vcc max is 5.5V.

Thus, if the minimum I2C VIH is 0.7*Vcc min, then it can support down to 2.03V for min I2C VIH.

Please kindly advise.

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  • 0
    •  Analog Employees 
    on Apr 14, 2021 2:53 PM

    Greetings Alee3,

       The datasheet describes these levels from the perspective that the user is using the devices' VCC as the pullup voltage level.  You should have no issues communicating using 3.3V.

    Thank you and best regards,

    Brad

  • Hi Brad,

    Thank you for your response.

    So what you are saying is that even though I have the supply voltage Vcc at 5.5V while the I2C SCL & SDA lines are pulled up to 3.3V, the LTC2991 shall properly detect the high and low logic levels of the I2C bus.

    If that is true, shouldn't the Datasheet list the min VIH for the I2C SCL & SDA lines as " 0.7*Vcc(min) " instead of " 0.7*Vcc "?

    Regards,

    Albert

  • +1
    •  Analog Employees 
    on Apr 16, 2021 8:16 PM in reply to alee3

    Greetings Albert,

       I looked into it a bit more, and the the VIH and VIL levels are indeed determined by the LTC2991 VCC.

    If the I2C bus is at 3.3V and the LTC2991 is powered at 5V VCC, then 3.3V is less than 5V * 0.7 = 3.5V.

    The VIL would be OK if driven from a 3.3V i2C bus. (3.3 * 0.3) < (5V * 0.3)

    It would probably work, but no guarantees as this is outside of the device's abs max.

    Best regards,

    Brad

Reply
  • +1
    •  Analog Employees 
    on Apr 16, 2021 8:16 PM in reply to alee3

    Greetings Albert,

       I looked into it a bit more, and the the VIH and VIL levels are indeed determined by the LTC2991 VCC.

    If the I2C bus is at 3.3V and the LTC2991 is powered at 5V VCC, then 3.3V is less than 5V * 0.7 = 3.5V.

    The VIL would be OK if driven from a 3.3V i2C bus. (3.3 * 0.3) < (5V * 0.3)

    It would probably work, but no guarantees as this is outside of the device's abs max.

    Best regards,

    Brad

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