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ISO1541 What if VDD2 is 0V

Gregory Lindstrom
Prodigy 70 points
Other Parts Discussed in Thread: ISO1541

Hi,

    I am using I2C to communicate with a remote sensor.  I want to turn the 3.3V to the sensor off between readings.  If I use a ISO1541 for isolation of the I2C bus I would have 3.3V on VDD1 and 0V on VDD2 in the "off" state.  The datasheet says that would put the device in "lockout".  I have three questions:

1.) Is this a valid way to try to use the device? 

2.) How much current is the device going to draw when Vdd1 is 3.3V and Vdd2 is 0V

3.) When VDD2 comes up to 3.3V is the device going to work or could it latch up?

                                                                                                                                          Thanks,

                                                                                                                                                    Greg

  • 0
    TI__Intellectual 2855 points

    Hi Gerg,

    Thanks for your post.

    If I correctly understand your query, the sensor and VDD2 will be supplied through the same controlled power supply.

    Please find the answers below:

    1.) Is this a valid way to try to use the device? 

    Ans : Yes, Device can be used in this state, but when the VDD2 supply is 0V the device enters lockout and the output of the device will be in high impedance mode. That needs to be handled on software end also to avoid possible bus contention.

    2.) How much current is the device going to draw when Vdd1 is 3.3V and Vdd2 is 0V

    Ans: Icc1 current drawn would be lower than the Max value mentioned in table 6.10 of datasheet.

    3. )When VDD2 comes up to 3.3V is the device going to work or could it latch up?

    Ans: When VDD2 > 2.8V the device exits undervoltage lockout and returns to normal functionality within 110us (Max).

    Regards

    Tejas 

  • 0 in reply to Tejas Hommaradi
    Prodigy 70 points

    Thank you Tejas,

         Correct same power supply.

    In lockout mode which max current are we talking about?  Is it the 10 µA max input leakage current (wishful thinking) or is it supply current in the mA range?   My sensor only draws about 1.5mA peak so.....

                                                                                                                                                                                                                     Thanks,

                                                                                                                                                                                                                            Greg

                                                                                                                                                                                                                                                       

  • 0 in reply to Gregory Lindstrom
    TI__Intellectual 2855 points
    Hi Greg,

    Thanks for confirming power supply details.

    I will carry out bench measurement for this configuration and get back to you with more details.


    Regards
    Tejas H
  • 0 in reply to Tejas Hommaradi
    TI__Guru 55795 points
    Hi Greg,

    Since Side1 and Side2 are isolated from each other, current consumption from both of the Sides should be independent from each other. Expected current consumption values of each Side (ICC1 and ICC2) are mentioned in Datasheet Section 7.5 and these are in mA.

    Input Leakage Current of 10µA is the current consumed by I/O pins (SDAx or SCL1) when it is functioning as an input channel and is not the same as ICC. Since VCC2 of ISO1541 is 0V in your case, ICC2 would also be 0mA whereas for VCC1 = 3.3V, ICC1 would typically be 2.1mA as stated in the datasheet.

    Regards,
    Koteshwar Rao
  • 0 in reply to Koteshwar Rao
    Prodigy 70 points
    That is what suspected and since I'm looking for uA level power this part is not going to work. Is there a part that you know of that would?
  • 0 in reply to Gregory Lindstrom
    TI__Guru 55795 points
    Hi Greg,

    Currently we do not have an I2C device that can consume less than 1mA and I am not aware of any I2C isolator in the market that can support less than 1mA of power consumption. If it is possible, one of the options you could consider looking at is to use a load switch to disconnect power to the isolator when device is not in use.

    Regards,
    Koteshwar Rao