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TXU0204-Q1: Partial Power Down

Shinji Tsuji
Genius 12938 points
Part Number: TXU0204-Q1

Hi team,

I have a question about the description in the datasheet P21 9.3.5 as below.

"It is recommended that the inputs are kept low before floating (disconnecting) either supply."

In my system, VCCA is active while VCCB cycle ON and OFF. It is possible that Ax inputs is driven from MCU while VCCB cycles ON and OFF.

What is the reason behind your recommendation(inputs kept low before floating) ? 

Is that OK to toggle inputs(Ax) while VCCB cycles ON and OFF?

regards,

  • 0
    Guru 272340 points

    [this answer is wrong; see below]

    The default state of the outputs (when active) is low. If the current state is high while one power supply is disconnected, I guess it is possible that the data signal is disabled before the internal OE signal, and that the output glitches low for a very short time before being disabled.

  • 0 in reply to Clemens Ladisch
    TI__Genius 12938 points

    Hi,

    Does it mean there might be a glitch on output (Short Low pulse) before it becomes Hi-Z?

    Is there any reliability concern?

    Could you advise how does the device behave in my case below?

    regards,

  • 0 in reply to Shinji Tsuji
    Guru 272340 points

    Yes, the output might drive low before becoming Hi-Z. The time might not be long enough for the output voltage actually to go all the way down, but I cannot test this here.

    The reliability of the TXU itself is not affected. The only question is whether the rest of the circuit can handle this.

  • 0 in reply to Clemens Ladisch
    TI__Genius 16725 points

    Hi Shinji,

    So the reason the datasheet recommends to drive inputs low prior to floating the supply is because if your inputs are in a high state when the supply is also in a high state but floating then a situation arises where the supply is not able to discharge to a low state. This causes there to be high supply currents in the supply that is still powered.

    However, if you are ramping the VccB supply to a 0V and then applying signals to the inputs then there is no issue. There will also be no glitch on the outputs, they will directly go Hi-Z when your VccB supply approaches 0V.

    Clemen's statement is accurate for our LXC family of translators which have dynamic pulldowns and will pull the outputs low prior to placing them in a Hi-Z state.

    Regards,

    Karan

  • 0 in reply to Karan Kotadia
    TI__Genius 12938 points

    Hi Karan,

    I am not sure if I understand correctly.

    Do you mean there will be a leakage path from VCCA(active) or INA(input) to VCCB(floating) that cause high supply current

    and also which keeps VCCB high?

    regards, 

  • +1 in reply to Shinji Tsuji
    TI__Genius 16725 points

    Hi Shinji,

    So no, the leakage path is from OutB/InB to VccB. So if your OutB/InB is High, then it will keep VccB high causing high IccA currents.

    If you are floating VccA while applying signals on InA/OutB then you will similarly see high IccB currents.

    But it seems like you are floating VccB while only INA is active, then you will see no issues.

    The datasheet statement is for simplicity to avoid the confusion you are seeing here.

    Regards,

    Karan