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TXS0102: Partial Power Down

May
Genius 9880 points
Part Number: TXS0102
Other Parts Discussed in Thread: TXB0108, TXS0108E

Hi Team,

My customer have questions regarding the IC,please see details below.

I would like to have the TXS0102 handle partial power down using its IOFF feature as described in SCEA044.  When in partial power down, how does the TXS0102 avoid the damage described in SCEA060?  Does the TXS0102 not have a VCCA-to-VCCB protection diode?  Or does the "IOFF feature" disable the VCCA-to-VCCB protection diode?  How does this work?  

Document references:    

From SCEA044: "The outputs are also disabled and put into a high-impedance state under partial power down conditions and this feature is referred to as a VCC isolation feature. If VCCB = 0 V, the A-port is disabled. Likewise, if VCCA = 0 V, the B-port is disabled. For the TXS type translators that do not have IEC level ESD protection (i.e. ±15-kV Air-Gap and ±8-kV Contact discharge), they are fully specified for partial-power-down applications using the IOFF feature. This IOFF circuitry disables the outputs, preventing damaging current backflow through the device when these devices are powered down."   

From SCEA060: "TXS and TXB translators require that VCCA be less than or equal to VCCB. When this requirement is violated, there is potential for the internal protection diode to become forward biased, resulting in increased current consumption and potential damage to the device. When using separate power supplies for VCCA and VCCB, special care must be taken if VCCA and VCCB will operate at the same voltage node. The system designer must ensure that power supply tolerances will not result in a potential difference between VCCA and VCCB large enough to bias the internal protection diode."

https://www.ti.com/lit/an/scea044/scea044.pdf
https://www.ti.com/lit/an/scea060/scea060.pd

Thank you.

Regards,
May

  • 0
    Guru 316920 points

    The IOFF protection applies to the I/O pins.

    The diode is between the supply pins. If only VCCB is powered down, a current can flow into it.

  • 0 in reply to Clemens Ladisch
    TI__Genius 9880 points

    Hi Clemens,

    SCEA044 states that TXS0102 is "fully specified for partial-power-down applications" including "If VCCB = 0 V." In this case where VCCB = 0 V, how does the TXS0102 avoid damage?

    Thank you.

    Regards,
    May

  • 0 in reply to May
    Guru 316920 points

    The current is not high enough to do damage.

  • 0 in reply to Clemens Ladisch
    TI__Genius 9880 points

    Hi Clemens,

    Thank you for the information, additional confirmation.

    Amount of current in TXS0102 datasheet Table 6.5:
    ICCA: With VCCA=3.6V and VCCB=0V, max ICCA is 2.2uA. So that seems perfectly safe.

    I am still confused why SCEA060 says this: "TXS and TXB translators require that VCCA be less than or equal to VCCB. When this requirement is violated, there is potential for the internal protection diode to become forward biased, resulting in increased current consumption and potential damage to the device."

    How could 2.2uA possibly damage the device? Is this applicable only for special  translator and what value of current can possibly damage the IC.
    Thank you.

    Regards,
    May

  • +1 in reply to May
    Guru 316920 points

    Figure 4 implies that the current never becomes large. There are larger currents only for the TXS0108E and TXB0108.