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SN74CB3T3257: Powered-off protection

user6290036
Prodigy 250 points
Part Number: SN74CB3T3257
Other Parts Discussed in Thread: SN74CBT3257C, SN74CB3Q3257
I want to use SN74CB3T3257 to switch A or B signal as shown below.
As shown below, if A and B are powered on but C is not powered, 5.5V will be applied to the signal line even though Vcc is not supplied to the IC.
After the above, does the SN74CB3T3257 operate normally when the power of C is turned on?
Since the maximum absolute rating of VIN is 7V and the recommended operating status of Vcc = 0V is not described, it is recognized that there is no problem.
3568.Figure.pptx
  • 0
    Guru 287350 points

    The recommended operation conditions of the SN74CB3T3257 do not allow a supply voltage larger than 3.6 V, and it clamps its outputs to its own VCC. You should not use the SN74CB3T3257 if you do not actually want level shifting.

    Just use a plain 5 V mux like the SN74CBT3257C. Its datasheet says:

    This device is fully specified for partial-power-down applications using Ioff. The Ioff feature ensures that damaging current will not backflow through the device when it is powered down. The device has isolation during power off.

    Please note that Ioff works only if the power supply is at 0 V, not when it's floating.

  • 0 in reply to Clemens Ladisch
    Prodigy 250 points
    Thank you for your reply.
    Please tell me 2 points.
    ① Please tell me about the case of using SN74CB3T3257.
    If EN is connected to Vcc via a pull-up resistor, is there no problem if 5.5V is applied when Vcc is not applied?

    ②Are there any other ICs that can satisfy this application (Power Point) at Vcc = 2.5V?
    We are concerned about when A and B are connected to the IC but C is not.
    Specifically, if C is not connected, there is a concern that 5.5V signals may be input from A and B without Vcc (2.5V) being supplied.
  • 0 in reply to user6290036
    Guru 287350 points

    1. The absolute maximum ratings and recommended operating conditions allow this.

    2. That is prevented by the Ioff feature.

  • 0 in reply to user6290036
    TI__Expert 5660 points

    Hello,

    1. For protection when voltage is applied to the control input pins when Vcc = 0V is typically referred to as fail-safe logic. You can learn more about this feature in this TI Precision Labs training video. 

    2. For protection when voltage is applied to the I/O signal pins when Vcc = 0V is typically referred to as powered-off protection. You can learn more about this feature in this TI Precision Labs training video.

    To confirm, do you need to pass 5.5V I/O signals while Vcc = 2.5V? If so, we need to find a device that supports input/output voltage beyond supply.

    Have you looked at SN74CB3Q3257?

    Best regards,
    Kate

  • 0 in reply to Kate Dickson
    Prodigy 250 points

    Thank you for your reply.

    >To confirm, do you need to pass 5.5V I/O signals >while Vcc = 2.5V？

    Yes.

    It seems SN74CB3Q3257 meets my request.

    In this case, it operates normally at Vcc = 2.5V, and there is no problem if 5.5V is applied to the signal line when Vcc = 0V, right?

  • 0 in reply to Clemens Ladisch
    Prodigy 250 points

    Let me confirm that my understanding is correct.

    Basically, if there is an Ioff function, there is no problem even if 5.5 V is applied to the signal line with Vcc = 0.

    However, depending on the maximum rating of the data sheet and the recommended operating conditions, if it is described on the assumption that Vcc is applied to the recommended operating conditions such as SN74CB3T3257, apply a 5.5 V signal with Vcc at 0 V It is not possible.

  • 0 in reply to user6290036
    TI__Expert 5660 points

    Hello,

    SN74CB3Q3257 is my recommendation for the following reasons:

    • With a supply voltage such as 2.5V, the switch supports up to 5.5V on the I/O signal pins (supports input/output beyond supply).
    • The switch is protected (high-impedance) when a signal up to 5.5V is applied to the I/O signal pins (powered-off protection).

    >>"Basically, if there is an Ioff function, there is no problem even if 5.5 V is applied to the signal line with Vcc = 0."

    If there is an Ioff specification (OFF leakage current) at the test condition of Vcc = 0V, the switch has powered-off protection (as indicated in my previous point).

    This specification also indicates that the switch is protected with I/O voltage up to 5.5V. This value may vary depending on the device and is indicated in the datasheet.

    Best regards,
    Kate

  • 0 in reply to Kate Dickson
    Prodigy 250 points
    Thank you for a very clear answer. I understand.
    Sorry, please tell me additionally.

    ①I want to know the ON resistance (Ron) when the SN74CB3Q3257 is used at Vcc = 2.5V and VS (VD) = 3.3 to 5.5V.
    Do you have a graph that shows the relationship between Vcc, VS, and Ron?
    ②Is there an order to apply voltage to OE, S, VDD terminal?
    ③Please let me know for reference.
    Is there an IC other than the SN74CBT3257C that operates at VDD = 5V instead of VDD = 2.5V in the same case as this question?
    I hope that 2.5V received from C will be boosted to 5V and used to increase IC options.
  • 0 in reply to user6290036
    Prodigy 250 points

    Sorry for writing continuously.

    Please tell me additionally.

    ④ When Vcc = 0V, does the SN74CB3Q3257 guarantee power-off protection up to 5.5V regardless of whether OE is 1 or 0?

  • 0 in reply to user6290036
    TI__Expert 5660 points

    Hi,

    Thank you for consolidating your questions to your single most recent post. You can expect a response from our team on that thread!