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AM3358: USB pins protection

Ulf Samuelsson
Expert 1010 points
Part Number: AM3358
Other Parts Discussed in Thread: TUSB4041I, , TS3USB30E, TPS65218

We intend to design a PCB containing a USB High Speed hub TUSB4041I where
one of the ports are connected directly to an AM3358 USB port configured as a device.

The AM3358 is powered from a backplane, which might be powered down

The USB hub is self-powered, but will also get power from the backplane if present.

Are the USB pins on the CPU protected?

We are worried about what happens if the CPU is powered down, and the USB hub is not.

Is there a problem with latchup or other problems which might affect long time reliability?

Best Regards
Ulf

  • 0
    TI__Mastermind 27890 points

    Hello Ulf,

    The USB signal pins are not fail-safe and as such should not be powered when the SoC is not powered. As you suggest, powering these pins in the absence of SoC power will result in the SoC being damaged.

    The USBn_VBUS pins are fail-safe and have no dependency on SoC power status.

  • 0 in reply to -DK-
    Expert 1010 points

    Does TI have any protection circuit that can be used to protect the pins?

    Example schematics?

    Can the CPU USB pins be powered from a bus power while the rest of the system is powered down?

    If the USB hub would only be active on request from the CPU the problem would be solved I guess,

    but does the TUSB4041 have any way of  powering down a port based on an input signal?

  • 0 in reply to Ulf Samuelsson
    TI__Mastermind 27890 points

    Hi,

    I'm not aware of a dedicated protection device for this sort of problem.

    The CPU USBn_VBUS pins are indeed fail-safe and can be powered when the CPU is powered-off. This is typical for use cases in which the CPU is operating as a USB Device and is connected to a USB Host while powered off. As mentioned earlier, the USB signal pins are not fail safe and cannot be powered when the CPU is otherwise un-powered.

  • 0 in reply to -DK-
    Expert 1010 points

    We are thinking about Analog Switches with a dual 2:1 Mux Functionality.

    The T.I. TS3USB30E looks good at first sight. We are not allowed to use QFN, VSON or similar.
    Has to have legs, and the VSSOP package is fine.

    The Analog Switch would be powered by the same voltage as the FT4232 USB device.
    The D+,D- would be connected to the FT4232.
    The Select pin "S" would be connected to PGOOD signal of the AM3358 Power Managment circuit (TPS65218).

    When S = 0, D+,D- are connected to D1+, D1- (which goes nowhere)

    When S = 1, D+,D- are connected to D2+, D2- (which is connected to the CPU)

    The selection would be made by the PGOOD signal of the AM335x Power Circuit.

    PGOOD becomes high only when the power to the CPU is stable.

    When power is not present to the CPU, or unstable, the PGOOD goes low and the analog switch is disconnected

    Could you discuss this solution with the TI apps guys responsible for the TS3USB30E and see if there are any problems with such a connection?

  • 0 in reply to Ulf Samuelsson
    TI__Mastermind 27890 points

    Hello Ulf,

    I'm moving this thread to the Interface forum. The TS3USB30E device is supported there.