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TCA6424: TCA6424 Hot Plugging

Part Number: TCA6424

Dear Team,

The problem currently encountered in this application is that the IO Pin is easily short-circuited when hot plugging (300kΩ high impedance becomes 130Ω)
Even if IO Pin is not dead, it's Pull High's impedance will be lower and lower (300kΩ==>60kΩ).
There are already TVS and Spike prevention solutions on every IO line.

Do you have any suggestion to solve it?

BR

Kevin

  • Hi Kevin,

    First, I'd make sure that the TVS solution has a peak clamping voltage that is suitable to protect the device from over-voltage conditions.  If the IOs are intended to toggle only at lower speeds, then you could also implement additional spike filtering using a shunt capacitance to ground on each pin.  You could also place series resistances on the IO lines in order to limit the current flowing into or out of the device during a fault condition, but you should be careful not to choose resistance values that are too high to interfere with normal operation.  In order to make sure output current is limited, it would also be useful if you could ensure that each port is configured as an input prior to the hot plug event (so that a low-impedance output does not become short-circuited).

    Regards,
    Max

  • Dear Max,

    Thanks for the promptly response!

    1. So normally, what is the rang of shunt C and series R you will suggest?

    2. Why is the input high impedance of IO getting lower and lower? Is it because the hot plug is damaged inside? Or because driving higher current will cause resistance lower?

    BR

    Kevin

  • Hi Kevin,

    It's hard to say, but my suspicion is that the input impedance is becoming lower because some damage has occurred to the internal circuitry that results in increased leakage to that pin.

    The shunt C range would depend on the signal bandwidth that is needed by the application.  Note that this capacitor will act with whatever series resistance exists in the application (including the effective output resistance of any driver circuit) to form a low-pass filter.  You should make sure that the filter isn't so strong that it degrades normal signaling.

    The series resistance range would affect bandwidth (as stated above) but will also introduce some DC loss.  You can compute the voltage drop across this resistance for your worst-case load current and determine whether or not it is too high for your application.

    Regards,
    Max

  • Dear Max

    If I series resistance on the Io pin, it can't pull low completely. It will have some risk.

    The circuit design and TVS solution as below picture. 

    If you have any suggestion kindly let me know.

    Thanks

  • Hi Kevin,

    What voltage does the IO get shorted to - is it just ground (as shown in the above illustration?)  If so, does the issue only occur when the output is high while short-circuited?  If so it seems the only solution would be to limit the current to within the absolute maximum IOH level of 25 mA.  If the series resistance cannot work, then is there any way to ensure the port is configured as an input during hot plug?  If not then you might consider placing a mux, buffer, or switch/FET/etc. in series with the signals so that the conduction path is only enabled after the two sides are properly mated together, but that will be a more complicated solution.

    Max