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SN74LV125A: we use buffer in STO circuit and this buffer cannot be recognized as a fail-safe part.

Long Wang
Prodigy 30 points
Part Number: SN74LV125A


Hello all,

I just encountered an old design, which now is under process of Safety certification with STO. 

Explanation of our circuit:

one 5V, which is generated from a circuit with input of STO signal, named as 5V_STO, has been connected as Vcc.

So the person from the certification local said, there is a danger, that the 5V_STO circuit can be failed, and then the high voltage, 500V in our case (by the way, this kind of circuit has been implemented in at least 50 thousand products over last 8 years, this kind of failure never happened, yeah, I know, it does not mean never.), can be transmitted further to buffer. Until here, I admit his argument. 

What he further said, leads me here to ask for your ideas. He said: "A buffer, like SN74LV125A, SHOULD never be accepted in a safety circuit, because it is not fail-safe."

I checked the safety products in TI list, there is no category of buffer. There might be either no fail-safe document about SN74LV125A, at least I did not find one. 

So here comes my question: Is SN74LV125A fail-safe or not, please give me some clues. Now I might use simulation tool to simulate the failure with 500V, but I am not sure, whether the spice model is really good at failure case simulation.

Thanks for your reading and further support.

Best regards,

Long

  • +1
    Guru 317480 points

    Applying 500 V exceeds the absolute maximum ratings, and will damage the device. And there is no guarantee what the effects are. (And no, a circuit simulator cannot model what happens when a chip blows up.)

    No safety documentation exists, and even if it did, it would just say "the pins could be shorted", so you'd have 500 V at the outputs.

    I do not know what your circuit does, so I cannot give recommendations. But if you want to use logic gates, you need to design your power supply and inputs so that 500 V does not reach them.

  • +1 in reply to Clemens Ladisch
    Prodigy 30 points

    Hi Clemens,

    thank you for your reply, we are now kind of changing our circuit, instead of buffer, we use digital isolator, which is listed in TI safety products.

    For buffer, if the damage effects not clarified by TI, we have to assume the worst effect, which as you said, short circuit to output.

    best regards,

    Long