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LM3478Q-Q1: FA/SD floating issue

Dave Chang
Expert 3615 points
Part Number: LM3478Q-Q1
Other Parts Discussed in Thread: LM3478

Hello support team,

Thanks for the help!

Our customer has a problem about the FA/SD pin setting.

As I know, if the FA/SD connects the resistor to ground, the function is set to the frequency adjustment; otherwise, the pull-high signal(>1.3V) is related to shutdown.

At customer's application, there will be a short period during the transition which will let FA/SD pin being floating.

Will it affect the circuit? 

If it needs the customer's schematic about LM3478, I will provide it with Email, thanks.

Best Regards,

Dave

  • 0
    TI__Genius 16115 points

    Hello Dave,

    It is not recommended to leave the FA/SD pin floating while the part is powerd.

    The customer should please follow the method that is described in the datasheet.

    Figure 26 shows how to avoid a high current through R_FA during shutdown mode.

    Best regards,
    Harry

  • 0 in reply to Harry Hofner
    TI__Expert 3615 points

    Hello Harry,

    I think you have misunderstood.

    There is a very short period of floating during the power-on sequence(us level).

    Will it affect the behavior?

    Thanks.

    Dave

  • +1 in reply to Dave Chang
    TI__Genius 16115 points

    Hello Dave,

    I am sure that I do understand correctly. I did not mean to say "leave it floating for longer".

    The point is that I cannot find anyone here who can tell me for sure at which point of time the part will actually "read" that resistor value.
    So, even if I had the exact timing from the customer, I could not guarantee if it would always work in a stable way or not.

    Therefore, the recommendation is to follow the principle of figure 26 in the datasheet.

    If - during that transition - the pin will be disconnected for a short period of time, the customer does anyway do some kind of switching.

    So why can they not switch the FETas shown in figure 26 instead?

    I don't believe that the floating pin would damage the part. Therefore, if there is no other way, they can always try their concept with the real hardware. But no guarantee from our side that this will work stable under all conditions.

    Best regards,
    Harry