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LSF0108: LSF0108 werid waveforms for A->B and B->A

Frank Xiao
Genius 12055 points
Part Number: LSF0108

Hi team,

My customer is using the LSF0108 now. We find some weird phenomenon and want to double check the reason.

A side is 1.05v, B side 1.8v. SPI : A side is master, no pull up. B side is slave with 499ohm pull up resistor.

picture 1:

A side to Bside.(purple is A side input, yellow is B output. we can find the purple has a back rise curve.

picture2：

 B side to A side( A side will have a negative undershoot) purple is A side output, yellow is B input.

Thanks.

  • 0
    TI__Guru* 97046 points

    Hi Frank,

    The first issue shows what happens when the output can't discharge as fast as the input. Since the input and output are connected together at lower input voltages, the output will force the input to rise some. At low voltage, the LSF device acts like a resistor between the A and B sides.

    I'm confused on the problem for the second set of images. The "undershoot" shown doesn't even go below zero volts -- what's the issue here?

  • 0 in reply to Emrys Maier
    TI__Genius 12055 points

    Hi Emrys,

    So based on your description. The two phenomenon is reasonable, correct? Do we have some methods to improve the first issue and second undershoot? Thanks.

  • 0 in reply to Frank Xiao
    TI__Guru* 97046 points

    Hi Frank,

    A small amount of capacitance could be added to the "A" side of the device - but I'm not sure why you would do that. The waveforms have excellent integrity and I can't think of any situation in which the shown signals would cause any problems.

    Is the customer seeing an issue?

  • 0 in reply to Emrys Maier
    TI__Genius 12055 points

    Hi Emrys,

    We need to consider whether phenomenon will make the device after LSF0108 think the voltage to low then high then low during power down. Although currently the waveform is ok, not sure whether other waveforms will be worse to cause some issues. Thanks. 

  • 0 in reply to Frank Xiao
    TI__Guru* 97046 points

    Hi Frank,

    The input signal is not on the output - thus it will not cause non-monotonicity.

    There is no undershoot, as I already pointed out. 

    These signals have excellent quality. There is no issue to troubleshoot here.