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Original question:

SN74CBTLV3257: SN74CBTLV3257PWR design issue for Voltage

SN74CBTLV3257: SN74CBTLV3257PWR design issue

Gary Chang
Intellectual 350 points
Part Number: SN74CBTLV3257
Other Parts Discussed in Thread: TMUX1574

Hello Team,

We use the SN74CBTLV3257 as the U7 MUX, in terms of signal performance, there is a difference compared to the PI3CH480QEX.

The VIL of PI3CH is only 0.2V, is this the leakage current difference of SN74CBTLV3257? 

Or do you have a better recommendation for the MUX?

Thanks

  • 0
    TI__Mastermind 27818 points

    Hey Gary,

    It's hard to determine if it's leakage given that neither datasheet actually specifies an on-leakage value. But that would make sense as a potential cause. Otherwise, the specs of both these devices are fairly close.
    The low voltage level is a bit high either way though. Is the line pulled up via a pullup resistor? You could reduce that value to hep this. Seems your pull up is a little too strong. 

    Thanks,
    Rami

  • 0 in reply to Rami Mooti1
    Intellectual 350 points

    Hello Rami,

    Thanks for your response. We currently do not have a pull-up resistor added.

    We previously added a 50K resistor, but it had no effect

    Thanks

  • 0 in reply to Gary Chang
    TI__Mastermind 27818 points

    Hey Gary,

    Sorry, I overlooked it was SPI so no pullup/pulldowns needed. So it does look like it may be a leakage related phenomena. 

    A pulldown could help the low voltage pulldown enough but you would probably be affecting the VIH then. Do you know the output impedances? You could try putting somewhere around a 200ohm pulldown (assuming roughly a 50ohm output impedance but I could be wrong) and see if the VIH is still high enough and the VIL is low enough for your use case. You could try shifting the pulldown around, could be up to 1kohm even (just depends on the output impedance here) to see if there is a sweet spot that your system will accept. 

    Although, I would recommend moving the TMUX1574 instead in this case to avoid the need of an external resistor. It's a drop in replacement to the 3257

    Thanks,
    Rami