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LSF0108: Enabling/Disabling Multiple LSF0108s

Part Number: LSF0108

Tool/software:

Hi,

I would like to enable and disable multiple LSF0108s with the same jumper. I believe I have biased the LSF0108s properly so that they can be disabled when the jumper is populated, but is there any issue when VREF_B and EN pins between multiple LSF0108s are shorted together when they are meant to be enabled? Could the VREF_Bs or ENs somehow contend with each other?

Thanks,

Matthew

  • Hi Matthew,

    I would recommend against doing so- the total amount of leakage current will increase after every LSF being enabled this way and cause a fluctuation in the VREFB + EN node, potentially causing translation issues. Each LSF0108 should have its own open-drain driver at the EN pin if it was to be EN/DIS at different times. 

    www.ti.com/.../5614304068001

    Regards,

    Jack

  • Hey Jack,

    Thanks for the recommendation.

    Just to make sure we're on the same page, we would like to enable all LSFs or disable all LSFs - we're not trying to enable some LSFs while disabling others.

    One thing I'm a little confused by is - while the total amount of leakage current would increase with every LSF being enabled this way, I was planning on adding a 200K ohm pullup for every single LSF with a shared VREF_B + EN pin as shown in the diagram. Shouldn't this result in the node biasing to the same voltage as if it weren't connected to other LSFs? Is there some sort of dynamic current draw on these pins while translating that would cause contention with the other LSFs?

    Thanks,

    Matthew 

  • The VREF_B/EN voltages of different chips can be different; they should not be connected together.

  • Hi Clemens,

    Thanks for clarifying this point. Alternatively to achieve more or less the same desire of a single jumper controlling enabling/disabling multiple LSFs, would there be an issue with the following? This is more or less inspired by Figure 9-1 of the datasheet where VCCB is used more like an enable signal.

    Thanks,
    Matthew

  • Hi Matthew,

    This is more acceptable then previous implementation, though note that each LSF0108 being used will sink in a current of (VREFB-VREFA-0.7V)/ 200kohms. It is important to check that the single jumper can accept this amount of current sourced.  Please note that the 5kohm pulldown should be removed here to prevent undesirable biasing to this node as well. 

    Regards,

    Jack

  • Hi Jack,

    Thanks for the response - the idea behind the 5K pulldowns was to weakly bias the node to GND in the case that the jumper is depopulated, such that the +5V is disconnected & to avoid VREF_B + EN possibly floating.

    I'm not certain I fully understand the concern regarding the biasing of the 5K pulldown - would we be better of leaving that node to be floating when the jumper is depopulated? Or is there an issue for when the jumper is populated and the +5V is essentially shorted to that node?

    Thanks,
    Matthew 

  • Hi Matthew,

    Upon closer eval, this implementation (5kohm pulldown) is acceptable as it will not influence the VREFB + EN node during translation. Customer would just have to pay closer attention to the additional leakage from this pulldown and if see it acceptable per their system requirements. 

    Regards,

    Jack

  • HI Jack,
    Thanks for confirming this.

    Thanks!

    Matthew