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LSF0108: tps386000 and lsf0108 pull up resistor issue

user1359349
Intellectual 450 points
Part Number: LSF0108
Other Parts Discussed in Thread: TPS386000,

hello

   i have a schematic ,the power sequence chip TPS38600 output need to connect the level translator,the TPS38600 power supply and pull up is 5V,and the LSF0108 need to translator to 3V3.

    acroding to the datasheet of TPS38600, i need to use a resistor which pull up the reset should be no smaller than 10k,but the lsf0108datasheet recommends the pull up resistor is 470R

which value will i need,thanks

  • 0
    TI__Guru* 97016 points
    Hi,
    The TPS386000 has open-drain outputs, which means that the outputs can be pulled-up to any voltage less than or equal to the devices limits -- in this case, 7V (Absolute Maximum Ratings table). You don't need a level translator for this application. Just connect the open drain to 3.3V via a pull-up resistor. As a side note, this voltage maximum is independent of the suply voltage of the device -- if you are inputting 5V or another voltage, you can still pull-up the outputs to a maximum of 7V.
  • 0 in reply to Emrys Maier
    Intellectual 450 points

    hello maier
    thanks for your replay,because on the PCBA,the ouput of tps386000 is already pull up to 5V,so there is a level translator between the tps386000 and cpld.
    now on the PCBA ,the pull up resistor have refered to lsf0108 datasheet,the value is 470R ,but when i read datasheet this days ,i have found ,the tps386000 recommends the pull up resistor on the ouput should not be smaller than 10k,
    please check it for me,if is ok for tps386000 and lsfg0108.do i need to change the value of the pull up resistor?

    please refer the refer to the following figure,thanks.

  • 0 in reply to user1359349
    TI__Guru* 97016 points
    The limiting factor here is the drive current of the TPS device. It looks like (from the datasheet) the device can only sink ~1mA of current into each RESET pin. That means your pull-up resistor needs to be sized to limit current to 1mA maximum.

    Doing some quick math, assuming a worst case of 5V + 10% = 5.5V, the resistor needs to be at least:

    R = 5.5/0.001 = 5.5 kohm.

    The short version of this -- if you just swap the pull-up to 10 kohm it should work fine.
  • 0 in reply to Emrys Maier
    Intellectual 450 points
    hello maier
    thank you very much,i will do it as your advise.