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SN74LVCH8T245: Going from SN74LVC8T245RHLR to SN74LVCH8T245RHLR

Albin Meli
Prodigy 10 points
Part Number: SN74LVCH8T245

We currently use SN74LVC8T245RHLR in our design and due to availability we were looking for replacement and landed on SN74LVCH8T245RHLR. those components are the same aside from SN74LVCH8T245RHLR having active bus-hold circuitry. Currently we have pullups and pulldowns in our circuit and we would love to know if:

  1. Can we move forward with using SN74LVCH8T245RHLR
  2. what the integral pullups/down resistor values are to see if the outputs of the driving devices can handle the extra current or remove the pullups from the board
  3. how much current will they sink/source when needed for a pullup / down
  • 0
    TI__Mastermind 22295 points

    Hi Albin,

    When using a device with bus-hold circuitry, pullup and pulldown aren't recommended. You can refer to this Application Note: Implications of Slow or Floating CMOS Inputs in section 4 where it goes more in depth about bus holds. 

  • 0 in reply to Albert Xu1
    Prodigy 10 points

    I understand that pullup and pulldown arent recommended and I understand bus holds. I'm trying to figure out what the impact of having pullup and pulldown be, and also what the integral pullup/pulldown resistor values are for the specific part list above.

  • 0 in reply to Albin Meli
    TI__Guru* 96681 points

    Hi Albin,

    A pull-up or pull-down resistor will create a voltage-divider with the bus-hold circuitry any time the bus holder is in the opposite state to the resistor's supply.

    For example:

    With a typical bus-hold circuit, the output resistance is around 1kohm. If you ahve a 10kohm pull-down resistor at the input and are not otherwise actively driving the line, there are two states this can be in:

    (1) The bus-hold circuit is in the low state, and there's no issue.

    (2) The bus-hold circuit is in the high state, and now the input voltage is pulled down to ~0.9 * VCC.

    #2 isn't much of an issue if you don't care what state the input is at, but if your intention of the pull-down is to put it in the low state, then you can see it won't work unless the resistor is very small (ie less than 1kohm).

    It's much better to just not use bus-hold if that's the case.