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SN74LVTH125: Active bus-hold circuitry clarification.

Genius 10142 points
Part Number: SN74LVTH125
Other Parts Discussed in Thread: SN74LV4T125, , SN74LV125

Hi Experts,

Good day. Due to supply issue, client would have to choose SN74LVTH125 as replacement for SN74LV4T125.

They are only using the part as a translator from 5V to 3.3V (Vcc = 3.3V). Infosheet of this alternative have this app note:

 As they have one input pin connected directly to ground, will this be a problem?
 
We are also looking at SN74LV125  and SN74LVC125. The output loading is in microamps (input to a CPLD). Are any of these viable?

Thanks for your guidance.

Regards,
Archie A.

  • Hi Archie,

    Emrys explains this very well in a previous question we had. 

    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.

  • Connecting a bus-hold input directly to ground is no problem, because there is no voltage divider.

    For translating to a lower voltage, you can use any device with overvoltage-tolerant inputs. Devices from the LV, LVC, or AHC families are fine.