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SN74LV244A: How to Know Output Resistor Need

Kazuki Itoh
Genius 9165 points
Part Number: SN74LV244A
Other Parts Discussed in Thread: SN74LV04A, SN74AHC541, SN74AHC244

Hi team,

I am considering if output resistor is needed to attenuate overshoot/undershoot in my system.

As SN74LV244A has very high speed tpd, very short transition time is expected.

How can I estimate transition time of SN74LV244A?

How can I determine output resistor value?

Also, I found the overshoot/undershoot of SN74LV04A is moderate compared to SN74LV244A so output resistor is usually not needed.

Is this just because SN74ALV244A has higher IOL and IOH than SN74LV04A?

Regards,

Itoh

  • 0
    Guru 243110 points

    tpd depends both on the internal switching speed and the strength of the output drivers. The '244 is a bus driver, so it has been designed with stronger outputs than the '04. Consider using a family with weaker drivers, e.g., SN74AHC244/SN74AHC541.

    The transition time can be estimated from the IBIS model.

    The resistor should be sized so that the sum of the device's output impedance and the resistor is equal to the trace's characteristic impedance. If you do not know the latter, just try out some resistors between about 20 Ω and 100 Ω and measure.