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SN74LVC1G139: Propagation delay

Daniel Flor
Intellectual 430 points
Part Number: SN74LVC1G139
Other Parts Discussed in Thread: SN74LVC1G29

Hello,

I need to use a fast decoder to be used in conjunction with an SRAM memory. As times are extremely critical, I looked for a decoder that was as fast as possible and also powered by 5V. The model I found was the SN74LVC1G139D, in the datasheet I observed the following information:

In section 6.6 (Switching Characteristics) it says that for a supply voltage of 5V, recommended operating free-air temperature, and CL = 15pF we have:

0.8 ns <= Tpd <= 3.6ns

Figure 1 of section 7 (Typical Characteristics) shows a graph, Propagation Delay x VCC, which shows Tplh and Tphl for the different paths within the decoder. According to the graph, for VCC = 5V the typical time would be close to 2 ns


The following questions arose that I would like to clarify:

  • Using 5V supply, what would be the typical time propagation delay ?
  • Within the minimum and maximum Tpd range, how Tpd behaves and what influences it? Is it random?
  • What to do for Tpd to be closer to 0.8ns?

Another important question:

   Is there a version or a similar decoder that have a Enable Low pin on it ?

Wit my best regards,

Daniel.

  • 0
    TI__Expert 6680 points

    Hi Daniel,

    It's difficult to say exactly what the propagation delay for a device will be, however TI guarantees that the delay will fall between that 0.8 & 3.6ns listed on the datasheet, when driving a capacitive load of 15pF. Typically, devices will tend to operate faster the closer they are to their maximum recommended operating voltage (recommended maximum VCC, not absolute maximum). Additionally, devices' propagation delay will depend on things like temperature, and load. See the below FAQ on capacitive loads for more information on propagation delay and how it relates to load capacitance:

    And unfortunately no, there is no device with an enable pin with the low tpd you're looking for.

    Thanks!

    Chad Crosby

  • 0 in reply to Chad Crosby
    Intellectual 430 points

    Thank you,  Chad Crosby. I found a good choice that has a smaller Tpd, it is not a 2x4, it is a 2x3, but can work too, it is the SN74LVC1G29. I'm interested in the low Tpd. I thought a circuit using logic gates but I didn't find one that  was fast enought. So I will try this IC.