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SN74AXC8T245: Propagation Delay

Jeff YANG
Mastermind 7875 points
Part Number: SN74AXC8T245
Other Parts Discussed in Thread: LSF0108

Tool/software:

Hi,

Our application as below, 

Singal direction VCCB->VCCA

CH2: VCCA=0.66V

CH1: VCCB=1.2V

We found the signal has been distortion data rate about >10Mhz.

I need check the propagation delay, right? The maximum frequecy can support is 1/88ns=11.3Mhz?

Is there have better suggestion for ~20Mhz application?


Thanks!

Jeff

  • 0
    Guru 317260 points

    1 / 88 ns = 11.3 Mbps = 5.7 MHz

    There is no other device that works at such low voltages.

  • 0 in reply to Clemens Ladisch
    Mastermind 7875 points

    Hi Clemens,

    I found LSF0108 seems can support 0.65V to 1.8V, and note1 mentioned "VREF1 supports 0.65 V to VREF2 - 0.6 V".

    So it may support 0.65V to 1.25V, right? thanks!

    Jeff

  • +1 in reply to Jeff YANG
    Guru 317260 points

    The LSF is a passive switch, so its propagation delay is very small.

    The LSF clamps all voltages higher then VREF1. When shifting to a higher voltage, rising edges are generated by the pull-up resistor at the output, but in your case, you want to output a lower voltage, so it will work even with a large-valued resistor.

    Please note that the LSF requires a minimum difference between the two reference voltages, but that VREF2 can use a voltage that is higher than the pull-up voltage. So you can use pull-up voltages of 0.65 V and 1.2 V, with VREF1 = 0.65 V and VREF2 connected to something like 1.8 V or 3.3 V.

  • 0 in reply to Clemens Ladisch
    Mastermind 7875 points

    Hi Clemens,

    We plan using LSF0108 to translate 0.65V to (1.5V or 1.8V).

    Do you have this two voltage translate test waveform tested in past can share to us?

    Thanks!

    Jeff

  • 0 in reply to Jeff YANG
    Guru 317260 points

    When translating to a higher voltage, you must use a pull-up resistor that is strong enough to generate a rising edge that is fast enough. (The actual value depends on the capacitance of the board, so I cannot give estimates or tests.) The 0.65 V device must be able to sink the current through the pull-up, which is likely to be a problem. For translating up, it might be a better idea to use the AXC.