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LSF0108: About rising and floating waveforms

Takanari Nishikawa
Prodigy 80 points

Part Number: LSF0108
Other Parts Discussed in Thread: SN74AXC1T45

Hi, I am thinking of using LSF0108.

Currently, we are conducting evaluations, but the rising and floating of the waveform is an issue.
The module specification is satisfactory, but we want to improve it.

Trying to change the pull-up resistance constant.
However, I am in trouble because of the barter of "floating of waveform" and "lack of rise".

Please tell me if there is a way to improve it.

Refer to the attached Excel for details.

LSF0108 Waveforms.xlsx

  • 0
    TI__Guru* 97146 points

    Hello Nishikawa-san,

    First, allow me to apologize for the extended time your post went without being answered. Generally, we will answer any post within 24 hours, however our tracking system was changed and we did not see this post until today.

    -

    The two issues you have identified are both expected performance trade-offs with using a passive clamp-FET based translator.

    The speed of the device can be increased by reducing the pull-up resistor size on the higher-voltage side of the device, however this will increase the output low-state voltage (V_OL, or what you are calling "floating of waveform"). To reduce the V_OL, the pull-up resistor must be reduced, or the driver must be stronger.

    The speed can also be increased by reducing the load capacitance, however that will typically be done by reducing the trace length on the higher-voltage side of the device, and cannot be easily achieved with a system that is already built.

    It may be the case that you cannot use an LSF translator for this application.

    -

    I would recommend to use an active translator such as the SN74AXC1T45 wherever possible. This will very significantly improve the output waveform, however it takes away the capability for auto-bidirectional signals.

    Where auto-direction is required, I would recommend using the TXB01xx series of translators whenever possible (when there are no pull-up or pull-down resistors and the driver is push-pull). Otherwise, I would recommend to use a TXS01xx series translator, which works similarly to the LSF, but adds some circuitry to speed up transition rates.